[Federal Register: October 14, 2003 (Volume 68, Number 198)]
[Rules and Regulations]
[Page 59249-59304]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr14oc03-30]
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Part II
Department of Transportation
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National Highway Traffic Safety Administration
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49 CFR Part 575
Consumer Information; New Car Assessment Program; Rollover Resistance;
Final Rule
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DEPARTMENT OF TRANSPORTATION
National Highway Traffic Safety Administration
49 CFR Part 575
[Docket No. NHTSA-2001-9663; Notice 3]
RIN 2127-AI81
Consumer Information; New Car Assessment Program; Rollover
Resistance
AGENCY: National Highway Traffic Safety Administration (NHTSA), DOT.
ACTION: Final policy statement.
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SUMMARY: The Transportation Recall Enhancement, Accountability, and
Documentation Act of 2000 requires NHTSA to develop a dynamic test on
rollovers by motor vehicles for the purposes of a consumer information
program, to carry out a program of conducting such tests, and, as these
tests are being developed, to conduct a rulemaking to determine how
best to disseminate test results to the public. This document modifies
NHTSA's rollover resistance ratings in its New Car Assessment Program
(NCAP) to include dynamic rollover tests after considering comments to
our previous document. The changes described in this document will
improve consumer information provided by NHTSA, but will not place
regulatory requirements on vehicle manufacturers.
DATES: NCAP rollover resistance ratings in the 2004 model year will be
determined using the system established by this document.
Petitions: Petitions for reconsideration must be received by
November 28, 2003.
FOR FURTHER INFORMATION CONTACT: For technical questions you may
contact Patrick Boyd, NVS-123, Office of Rulemaking, National Highway
Traffic Safety Administration, 400 Seventh Street, SW., Washington, DC
20590 and Dr. Riley Garrott, NVS-312, NHTSA Vehicle Research and Test
Center, P.O. Box 37, East Liberty, OH 43319. Mr. Boyd can be reached by
phone at (202) 366-6346 or by facsimile at (202) 493-2739. Dr. Garrott
can be reached by phone at (937) 666-4511 or by facsimile at (937) 666-
3590.
SUPPLEMENTARY INFORMATION:
I. Executive Summary
II. Safety Problem
III. Background
A. Existing NCAP Program and the TREAD Act
B. National Academy of Sciences Study
IV. Notice of Proposed Rulemaking
V. Results of Dynamic Maneuver Tests of 25 Vehicles
A. J-Turn Maneuver
B. Fishhook Maneuver
C. Loading Conditions
D. Test Results
VI. Rollover Risk Model
VII. Comments to the Previous Notice
A. Combined or Separate Rollover Resistance Ratings
B. Crash Avoidance Technologies
C. The J-Turn and Fishhook Maneuvers
D. Tire Wear
E. Pavement Temperature
F. Surface Friction
G. Steering Reversal
H. Fifteen-Passenger Vans
I. Tip-up Criterion
J. Testing of Passenger Cars vs. Light Trucks
K. Testing with Electronic Stability Control Systems
VIII. Final Form for Rollover Resistance Ratings `` Alternative I
A. Combined Ratings
B. Dynamic Testing
C. Demonstration Program
IX. Cost Benefit Statement
X. Rulemaking Analyses and Notices
A. Executive Order 12866
B. Regulatory Flexibility Act
C. National Environmental Policy Act
D. Executive Order 13132 (Federalism)
E. Unfunded Mandates Act
F. Civil Justice Reform
G. Paperwork Reduction Act
H. Plain Language
Appendix I. Fishhook Test Protocol
Appendix II. Development of Logistic Regression Risk Model
I. Executive Summary
While the total number of highway fatalities has remained
relatively stable over the past decade, the number of rollover deaths
has risen substantially. According to NHTSA's National Center for
Statistics and Analysis, from 1991 to 2001 the number of passenger
vehicle occupants killed in all motor vehicle crashes increased 4
percent, while fatalities in rollover crashes increased 10 percent. In
the same decade, passenger car occupant fatalities in rollovers
declined 15 percent while rollover fatalities in light trucks increased
43 percent. In 2001, 10,138 people died in rollover crashes, a figure
that represents 32 percent of occupant fatalities for the year.
In response to that trend, NHTSA has been evaluating rollover
testing since 1993. In 2001, NHTSA began publishing rollover rating
information for consumers, supplementing New Car Assessment Program
(NCAP) frontal crashworthiness ratings that began in 1979 and side
impact ratings that began in 1997.
When Congress approved the ``Transportation Recall, Enhancement,
Accountability and Documentation (TREAD) Act of November 2000'',
Section 12 directed the Secretary of Transportation to ``develop a
dynamic test on rollovers by motor vehicles for a consumer information
program; and carry out a program conducting such tests. As the
Secretary develops a [rollover] test, the Secretary shall conduct a
rulemaking to determine how best to disseminate test results to the
public.''
On July 3, 2001, NHTSA published a Request for Comments notice (66
FR 35179) discussing a variety of dynamic rollover tests that we had
chosen to evaluate in our research program and what we believed were
their potential advantages and disadvantages.
We published a Notice of Proposed Rulemaking on October 7, 2002 (67
FR 62528) that proposed alternative ways of using the dynamic maneuver
test results in consumer information on the rollover resistance of new
vehicles.
Beginning with rollover ratings for the 2004 model year, NHTSA will
combine a vehicle's Static Stability Factor (SSF) measurement with its
performance in the so-called ``Fishhook'' maneuver. The so-called ``J-
Turn'' dynamic test maneuver discussed in previous notices will be not
be used by NHTSA for rating rollover resistance. Our analysis has found
that the J-Turn maneuver test does not add any meaningful information
to what is obtained from the fishhook maneuver test alone (see Appendix
II.B). The predicted rollover rate will be translated into a five-star
rating system that is the same as the one now in use: One star is for a
rollover rate greater than 40 percent; two stars, between 30 and 39
percent; three stars, between 20 and 29 percent; four stars, between 10
and 19 percent; and five stars for 10 percent or less.
This decision maximizes the vehicle information used to make the
rollover rate prediction and will allow us to ensure that rollover NCAP
information corresponds even more closely to real-world rollovers. We
have also decided to present our rollover information as a single
combined rollover rating that most commenters agreed would be more
understandable to consumers.
This document also includes a test procedure (Appendix I) for
conducting vehicle maneuver tests, and discusses testing regimes that
have been incorporated to minimize variability in test data.
II. Safety Problem
Rollover crashes are complex events that reflect the interaction of
driver, road, vehicle, and environmental factors. We can describe the
relationship between these factors and the risk of rollover using
information from the agency's crash data programs. We limit our
discussion here to light vehicles,
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which consist of (1) passenger cars and (2) multipurpose passenger
vehicles and trucks under 4,536 kilograms (10,000 pounds) gross vehicle
weight rating.\1\
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\1\For brevity, we use the term ``light trucks'' in this
document to refer to vans, minivans, sport utility vehicles (SUVs),
and pickup trucks under 4,536 kilograms (10,000 pounds) gross
vehicle weight rating. NHTSA has also used the term ``LTVs'' to
refer to the same vehicles.
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According to the 2001 Fatality Analysis Reporting System (FARS),
10,138 people were killed as occupants in light vehicle rollover
crashes, which represent 32 percent of the occupants killed that year
in crashes. Of those, 8,407 were killed in single-vehicle rollover
crashes. Seventy-eight percent of the people who died in single-vehicle
rollover crashes were not using a seat belt, and 64 percent were
partially or completely ejected from the vehicle (including 53 percent
who were completely ejected). FARS shows that 54 percent of light
vehicle occupant fatalities in single-vehicle crashes involved a
rollover event.
Using data from the 1997-2001 National Automotive Sampling System
(NASS) Crashworthiness Data System (CDS), we estimate that 281,000
light vehicles were towed from a police-reported rollover crash each
year (on average), and that 30,000 occupants of these vehicles were
seriously injured or killed (defined as any fatality or an injury with
an Abbreviated Injury Scale (AIS) rating of at least AIS 3).\2\ Of
these 281,000 light vehicle rollover crashes, 225,000 were single-
vehicle crashes. (The NCAP rollover resistance ratings estimate the
risk of rollover if a vehicle is involved in a single-vehicle crash.)
Sixty-one percent of those people who suffered a serious injury in
single-vehicle towaway rollover crashes were not using a seat belt, and
49 percent were partially or completely ejected (including 40 percent
who were completely ejected). Estimates from NASS CDS indicate that 80
percent of towaway rollovers were single-vehicle crashes, and that 83
percent (168,000) of the single-vehicle rollover crashes occurred after
the vehicle left the roadway. An audit of 1992-96 NASS CDS data showed
that about 95 percent of rollovers in single-vehicle crashes were
tripped by mechanisms such as curbs, soft soil, pot holes, guard rails,
and wheel rims digging into the pavement, rather than by tire/road
interface friction as in the case of untripped rollover events.
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\2\A broken hip with splintering of the bone is an example of an
AIS 3 injury.
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According to the 1997-2001 NASS General Estimates System (GES)
data, 62,000 occupants annually received injuries rated as K or A on
the police KABCO injury scale in rollover crashes. (The police KABCO
scale calls A injuries ``incapacitating,'' but their actual severity
depends on local reporting practice. An ``incapacitating'' injury may
mean that the injury was visible to the reporting officer or that the
officer called for medical assistance. A K injury is fatal.) The data
indicate that 215,000 single-vehicle rollover crashes resulted in
49,000 K or A injuries. Fifty percent of those with K or A injury in
single-vehicle rollover crashes were not using a seat belt, and 24
percent were partially or completely ejected from the vehicle
(including 21 percent who were completely ejected). Estimates from NASS
GES indicate that 13 percent of light vehicles in police-reported
single-vehicle crashes rolled over. The estimated risk of rollover
differs by light vehicle type: 10 percent of cars and 10 percent of
vans in police-reported single-vehicle crashes rolled over, compared to
18 percent of pickup trucks and 27 percent of SUVs. The percentages of
all police-reported crashes for each vehicle type that resulted in
rollover were 1.7 percent for cars, 2.0 percent for vans, 3.8 percent
for pickup trucks and 5.5 percent for SUVs as estimated by NASS GES.
III. Background
A. Existing NCAP Program and the TREAD Act
NHTSA's NCAP program has been publishing comparative consumer
information on frontal crashworthiness of new vehicles since 1979, on
side crashworthiness since 1997, and on rollover resistance since
January 2001 (66 FR 3388). This notice does not establish a new
consumer information program on rollover resistance ratings. Rather, it
refines our existing rollover resistance rating program in accordance
with the requirements of the TREAD Act and the recommendations of the
National Academy of Sciences.
The present NCAP rollover resistance ratings are based on the
Static Stability Factor (SSF) of a vehicle, which is the ratio of one
half its track width to its center of gravity (c.g.) height (see http://frwebgate.access.gpo.gov/cgi-bin/leaving.cgi?from=leavingFR.html&log=linklog&to=http://www.nhtsa.dot.gov/hot/rollover/
for ratings and explanatory
information). After an evaluation of some driving maneuver tests in
1997 and 1998, we chose to use SSF instead of any driving maneuvers to
characterize rollover resistance. As we explained in our notices
establishing rollover NCAP, we chose SSF as the basis of our ratings
because it represents the first order factors that determine vehicle
rollover resistance in the vast majority of rollovers which are tripped
by impacts with curbs, soft soil, pot holes, guard rails, etc. or by
wheel rims digging into the pavement. In contrast, untripped rollovers
are those in which tire/road interface friction is the only external
force acting on a vehicle that rolls over. Driving maneuver tests
directly represent on-road untripped rollover crashes, but such crashes
represent less than five percent of rollover crashes.\3\
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\3\ NHTSA Reseach Note, ``Passenger Vehicles in Untripped
Rollovers,'' September 1999.
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At the time, we believed it was necessary to choose between SSF and
driving maneuver tests as the basis for rollover resistance ratings.
SSF was chosen because it had a number of advantages: it is highly
correlated with actual crash statistics; it can be measured accurately
and inexpensively and explained to consumers; and changes in vehicle
design to improve SSF are unlikely to degrade other safety attributes.
We also considered the fact that an improvement in SSF represents an
increase in rollover resistance in both tripped and untripped
circumstances while maneuver test performance can be improved by
reduced tire traction and certain implementations of electronic
stability control that we believe are unlikely to improve resistance to
tripped rollovers.
Congress funded NHTSA's rollover NCAP program, but directed the
agency to enhance the program. Section 12 of the ``Transportation
Recall, Enhancement, Accountability and Documentation (TREAD) Act of
November 2000'' directs the Secretary to ``develop a dynamic test on
rollovers by motor vehicles for a consumer information program; and
carry out a program conducting such tests. As the Secretary develops a
[rollover] test, the Secretary shall conduct a rulemaking to determine
how best to disseminate test results to the public.'' The rulemaking
was to be carried out by November 1, 2002.
On July 3, 2001, NHTSA published a Request for Comments notice (66
FR 35179) regarding our research plans to assess a number of possible
dynamic rollover tests. The notice discussed the possible advantages
and disadvantages of various approaches that had been suggested by
manufacturers, consumer groups, and NHTSA's prior research. The driving
maneuver tests to be evaluated fit into two broad categories: closed-
loop maneuvers in which all test vehicles attempt to follow the same
path; and open-loop maneuvers in which all test vehicles are given
equivalent steering inputs. The
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principal theme of the comments was a sharp division of opinion about
whether the dynamic rollover test should be a closed loop maneuver test
like the ISO 3388 double lane change that emphasizes the handling
properties of vehicles or whether it should be an open loop maneuver
like a J-Turn or Fishhook that are limit maneuvers in which vulnerable
vehicles would actually tip up. Ford recommended a different type of
closed loop lane change maneuver in which a path-following robot or a
mathematical correction method would be used to evaluate all vehicles
on the same set of paths at the same lateral acceleration. It used a
measurement of partial wheel unloading without tip-up at 0.7g lateral
acceleration as a performance criterion in contrast to the other closed
loop maneuver tests that used maximum speed through the maneuver as the
performance criterion. Another unique comment was a recommendation from
Suzuki to use a sled test developed by Exponent Inc. to simulate
tripped rollovers.
The subsequent test program (using four SUVs in various load
conditions and with and without electronic stability control enabled on
two of the SUVs) showed that open-loop maneuver tests using an
automated steering controller could be performed with better
repeatability of results than the other maneuver tests. The J-Turn
maneuver and the Fishhook maneuver (with steering reversal at maximum
vehicle roll angle) were found to be the most objective tests of the
susceptibility of vehicles to maneuver-induced on-road rollover. Except
for the Ford test, the closed loop tests were found not to measure
rollover resistance. Instead, the tests of maximum speed through a
double lane change responded to vehicle agility. None of the test
vehicles tipped up during runs in which they maintained the prescribed
path even when loaded with roof ballast to experimentally reduce their
rollover resistance. The speed scores of the test vehicles in the
closed loop maneuvers were found to be unrelated to their resistance to
tip-up in the open-loop maneuvers that actually caused tip-up. The test
vehicle that was clearly the poorest performer in the maneuvers that
caused tip-ups achieved the best score (highest speed) in the ISO 3388
and CU short course double lane change, and one vehicle improved its
score in the ISO 3388 test when roof ballast was added to reduce its
rollover resistance.
Due to the non-limit test conditions and the averaging necessary
for stable wheel force measurements, the wheel unloading measured in
the Ford test appeared to be more quasi-static (as in driving in a
circle at a steady speed or placing the vehicle on a centrifuge) than
dynamic. Sled tests were not evaluated because we believed that SSF
already provided a good indicator of resistance to tripped rollover.
B. National Academy of Sciences Study
During the time NHTSA was evaluating dynamic maneuver tests in
response to the TREAD Act, the National Academy of Sciences (NAS) was
conducting a study of the four SSF-based rollover resistance ratings
and was directed to make recommendations regarding driving maneuver
tests. We expected the NAS recommendations to have a strong influence
on TREAD-mandated changes to NCAP rollover resistance ratings.
When NHTSA proposed the present SSF rollover resistance ratings in
June 2000 (65 FR 34998), vehicle manufacturers generally opposed it
because they believed that SSF as a measure of rollover resistance is
too simple since it does not include the effects of suspension
deflections, tire traction and electronic stability control (ESC). In
addition, the vehicle manufacturers argued that the influence of
vehicle factors on rollover risk is too slight to warrant consumer
information ratings for rollover resistance. In the conference report
of the FY2001 DOT Appropriations Act, Congress permitted NHTSA to move
forward with its rollover rating program, but directed the agency to
fund a National Academy of Sciences (NAS) study on vehicle rollover
ratings. The study topics were ``whether the static stability factor is
a scientifically valid measurement that presents practical, useful
information to the public including a comparison of the static
stability factor test versus a test with rollover metrics based on
dynamic driving conditions that may induce rollover events.'' The
National Academy's report was completed and made available at the end
of February 2002.
The NAS study found that SSF is a scientifically valid measure of
rollover resistance for which the underlying physics and real-world
crash data are consistent with the conclusion that an increase in SSF
reduces the likelihood of rollover. It also found that dynamic tests
should complement static measures, such as SSF, rather than replace
them in consumer information on rollover resistance. The dynamic tests
the NAS recommended would be driving maneuvers used to assess
``transient vehicle behavior leading to rollover.''
The NAS study also made recommendations concerning the statistical
analysis of rollover risk and the representation of ratings. It
recommended that we use logistic regression rather than linear
regression for analysis of the relationship between rollover risk and
SSF, and it recommended that we consider a higher-resolution
representation of the relationship between rollover risk and SSF than
is provided by the current five-star rating system.
We published a Notice of Proposed Rulemaking on October 7, 2002 (67
FR 62528) that proposed alternative ways of using the dynamic maneuver
test results in consumer information on the rollover resistance of new
vehicles. We chose the J-Turn and Fishhook maneuver (with roll rate
feedback) as the dynamic maneuver tests because they were the type of
limit maneuver tests that could directly lead to rollover as
recommended by the NAS. We also proposed to use a logistic regression
analysis to determine the relationship between vehicle properties and
rollover risk, as recommended by the NAS. The resulting rollover
resistance ratings were proposed to be part of NHTSA's New Car
Assessment Program (NCAP). Also, we proposed two methods for presenting
rollover resistance ratings for consumer information.
IV. Notice of Proposed Rulemaking
The TREAD Act calls for a rulemaking to determine how best to
disseminate rollover test results to the public, and our Notice of
Proposed Rulemaking (NPRM) of October 7, 2002 (67 FR 62528) proposed
two alternatives for using the dynamic test results in consumer
information on the rollover resistance of new vehicles. In this case
the term ``rulemaking'' refers more to the process than to the product.
This document does not amend the Code of Federal Regulations, but
establishes NHTSA's policy on consumer information regarding the
rollover resistance program. As mentioned above, this program places no
requirements on vehicle manufacturers, only some on NHTSA.
While the TREAD Act calls for a rulemaking to determine how best to
disseminate the rollover test results, the development of the dynamic
rollover test is simply the responsibility of the Secretary. Based on
NHTSA's recent research to evaluate rollover test maneuvers, the
National Academy of Sciences' study of rollover ratings, comments to
the July 3, 2000 notice, extensive consultations with experts from the
vehicle industry, consumer groups and academia, and NHTSA's
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previous research in 1997-8, the agency chose the J-Turn and the
Fishhook maneuvers as dynamic rollover tests. They are the limit
maneuver tests that NHTSA found to have the highest levels of
objectivity, repeatability and discriminatory capability. The document
announced that vehicles would be tested in two load conditions using
the J-Turn at up to 60 mph and the Fishhook maneuver at up to 50 mph.
Both maneuvers would be conducted with an automated steering
controller, and the reverse steer of the Fishhook maneuver would be
timed to coincide with the maximum roll angle to create an objective
``worst case'' for all vehicles regardless of differences in resonant
roll frequency. Figures 1 and 2 illustrate the open-loop steering wheel
motions characterizing these maneuvers. The light load condition would
be the weight of the test driver and instruments, approximating a
vehicle with a driver and one front seat passenger. The notice
announced that the heavy load condition would add additional 175 lb
manikins in all rear seat positions.
The National Academy of Sciences recommended that dynamic maneuver
tests be used to supplement rather than replace Static Stability Factor
in consumer information on rollover resistance. NHTSA proposed two
alternatives for consumer information ratings on vehicle rollover
resistance that included both dynamic maneuver test results and Static
Stability Factor. The first alternative was to include the dynamic test
results as vehicle variables along with SSF in a statistical model of
rollover risk that would combine their predictive power. This is
conceptually similar to the present ratings in which a statistical
model is used to distinguish between the effects of vehicle variables
and demographic and road use variables recorded for state crash data on
a large number of single-vehicle crashes. The National Academy of
Sciences recommended using a logistic regression model for this
purpose. Such a model would be used to predict the rollover rate in
single-vehicle crashes for a vehicle considering both its dynamic
maneuver test performance and its Static Stability Factor for an
average driver population (as a common basis of comparison).
Under the first alternative, the ``star rating'' of a vehicle would
be based on its rollover rate in single-vehicle crashes predicted by a
statistical model. The format would be the same as for the present
rollover ratings (for example, one star for a predicted rollover rate
in single-vehicle crashes greater than 40 percent and five stars for a
predicted rollover rate less than 10 percent). The present rollover
ratings are based on a linear regression model using state crash
reports of 241,000 single-vehicle crashes of 100 make/model vehicles.
We proposed to replace the current rollover risk model with one that
uses the performance of the vehicle in dynamic maneuver tests as well
as its SSF to predict rollover risk. The performance of a vehicle in
dynamic maneuver tests would be simply whether it tipped up or not in
each of the four maneuver/load combinations.
In order to compute this logistic model for rollover risk, it is
necessary to have the dynamic maneuver test results as well as SSF for
a number of vehicles with rollover rates established by state crash
reports of single-vehicle crashes. We had the SSF measurements and
established rollover rates for the 100 make/model vehicles upon which
we based the static rating system but not their dynamic maneuver test
results. Thus, we asked for comment on the suitability of a rating
method that combines static and dynamic vehicle properties in a single
rating and on the validity of logistic regression analysis for the risk
model that combines the properties in a way that is predictive of real-
world crash experience.
The NPRM notice announced that we were going to perform the dynamic
maneuver tests on about 25 of the 100 make/model vehicles for which we
had SSF measurements and substantial state crash data. Time and budget
constraints would not permit testing all 100 vehicles. With these
dynamic maneuver test results and our existing crash and SSF
information we would be able to compute the new risk model using a
standard statistical package of computer programs (SAS) for logistic
regression analysis. This final document presents the dynamic maneuver
test results for 24 of the 100 vehicles, chosen to span the SSF range
and to represent high production vehicles of each type (passenger car,
van, pickup truck and sport utility vehicle (SUV)). An additional SUV
with a lower SSF than found among the 100 vehicles was also included.
The resulting risk model is presented in this document.
The second alternative we proposed was to have separate ratings for
Static Stability Factor and for dynamic maneuver test performance.
Dynamic maneuver tests directly represent on-road untripped rollovers.
Under this alternative, the dynamic maneuver test performance would be
used to rate resistance to untripped rollovers in a qualitative scale.
Barring unforeseen results of the dynamic maneuver tests of the 25
vehicle group, the obvious qualitative scale would be: A for no tip-
ups, B for tip-up in one maneuver, C for tip-ups in two maneuvers, D
for tip-ups in three maneuvers and E for tip-ups in all four maneuvers/
load combinations.
A statistical risk model is not possible for untripped rollover
crashes, because they appear to be relatively rare events and they
cannot be reliably identified in state crash reports. For this
alternative, the current Static Stability Factor based system would be
used to rate resistance to tripped rollovers (since we believe most of
the rollovers reported in the state crash reports are tripped). Again
we asked for comments on the usefulness and validity of the concept in
the NPRM notice, but we could not offer examples of actual vehicle
ratings because the tests had not yet been conducted.
V. Results of Dynamic Maneuver Tests of 25 Vehicles
This section presents an overview of the test maneuvers and the
results for 25 vehicles that were used to develop the logistic
regression risk model. A more extensive account of the test program is
contained in the Phase VI and VII Report that has been placed in Docket
NHTSA-2001-9663. A detailed description of how we will perform the
maneuver tests for NCAP ratings is contained in Appendix I.
The NHTSA J-Turn and Fishhook (with roll rate feedback) maneuver
tests were performed for 25 vehicles representing four vehicle types
including passenger cars, vans, pickup trucks and SUVs. We chose mainly
high production vehicles that spanned a wide range of SSF values, using
vehicles NHTSA already owned where possible. Except for four 2001 model
year vehicles NHTSA purchased new, the vehicle suspensions were rebuilt
with new springs and shock absorbers, and other parts as required for
all the other vehicles included in the test program.
A. J-Turn Maneuver
The NHTSA J-Turn maneuver represents an avoidance maneuver in which
a vehicle is steered away from an obstacle using a single input. The
maneuver is similar to the J-Turn used during NHTSA's 1997-98 rollover
research program and is a common maneuver in test programs conducted by
vehicle manufacturers and others. Often the J-Turn is conducted with a
fixed steering input (handwheel angle) for all test vehicles. In its
1997-98 testing, NHTSA used a fixed handwheel angle of 330 degrees. In
the testing that preceded the NPRM notice, we developed an objective
method of specifying equivalent handwheel angles
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for J-Turn tests of various vehicles, taking into account their
differences in steering ratio, wheelbase and linear range understeer
properties. (See NHTSA's Phase IV report docketed with the NPRM notice
as item 38 in Docket No. NHTSA 2001-9663). Under this method, one first
measures the handwheel angle that would produce a steady-state lateral
acceleration of 0.3 g at 50 mph on a level paved surface for a
particular vehicle. In brief, the 0.3 g value was chosen because the
steering angle variability associated with this lateral acceleration is
quite low and there is no possibility that stability control
intervention could confound the test results. Since the magnitude of
the handwheel position at 0.3 g is small, it must be multiplied by a
scalar to have a high maneuver severity. In the case of the J-Turn, the
handwheel angle at 0.3 g was multiplied by eight. When this scalar is
multiplied by the average handwheel angle at 0.3 g (observed during
NHTSA's 1997-98 rollover research program), the result is approximately
330 degrees. Figure 1 illustrates the J-Turn maneuver in terms of the
automated steering inputs commanded by the programmable steering
machine. The rate of the handwheel turning is 1000 degrees per second.
To begin the maneuver, the vehicle was driven in a straight line at
a speed slightly greater than the desired entrance speed. The driver
released the throttle, coasted to the target speed, and then triggered
the commanded handwheel input. The nominal maneuver entrance speeds
used in the J-Turn maneuver ranged from 35 to 60 mph, increased in 5
mph increments until a termination condition was achieved. Termination
conditions were simultaneous two inch or greater lift of a vehicle's
inside tires (two-wheel lift) or completion of a test performed at the
maximum maneuver entrance speed without two-wheel lift. If two-wheel
lift was observed, a downward iteration of vehicle speed was used in 1
mph increments until such lift was no longer detected. Once the lowest
speed for which two-wheel lift could be detected was isolated, two
additional tests were performed at that speed to monitor two-wheel lift
repeatability.
B. Fishhook Maneuver
The second maneuver test, the fishhook maneuver, uses steering
inputs that approximate the steering a driver acting in panic might use
in an effort to regain lane position after dropping two wheels off the
roadway onto the shoulder. In the NPRM notice, we described it as a
road edge recovery maneuver. As pointed out by some commenters, it is
performed on a smooth pavement rather than at a road edge drop-off, but
its rapid steering input followed by an over-correction is
representative of a general loss of control situation. The original
version of this test was developed by Toyota, and variations of it were
suggested by Nissan and Honda. NHTSA has experimented with several
versions since 1997, and the present test includes roll rate feedback
in order to time the counter-steer to coincide with the maximum roll
angle of each vehicle in response to the first steer.
Figure 2 describes the Fishhook maneuver in terms of the automated
steering inputs commanded by the programmable steering machine and
illustrates the roll rate feedback. The initial steering magnitude and
countersteer magnitudes are symmetric, and are calculated by
multiplying the handwheel angle that would produce a steady state
lateral acceleration of 0.3 g at 50 mph on level pavement by 6.5. The
average steering input is equivalent to the 270 degree handwheel angle
used in earlier forms of the maneuver but, as in the case of the J-
Turn, the procedure above is an objective way of compensating for
differences in steering gear ratio, wheelbase and understeer properties
between vehicles. The fishhook maneuver dwell times (the time between
completion of the initial steering ramp and the initiation of the
countersteer) are defined by the roll motion of the vehicle being
evaluated, and can vary on a test-to-test basis. This is made possible
by having the steering machine monitor roll rate (roll velocity). If an
initial steer is to the left, the steering reversal following
completion of the first handwheel ramp occurs when the roll rate of the
vehicle first equals or goes below 1.5 degrees per second. If an
initial steer is to the right, the steering reversal following
completion of the first handwheel ramp occurs when the roll rate of the
vehicle first equals or exceeds -1.5 degrees per second. The handwheel
rates of the initial steer and countersteer ramps are 720 degrees per
second.
To begin the maneuver, the vehicle was driven in a straight line at
a speed slightly greater than the desired entrance speed. The driver
released the throttle, coasted to the target speed, and then triggered
the commanded handwheel input described in Figure 2. The nominal
maneuver entrance speeds used in the fishhook maneuver ranged from 35
to 50 mph, increased in 5 mph increments until a termination condition
was achieved. Termination conditions included simultaneous two inch or
greater lift of a vehicle's inside tires (two-wheel lift) or completion
of a test performed at the maximum maneuver entrance speed without two-
wheel lift. If two-wheel lift was observed, a downward iteration of
vehicle speed was used in 1 mph increments until such lift was no
longer detected. Once the lowest speed for which two-wheel lift could
be detected was isolated, two additional tests were performed at that
speed to check two-wheel lift repeatability.
C. Loading Conditions
The vehicles were tested in each maneuver in two load conditions in
order to create four levels of stringency in the suite of maneuver
tests. The light load was the test driver plus instrumentation in the
front passenger seat, which represented two occupants. A heavier load
was used to create a higher level of stringency for each test. In our
NPRM, we announced that the heavy load would include 175 lb
anthropomorphic forms (water dummies) in all rear seat positions.
During the test of the 25 vehicles, it became obvious that heavy load
tests were being run at very unequal load conditions especially between
vans and other vehicles (two water dummies in some vehicles but six
water dummies in others). While very heavy passenger loads can
certainly reduce rollover resistance and potentially cause special
problems, crashes at those loads are too few to greatly influence the
overall rollover rate of vehicles. Over 94% of van rollovers in our
293,000 crash database occurred with five or fewer occupants, and over
99% of rollovers of other vehicles occurred with five or fewer
occupants. The average passenger loads of vehicles in our crash
database was less than two: 1.81 for vans; 1.54 for SUVs; 1.48 for
cars; and 1.35 for pickup trucks. In order to use the maneuver tests to
predict real-world rollover rates, it seemed inappropriate to test the
vehicles under widely differing loads that did not correspond to the
real-world crash statistics. Therefore, the tests used to develop a
statistical model of rollover risk were changed to a uniform heavy load
condition of three water dummies (representing a 5-occupant loading)
for all vehicles capable of carrying at least five occupants. Some
vehicles were loaded with only two water dummies because they were
designed for four occupants. For pickup trucks, water dummies were
loaded in the bed at approximately the same height as a passenger in
the front seat.
To avoid disruption, the tests were completed under the original
loading
[[Page 59255]]
plan. Then we conducted tests at a 5-occupant heavy load only for those
vehicles in which loading differences might influence tip-up. If the
vehicle had completed the maneuver without tip-up with more than three
water dummies in the rear it was not necessary to retest at a lighter
load. Likewise, if the vehicle tipped up in the light load (no water
dummies) condition, it was not necessary to retest with three water
dummies in the rear. We have never observed a vehicle for which a
greater passenger load improved performance in a tip-up test.
D. Test Results
The test results in Table 1 reflect the performance either measured
or imputed as described for a heavy-load condition representing 5
occupants except for the Ford Explorer 2DR, the Chevrolet Tracker and
Metro that were designed for only four occupants, and the Honda CRV,
Honda Civic and Chevrolet Cavalier that could not be loaded to the 5
occupant level without exceeding a gross axle weight rating because of
the additional weight of the outriggers.
Note that Table 1 includes some results collected during tests
performed with alternative steering angles. Although the steering
angles used during these tests were still based on the handwheel angle
that would produce a steady-state lateral acceleration of 0.3 g at 50
mph on a level paved surface, the scalars used to calculate the
steering angles were smaller. These tests were performed because, for
some vehicles, the methods used to calculate the steering inputs used
in the J-Turn and/or Fishhook maneuvers can produce ``excessive''
steering--steering angles so great that maneuver severity is actually
reduced (i.e., the lateral force capability of the tires is exceeded).
As an example, consider the Ford Ranger 4WD and Aerostar. These
vehicles required a reduction of the J-Turn steering scalar from 8.0 to
7.0 (Ranger 4WD) or 6.0 (Aerostar) before J-Turn steering was able to
produce two-wheel lift.
Table 1.--Dynamic Maneuver Test Results (the Check Mark Indicates Tip-Up Observed)
----------------------------------------------------------------------------------------------------------------
Nominal
Model range/make/ static Fishhook Fishhook J-turn light J-turn heavy
Veh. group number model stability light (FL) heavy (FH) (JL) (2 (JH) (5
factor (2 occ.) (5 occ.) occ.) occ.)
----------------------------------------------------------------------------------------------------------------
'92-'00 Mitsubishi 0.95 [bcheck] [bcheck] ............ [bcheck]
Montero 4WD.
47..................... '95-'03 Chevrolet 1.02 [bcheck] [bcheck] ............ [bcheck]
Blazer 2WD.
43..................... '95-'01 Ford 1.06 ............ ............ ............ ............
Explorer 2dr 2WD.
44..................... '95-'01 Ford 1.06 ............ [bcheck] ............ ............
Explorer 4dr 4WD.
66..................... '96-'00 Toyota 1.06 ............ [bcheck] ............ ............
4Runner 4WD.
89..................... '93-'97 Ford 1.07 [bcheck] [bcheck] [bcheck] [bcheck]
Ranger p/u 4WD.
58..................... '88-'97 Jeep 1.08 ............ ............ ............ ............
Cherokee 4WD.
59..................... '95-'02 Acura SLX/ 1.09 [bcheck] [bcheck] [bcheck] [bcheck]
Isuzu Trooper 4WD.
70..................... '88-'98 Ford 1.10 [bcheck] [bcheck] [bcheck] [bcheck]
Aerostar 2WD.
74..................... '88-'02 Chevrolet 1.12 ............ [bcheck] ............ ............
Astro 2WD.
53..................... '89-'98 Chevrolet/ 1.13 ............ [bcheck] ............ ............
Geo Tracker 4WD.
91..................... '88-'98 Chevrolet 1.14 ............ ............ ............ ............
K1500 p/u 4WD.
88..................... '93-'97 Ford 1.17 ............ [bcheck] ............ [bcheck]
Ranger p/u 2WD.
85..................... '97-'02 Ford F-150 1.18 ............ ............ ............ ............
p/u 2WD.
54..................... '97-'01 Honda CR-V 1.19 [bcheck] [bcheck] ............ [bcheck]
4WD.
83..................... '88-'96 Ford F-150 1.19 ............ ............ ............ ............
p/u 2WD.
67..................... '88-'95 Dodge 1.21 ............ ............ ............ ............
Caravan/Plymouth
Voyager 2WD.
90..................... '88-'98 Chevrolet 1.22 ............ ............ ............ ............
C1500 p/u 2WD.
68..................... '96-'00 Dodge 1.23 ............ ............ ............ ............
Caravan/Plymouth
Voyager 2WD.
73..................... '95-'98 Ford 1.24 ............ ............ ............ ............
Windstar 2WD.
22..................... '95-'01 Chevrolet/ 1.29 ............ ............ ............ ............
Geo Metro.
19..................... '88-'94 Chevrolet 1.32 ............ ............ ............ ............
Cavalier.
18..................... '91-'96 Chevrolet 1.40 ............ ............ ............ ............
Caprice.
7...................... '88-'95 Ford 1.45 ............ ............ ............ ............
Taurus.
26..................... '92-'95 Honda 1.48 ............ ............ ............ ............
Civic.
--------------
Total Tip-ups.......... .................. ........... 6 11 3 7
----------------------------------------------------------------------------------------------------------------
During some Fishhook tests, excessive steering caused some vehicles
to reach their maximum roll angle response to the initial steering
input before it had been fully completed (this is essentially
equivalent to a ``negative'' T1 in Figure 2). Since dwell
time duration can have a significant effect on how the Fishhook
maneuver's ability to produce two-wheel lift, we believe that excessive
steering may stifle the most severe timing of the counter steer for
some vehicles. In an attempt to better insure high maneuver severity, a
number of vehicles that did not produce two-wheel lift with steering
inputs calculated with the 6.5 multiplier were also tested with lesser
steering angles by reducing the multiplier to 5.5. This change reduced
the likelihood of excessive steering, and increased the dwell times
observed during the respective maneuvers. In the case of the Ford
Ranger 4x2, Fishhook maneuvers with steering inputs based on the
reduced multiplier were able to produce two-wheel lift. Such lift was
not observed when the original steering was used (i.e., when a
multiplier of 6.5 was used). We have modified the Fishhook test
procedure to include tests at the steering angle determined by the 5.5
multiplier for vehicles that do not tip up using the original steering
angle determination.
Each test vehicle in Table 1 represented a generation of vehicles
whose model year range is given. Twenty-four of the vehicles were taken
from 100 vehicle groups whose 1994-98 crash statistics in six states
were the basis of the present SSF based rollover resistance ratings.
The vehicle group numbers used to identify these vehicles in the prior
notices (65 FR 34998 and 66 FR 3388) are given for convenience. The
nominal SSFs used to describe the vehicle groups in the prior
statistical studies are given. While there were some variations between
the SSFs of the
[[Page 59256]]
individual test vehicles and the nominal vehicle group SSF values, the
nominal SSFs were retained for the present statistical analyses because
they represent vehicles produced over a wide range of years in many
cases and provide a simple comparison between the risk model presented
in this document and that discussed in the previous notices.
The check marks under the various test maneuver names indicate
which vehicles tipped up during the tests. Eleven of the twenty-five
vehicles tipped up in the Fishhook maneuver conducted in the heavy
condition. The heavy condition represented a five-occupant load for all
vehicles except the six mentioned above that were limited to a four-
occupant load by the vehicle seating positions and GVWR. All eleven
were among the sixteen test vehicles with SSFs less than 1.20. None of
the vehicles with higher SSFs tipped up in any test maneuver. The
fishhook test under the heavy load clearly had the greatest potential
to cause tip-up. The groups of vehicles that tipped up in other tests
were subsets of the larger group of eleven that tipped up in the
fishhook heavy test. There were seven vehicles in the group that tipped
up in the J-Turn heavy test, six of which also tipped up in the
Fishhook light test. The J-Turn light test had the least potential to
tip up vehicles. Only three vehicles tipped up, all of which had tipped
up in every other test.
VI. Rollover Risk Model
In its study of our rating system for rollover resistance
(Transportation Research Board Special Report 265), the National
Academy of Sciences (NAS) recommended that we use logistic regression
rather than linear regression for analysis of the relationship between
rollover risk and SSF. Logistic regression has the advantage that it
operates on every crash data point directly rather than requiring that
the crash data be aggregated by vehicle and state into a smaller number
of data points. For example, we now have state data reports of about
293,000 single-vehicle crashes of the hundred vehicle make/models
(together with their corporate cousins) whose single-vehicle crashes we
have been tracking in six states. The logistic regression analysis of
this data would have a sample size of 293,000, producing a narrow
confidence interval on the repeatability of the relationship between
SSF and rollover rate. In contrast, the linear regression analysis
operates on the rollover rate of the hundred vehicle make/models in
each of the six states. It produces a maximum sample size of only 600
(100 vehicles times six states) minus the number of samples for which
fewer than 25 crashes were available for determining the rollover rate
(a data quality control practice). Confidence limits computed for a
data sample size of 600 will be much greater than those based on a
sample size of 293,000. On average, each sample in the linear
regression analysis was computed from over 400 crash report samples.
However, ordinary techniques to compute the confidence intervals of
linear regression results do not take into account the actual sample
size represented by aggregated data. The statistical model created to
combine SSF and dynamic test information in the prediction of rollover
risk was computed by means of logistic regression as recommended by the
NAS. Logistic regression is well suited to the correlation with crash
data of vehicle properties that include both continuous variables like
SSF and binary variables like tip-up or no tip-up in maneuver tests.
We had previously considered logistic regression during the
development of the SSF based rating system (66 FR 3388, January 12,
2001, p.3393), but found that it consistently under-predicted the
actual rollover rate at the low end of the SSF range where the rollover
rates are high. The NAS study acknowledged this situation and gave the
example of another analysis technique (non-parametric) that made higher
rollover rate predictions at the low end of the SSF scale. In the NPRM,
we discussed our plan to first examine ways to improve the fit of the
logistic regression model to the actual rollover rates in the simpler
model with SSF as the only vehicle attribute before expanding the
logistic regression model to predict rollover rates using maneuver test
results and SSF as vehicle attributes. In this way, the addition of
maneuver test results is more likely to have an effect that reflects
the additional information they represent on rollover causation.
Appendix II discusses the details of seeking a mathematical
transformation of SSF to improve the accuracy of logistic regression
models. We found that logistic regression on the transformation
``Log(SSF-0.9)'' rather than on SSF directly computed a risk model
whose predictions of rollovers per single-vehicle crash more closely
matched the relationship between vehicle SSF and actual rollover rates
observed in state crash data. We sought to optimize the accuracy of the
predictions in the SSF range between 1.0 and 1.25 that includes the
vehicles with the highest rollover rates, even at the expense of
accuracy in predicting the low rollover rates at high end of the SSF
scale. The risk model that resulted from this exercise is equivalent to
the SSF-based rating system used for 2001-2003 NCAP rollover resistance
ratings except that it was computed using logistic regression rather
than linear regression as the statistical technique. Figure 3 compares
the logistic regression model and linear regression model formerly used
for NCAP ratings. The linear regression model is not in the form of a
straight line because it also operated on a transformation of SSF
(Log(SSF) in this case). The logistic regression model is the more
accurate at lower half of the SSF range, and the linear regression
model is the more accurate at the upper half of the SSF range. The two
curves are quite similar.
A good logistic regression risk model using SSF only was the
starting point for models using dynamic variables together with SSF.
The dynamic maneuver test results (tip-up or no tip-up in each
maneuver/load combination in Table 1) were used as four binary dynamic
variables in the logistic regression analysis. The dynamic variables
were entered in addition to SSF to describe the vehicle. The same
driver and road variables from state crash reports discussed above were
used. The state crash report data for twenty four of the vehicles used
in the logistic regression analysis with dynamic maneuver test
variables was a subset of the database of 293,000 single-vehicle
crashes described above. One extra vehicle was added for the maneuver
tests that was not among the 100 vehicle groups we had studied
previously, but state crash report data from the same years and states
was obtained for it. However, the database with SSF and dynamic
maneuver test was much smaller than the 293,000 sample size available
for the logistic regression model with SSF only. Its sample size was
96,000 single-vehicle crashes of 25 vehicles including 20,000
rollovers. Appendix II contains a more detailed discussion.
First, we tried each dynamic variable separately in conjunction
with SSF. The models using variables for performance in the Fishhook
heavy and J-Turn heavy maneuvers predicted a greater rollover risk for
those vehicles that tipped up in the maneuver test. However, the models
using variables for performance in the Fishhook light and J-Turn light
maneuvers predicted a greater rollover risk for vehicles that did not
tip up.
We do not believe vehicles that tip up in the least severe
maneuvers are actually safer than those that do not tip up. A more
rational interpretation is that the numbers of vehicle tipping up
[[Page 59257]]
in these maneuvers were too few to establish a definitive correlation.
Only three vehicles tipped up in the J-Turn light maneuver, and six
vehicles tipped up in the Fishhook light maneuver. Only one more
vehicle tipped up in the J-Turn heavy maneuver than in the Fishhook
light, and the prediction of the model with J-Turn heavy was consistent
with expectations that tip-up in the test predicts greater rollover
risk. However, the extra vehicle in the J-Turn heavy tip-up group was
the Ford Ranger 2 WD with a very large sample size of over 8,000
single-vehicle crashes (nearly 10 percent of the entire data base).
Next we computed a logistic regression model combining SSF with the
dynamic variables for both maneuvers, Fishhook heavy and J-Turn heavy,
that were observed to have a directionally correct result when entered
into the model individually. The variable for J-Turn heavy was rejected
by the logistic regression program as not statistically significant in
the presence of the Fishhook heavy variable. In other words, the
predictions based on tip-up in the Fishhook heavy maneuver do not
change whether or not the vehicle also tips up in the J-Turn heavy
maneuver.
Figure 4 shows the final model that uses Fishhook heavy as the only
necessary dynamic variable. This model has a risk prediction for
vehicles that tip up in the dynamic maneuver tests based on the
greatest number of vehicles possible in our 25 vehicle data base. All
11 vehicles that tipped up in any maneuver are represented on the tip-
up curve, and the 14 vehicles without tip-up are represented on the
other curve. The risk curve in Figure 4 representing vehicles that
tipped up in the Fishhook heavy maneuver is very similar to the
logistic regression model based on SSF only in Figure 3 (that was based
on the rollover rates of 100 vehicles). This result is logical because
the SSF only model was optimized for best fit in the 1.00 to 1.25 SSF
range that included all vehicles tipping up in dynamic maneuver tests.
Also, the fact that the risk curve of the logistic regression model in
Figure 3 that was based on the SSF of 100 vehicles closely matches the
risk curve in Figure 4 that was based on 11 vehicles that tipped up in
the dynamic tests suggests that the curve in Figure 4 is robust.
However, the small difference in Figure 4 between the risk curve for
vehicles that tip up in the dynamic test and the risk curve for those
that do not tip up suggests that the predictive power of tip-up in the
dynamic test may not be great.
Our testing and logistic regression analysis was sufficient to
assign a greater rollover risk to vehicles that tipped up in the most
severe maneuver than to those that did not tip up at all. However, the
extra risk was small, and we were not able to distinguish a rollover
risk difference between vehicles that tipped up in the less severe
Fishhook maneuver with a two occupant load from those that tipped up
only with a five occupant load. In general, vehicles that tip up in the
Fishhook maneuver with a two occupant load also tip up at a slower
entry speed in the Fishhook maneuver with a five occupant load than
those that do not. Therefore, our data does not allow us to distinguish
rollover risk differences between vehicles on the basis of maneuver
entry speed for tip-up. The objective of using different load
conditions and different maneuvers instead of different speeds in a
single maneuver to provide a range of test severity was to reduce the
sensitivity of the result to extraneous factors such as tire wear.
It is noteworthy that the final rollover risk model required
results from only the fishhook maneuver. This is an advantage from the
standpoint of minimizing the practical problems of the effects of tire
wear during a test series and of deviations from uniformity of surface
friction at a test facility. The fishhook maneuver produces less wear
on the test tires and requires only about 2 or 3 lane widths of uniform
test surface versus 10 or more lane widths for the J-Turn maneuver. The
commenters also considered it more representative of a real driving
situation than the J-Turn.
VII. Comments to the NPRM Notice and Agency Response
We received 39 comments to the NPRM notice from vehicle
manufacturers, equipment suppliers, test labs, public interest groups,
the National Transportation Safety Board, the Insurance Institute for
Highway Safety, attorneys, and members of the public. Mainly, the
comments addressed whether the static and dynamic measurements should
be used for separate ratings of rollover resistance or for a combined
rating based on a risk model. The nature of the dynamic maneuver tests,
testing of 15-passenger vans, and several practical testing issues such
as the extraneous effects of tire wear, surface condition and ambient
temperature were also addressed. The notice also introduced the related
subject of handling ratings that was not part of the TREAD Act
requirements. We received a number of valuable comments on handling
tests, and we are still soliciting information. However, the subject of
this notice is confined to the TREAD Act requirements for dynamic
rollover ratings.
A. Combined or Separate Rollover Resistance Ratings
The main question posed in the NPRM notice was whether the rollover
resistance ratings should reflect the combined statistical power of SSF
and dynamic tests for predicting rollover risk or whether ratings of
rollover risk using SSF alone should continue, supplemented with a
qualitative comparison of dynamic test performance. The document gave
alternative A as a risk model determined by logistic regression
analysis of state crash reports of single-vehicle crashes for about 25
vehicles with known SSF and dynamic test results. That process led to
the risk model described in Section VI, however the mathematical
calculation of the model could not be performed until the completion of
a lengthy dynamic test program. Alternative B in the notice was a
continuation of rollover risk prediction using SSF-only plus
qualitative separate dynamic scores of A, B, C, D, or E signifying the
number of maneuvers in which the vehicle tripped up without a risk
interpretation.
Commenters representing TRW Automotive, National Automobile Dealers
Association (NADA), General Motors (GM), Alliance of Automobile
Manufacturers (Alliance), Association of International Automobile
Manufacturers (AIAM), Insurance Institute for Highway Safety (IIHS),
Bosch, Consumers Union, Advocates for Highway and Auto Safety
(Advocates), Toyota, Continental-Teves and Public Citizen remarked
directly on the question of combined versus separate use of SSF and
dynamic maneuver tests in rollover resistance ratings. Except for
Continental-Teves and Bosch, the commenters were in favor of ratings
that combined the SSF and dynamic maneuver tests in a single rating.
Consumers Union specifically supported the logit risk model operating
on a moderate risk scenario (in which rollover rates vary in the
approximate range of 0.075 to 0.55 across the range of vehicles) as a
way of combining the SSF and dynamic maneuver tests. It commented that
using the risk model it described was consistent with the
recommendations of the NAS study. We believe the risk model we have
developed is consistent with recommendation of NAS and Consumers Union.
It is the logit model with the risk scenario (of demographic and road
condition variables) that represents the average crash conditions of
293,000 actual single-vehicle crashes.
[[Page 59258]]
It produces predicted rollover rates in the range of 0.09 to 0.50 for
vehicles ranging from tip-up to no tip-up in maneuvers and from 1.0 to
1.55 in SSF.
The other commenters in favor of combined ratings were primarily
concerned that separate ratings would be too confusing to serve as
consumer information. They believed a combined rating was the only
viable option, but they did not comment specifically on the means used
by NHTSA to develop the combined risk model. IIHS and the Alliance
(along with Carr Engineering) suggested that another comment period
following the notice containing the actual model (as opposed to the
example given in the NPRM notice) would be necessary. GM suggested that
the risk model be developed through a collaborative effort along the
lines of the Motor Vehicle Safety Research Advisory Committee, and the
Alliance suggested a working-level dialog between NHTSA and the auto
industry to develop the risk model. TRW supported a single rating that
would be computed on the basis of the SSF only model with a
predetermined number of stars added or subtracted for dynamic maneuver
performance (determined without a statistical relationship to risk).
Advocates expressed wariness that the combined rating could be
misleading to consumers unless it corresponded to real-world rollover
rates. Public Citizen preferred the combined rating developed from a
risk model. It was concerned that consumers would focus more attention
on the dynamic maneuvers in separate ratings although the tests
represent an event (on-road untripped rollover) that occurs in less
than 5 percent of actual rollover crashes.
Continental-Teves and Bosch prefer separate ratings for SSF and
dynamic maneuver tests. Continental-Teves stated that ``the relative
effects of SSF and dynamic performance are not well understood, and may
not be the same for every vehicle or every driver.'' Bosch stated that
``static and dynamic ratings should be separate, as they are both
equally important with regards to indicating stability and safety of
the vehicle.'' Bosch further explained that `` a combined rating may
not adequately show the influence of such systems [Electronic Stability
Control and Rollover Mitigation] which in turn would not encourage
manufacturers to add systems to vehicles that increase overall vehicle
safety in potential rollover as well as many other situations.''
B. Crash Avoidance Technologies
Some of the stated expectations of the commenters about rollover
resistance ratings are unrealistic. The rollover resistance ratings
predict the likelihood of a single-vehicle crash becoming a rollover.
They do not predict the likelihood of the vehicle becoming involved in
a single-vehicle crash. Similarly, the frontal and side NCAP
crashworthiness ratings do not predict the likelihood of the vehicle
striking an object head-on or being struck from the side. The Alliance
comment anticipates the dilemma. While conceding that SSF is strongly
correlated with a tripped rollover once the vehicle is already off-
road, it states that `` the likelihood of being involved in a single-
vehicle crash in the first place `` particularly one involving off-road
excursion `` is influenced much more by demographic and environmental
influences than is the scenario examined for SSF purposes.'' The
scenario used in the combined risk model is the same scenario used in
the SSF model, namely the average demographic and environmental
variables reported by the states for the entire 293,000 single-vehicle
crash data base we have collected. We think this is the best scenario
to characterize single-vehicle crashes.
The Alliance is concerned that our model ``may fail to account for
potentially beneficial technologies for avoiding single-vehicle and
rollover crashes, such as electronic stability control and variable
ride high suspension systems.'' Its concern is unnecessary for variable
ride-height suspension systems, which will be tested in the highway
rather than off-road height for both SSF and dynamic maneuver tests,
and the technology will certainly improve the rating of vehicles so
equipped.
However, the Alliance is right that the model does not predict the
risk of a single-vehicle crash. NHTSA has been very clear in public
notices, consumer information and web site presentations that neither
the SSF risk model nor the proposed combined SSF and dynamic maneuver
risk model predict the risk of having a single-vehicle crash. From the
standpoint of rollover resistance, single-vehicle crashes are a measure
of exposure. The prediction is of the risk of a rollover resulting from
the exposure of the vehicle to a single-vehicle crash. The risk of
rollover in the event of a single-vehicle crash is strongly influenced
by vehicle properties, but the vehicle properties of modern vehicles
have far less influence in comparison to demographic and environmental
factors regarding the risk of a single-vehicle crash in the first
place. However, electronic yaw stability control may provide a real-
world reduction in single-vehicle crashes.
We have been optimistic about the potential of electronic yaw
stability control to reduce single-vehicle crashes. NHTSA's consumer
information identifies its availability as standard or optional
equipment on individual vehicles and explains how it operates to help a
driver maintain control in extreme circumstances. One of the reasons we
are exploring the possibility of NCAP handling ratings is to describe
the effect of yaw stability control on handling predictability.
However, the technology has not been in widespread use long enough to
produce much crash evidence for the evaluation of its real-world
effectiveness in preventing single-vehicle crashes. Our previous
attempts at evaluating its effectiveness were thwarted by insufficient
data.
Part of the motivation for the NAS study of NHTSA's SSF-based
rollover resistance ratings was the Alliance's concern that yaw
stability control was not being considered. In its public oral
presentation to the NAS study committee in May 2001, NHTSA said it did
not expect yaw stability control to have a large effect on the risk of
rollover given a single-vehicle crash. In its view, the large majority
of rollovers were the result of various types of tripping, and SSF
represented the most important vehicle attributes in those
circumstances. NHTSA believes that the greatest potential effect of yaw
stability control was in reducing single-vehicle crashes in the first
place. Therefore, we suggested to the committee that rather than trying
to predict rollovers per single-vehicle crash with dynamic maneuver
tests, we should keep SSF for that purpose and adjust the comparative
risk for vehicles with yaw stability control by the effect of yaw
stability control to reduce exposure to single-vehicle crashes.
However, establishing the effectiveness of yaw stability control would
require data not available for at least two or three more years.
Neither the NAS committee nor the Alliance, which was active in
providing the committee information, expressed interest in this
suggestion. But the present comments indicate that finding a way to
include the crash avoidance potential of yaw stability control is a
principal concern of the Alliance and several suppliers of these
systems.
IIHS's comment also shows an expectation of more than what is
possible for a rollover resistance rating. It discusses a comparison of
the 1997 Jeep Grand Cherokee and 1997 Toyota 4Runner made in one its
reports. In that report, the Toyota had four times the number of fatal
rollovers per 100,000
[[Page 59259]]
registered vehicles as the Jeep, but they had very similar SSFs. They
also had very similar rollover rates in terms of rollovers per single-
vehicle crash that were consistent with their SSFs. IIHS expects a good
dynamic rating to show a large difference between the Grand Cherokee
and the 4Runner. That will not be possible because differences in
dynamic maneuver test performance predict only small differences in
rollover rate, and, in fact, there is not a large difference in
rollover rate between these vehicles in terms of rollovers per single-
vehicle crash in our six state crash data base. The difference is in
the definition of rollover rate. A rollover rate in terms of fatal
rollovers per 100,000 vehicles depends on the rate of single-vehicle
crashes per 100,000 vehicles and on the occurrence of a fatality in the
rollover as well as on the rate of rollover per single-vehicle crash.
The first two of these factors depend primarily on demographic and
environmental influences and can mask actual differences or
similarities between vehicles as in this case. Neither vehicle had yaw
stability control, which would have created a plausible vehicle-related
difference in single-vehicle crash rate. The difference in fatality
rate could involve crashworthiness features, or particularly in the
case of rollover, it could merely reflect the seat belt wearing habits
of a risk taking demographic that also experienced a higher rate of
single-vehicle crashes. The rate of rollovers per single-vehicle crash
is much less sensitive to demographic influences than is the rate of
fatal rollovers per 100,000 vehicles.
Carr Engineering and Suzuki commented that the agency was not
following the recommendations of the NAS study by performing J-Turn and
Fishhook maneuver tests. They believe that the NAS recommended handling
tests to assess loss of control potential rather than limit maneuvers
to assess the resistance of the vehicle to actual on-road tip-up. We
agree that the language of the NAS study report is somewhat ambiguous.
That is why we included in our NPRM notice the clarification the NAS
study panel gave us during the presentation of the report to NHTSA in
response to our direct questions about J-Turn and Fishhook tests versus
handling tests. The NAS study committee clarified that it envisioned
dynamic maneuver tests as limit maneuvers where loss of control and
actual on-road vehicle tip-up can be expected for vulnerable vehicles.
The NAS study panel stated it was not in a position to recommend a
specific test because that would require study of discriminatory
capability, repeatability and other properties, but J-Turns and
Fishhooks were of the type of tests it had in mind. Two outside experts
in vehicle dynamics and testing reviewed our test plan before the Phase
VI test of the 25 vehicles. One had been a member of the NAS study
committee. Once again, we were assured that our tests were consistent
with the NAS recommendations.
We believe that both our test selection and our analysis method of
developing a rollover risk model to combine SSF and dynamic test
results are entirely consistent with the recommendations of the NAS
study and therefore appropriate to satisfy the requirements of the
TREAD Act. We agree that it is important to inform consumers of the
effectiveness of yaw stability control in reducing single-vehicle
crashes, and we will determine its effectiveness from crash report data
as sufficient data becomes available.
C. The J-Turn and Fishhook Maneuvers
There were a number of comments regarding the J-Turn and Fishhook
test protocols from the Alliance, GM, Toyota, Honda, Nissan, Renfroe
Engineering, Carr Engineering, Mechanical Systems Analysis Inc, and
Automotive Testing Inc. In addition, Ford made a detailed presentation
elaborating on some of the subjects introduced in the Alliance comment.
The Ford presentation material was placed in Docket NHTSA-2001-9663.
A number of the commenters objected to the J-Turn maneuver because
they thought it was not representative of real driving, involved too
fast a steering movement, or was redundant. Since its results were not
used in the risk model, we agree that it is redundant. As a result, we
are no longer planning to use it in the NCAP testing program.
Except for Suzuki, Carr Engineering and Ford, those who commented
on the maneuver tests supported the Fishhook maneuver. Carr Engineering
and Advocates objected to calling the Fishhook maneuver a road edge
recovery test as we had done in the NPRM notice. While the Fishhook
maneuver includes steering commands like a crash involving road edge
recovery, it is performed on a smooth uniform surface instead of one
with vertical drop-offs and friction coefficients differences that
exist at road edges. To accommodate these concerns, we will refer to
the maneuver as the Fishhook.
D. Tire Wear
The effect of tire wear on test results and the tire changing
protocol was addressed by several commenters. Tire shoulder wear during
limit maneuver tests is much more severe than in ordinary driving and
has the effect of increasing the lateral acceleration capability of the
vehicle. After a number of tests, the tire wear causes the vehicle to
tip up more easily, and there is concern that a vehicle with test-worn
tires does not represent a typical street driven vehicle. In the 25
vehicle tests, new tires were used for each maneuver (FH, FL, JH, JL)
which limited the tires to no more than 6 runs in each direction (4 for
Fishhooks) before detecting tip-up if it occurred.
Ford gave an example using a Ford Ranger 4WD that was apparently
known to tip up at 53 mph with worn tires in a J-Turn test. The vehicle
was equipped with new tires and tested repeatedly at 53 mph. It did not
tip up during the first three runs, but during the fourth run a large
increase in lateral acceleration and sideslip angle occurred and the
vehicle tipped up. It continued this behavior for two subsequent runs,
and the tires exhibited a large amount of shoulder wear after only six
runs. We have noticed similar tire wear effects, but not in so few
runs. The J-Turn tests are of much longer duration than Fishhook tests
and produce more wear per run. Also tests run at lower speeds
approaching tip-up speed produce less wear than tests performed at a
higher speed just below the tip-up speed. Ford's example of a worst
case in which the tire wear of just three runs changed vehicle behavior
from no tip to tip-up is an effective illustration of the tire wear
problem.
We believe this problem is much less acute for Fishhook tests. We
performed a similar experiment using a 2001 Ford Explorer 4 door 4WD
that we knew would tip up at 40 mph on worn tires in a Fishhook
maneuver. We performed 18 test runs without tip-up and then experienced
a 20 degree tip-up against the outriggers on the nineteenth run. We
performed three more runs and experienced two more tip-ups. Renfroe
Engineering also commented about tire wear effects citing an UMTRI
study in which lateral tire forces remained steady for about 10 runs
and then increased to a maximum force at about 20 runs.
Ford suggested a tire change protocol to limit tire wear. We intend
to test a number of vehicles in the summer of 2003. During these tests
we will use the tire change protocol of Appendix I because we believe
this appropriately limits the effect of tire wear. However, we intend
to confirm tip-ups using new (broken in but not worn) tires when
appropriate to make sure that the
[[Page 59260]]
vehicle scores have not been affected by tire wear. We will consider
the results of this exercise in deciding whether any changes in the
tire change protocol are necessary.
E. Pavement Temperature
The Alliance and Toyota commented on the potential effect of
pavement temperature on Fishhook maneuver results. Toyota has observed
increases in pavement friction as an apparent consequence of increases
in pavement temperature. It also supplied a computer simulation of
Fishhook tests that showed a large decrease in the speed at tip-up with
increases in surface friction. Taken together, Toyota's information
predicts a decrease in tip-up speed in a Fishhook maneuver of over 15
mph for a 70 degree F increase in pavement temperature. While the risk
model for ratings does not depend on tip-up speed, the temperature
effects predicted by Toyota would prevent most of the vehicles that
tipped up in a summer test from having tip-up in a winter test. NHTSA
ran a number of tests to evaluate the temperature sensitivity of J-Turn
and Fishhook tests (NHTSA Technical Report ``Testing to Determine the
Effects of Ambient Temperature on Dynamic Rollover Testing'', docketed
with this notice). We tested the 2001 Toyota 4Runner 4WD (with and
without yaw stability control enabled) and the 2001 Chevrolet Blazer
2WD on the same test track during cold, moderate and hot ambient
temperature. The difference between cold and hot ambient temperature
was about 60 degrees F. We do not have pavement temperatures, but there
is no reason to believe that the range of pavement temperature is less
than the range of ambient temperature. The whole test procedure
including the determination of handwheel angles based on the 0.3g
steady state curve was repeated at each temperature. The results are
given in Table 2. Every test that failed to cause tip-up in cold
weather also failed to cause tip-up in hot weather, and the two tests
that caused tip-up in hot weather also caused tip-up in cold weather.
Thus, the temperature effect predicted by the commenters did not occur.
The tip-up speeds for the Blazer in the right and left Fishhooks
repeated to within 1 mph despite differences in ambient temperature of
60 degrees F, seasonal differences in pavement surface, and the use of
three different sets of tires. The only temperature effect observed was
that the Blazer tipped up in the J-Turn in cold weather but did not in
the moderate and hot weather tests. This is the opposite of the
temperature effect predicted by the commenters and occurred during a
maneuver we no longer intend to use. We do not think it is necessary to
set tight surface temperature limits on the test protocol as suggested
by the commenters.
Table 2.--Results From NHTSA J-Turn and Fishhook Tests at Various Ambient Temperature Conditions.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Initial Steer Left Initial Steer Right
-----------------------------------------------------
Ambient Commanded Wheel lift, Wheel Lift,
Test vehicle and configuration Test maneuver Test condition temperature handwheel front/rear Maneuver front rear Maneuver
([deg]F) angle (inches) entrance (inches) entrance
(degrees) ---------------- speed ---------------- speed
Front Rear (mph) Front Rear (mph)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Toyota 4Runner, VSC disabled..... NHTSA.............. Cold............. 30 345 0 0 62.1 0 0 61.7
J-Turn\1\..........
Moderate......... 79 354 0 0 60.4 0 0 60.0
Hot.............. 87 358 0 0 61.8 0 0 60.3
Fishhook\2\........ Cold............. 32 280 1 0 51.1 0 1 51.7
Moderate......... 74-73 287 0 0 48.0 0 0 48.5
Hot.............. 89 290 1 0 51.4 0 0 50.8
Toyota 4Runner, VSC enabled...... NHTSA.............. Cold............. 28 345 0 0 61.8 0 0 62.4
J-Turn\1\..........
Moderate......... 75 354 0 0 59.4 0 0 58.2
Hot.............. 90 358 0 0 61.9 0 0 61.6
Fishhook\2\........ Cold............. 31 280 0 0 51.3 0 0 51.7
Moderate......... 72 287 0 0 48.8 0 0 50.1
Hot.............. 90 290 0 0 50.7 0 0 51.3
Chevrolet Blazer................. NHTSA.............. Cold............. 29 381 5-8 5-8 58.0 5-8 5-8 54.8
J-Turn\1,3\........
Moderate......... 83 401 0 0 60.9 0 0 62.2
Hot.............. 86 392 0 0 60.3 0 0 59.4
Fishhook\2,3\...... Cold............. 30 309 5-8 5-8 40.2 2-3 2-3 39.1
Moderate......... 74 326 3-4 3-4 40.3 4-5 4-5 40.1
Hot.............. 90 319 2-3 2-3 39.4 2-3 2-3 38.8
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ NHTSA J-Turn maximum nominal entrance speed was 60 mph.
\2\ Fishhook maximum nominal entrance speed was 50 mph.
\3\ Two-wheel lift =2 inches was observed during tests highlighted in bold.
F. Surface Friction
A practical problem for the repeatability of any limit maneuver
test is the possibility that the surface friction properties of the
test track will change. Ford commented that computer simulations of
several of its SUVs showed that a change in surface coefficient of 0.05
would change the tip-up speed in a fishhook test by as much as 12 mph
in one example (6 mph and 4 mph respectively for two other example
vehicles). It also commented that a seasonal variation in surface
coefficient of 0.05 could be typical of test tracks, and that its own
test track exhibited a long-term trend of an increase in coefficient of
0.02 per year (which would change the tip-up speed of the first example
vehicle by 8 mph in Ford's simulation). Ford's simulations are even
more pessimistic than Toyota's regarding the possibility of repeatable
Fishhook tip-up speeds given normal variations in surface properties
and temperatures. However, we have not observed these large variations
in tip-up speed in actual tests. The very close repeatability of tip-up
speed for the Blazer in Table 2 extended over likely
[[Page 59261]]
seasonal changes in the pavement as well as changes in ambient
temperature.
Additionally, NHTSA performed a study using the same 4Runner and
Blazer mentioned above for J-Turn and Fishhook tests at Daimler
Chrysler's Arizona Proving Grounds (APG) and General Motors Desert
Proving Grounds (DPG) as well as TRC of Ohio, where our maneuver test
development has been conducted (NHTSA Technical Report ``Testing to
Determine the Effects of Surface Variability on Dynamic Rollover
Testing'', docketed with this notice). Table 3 shows the peak and slide
braking coefficients (multiplied by 100) measured at these facilities.
Table 3.--Friction Numbers for all Test Facilities
----------------------------------------------------------------------------------------------------------------
Peak braking coefficient Skid number
Test facility ---------------------------------------------------------------------------
Dry Wet Dry Wet
----------------------------------------------------------------------------------------------------------------
TRC................................. 94-96 69-83 81-84 47-54
DPG................................. 86-93 74-77 83-85 60-64
APG................................. 90-93 75-80 81-84 56-59
----------------------------------------------------------------------------------------------------------------
Table 4 shows the results of the maneuver tests. As in Table 2, the
vehicles were loaded with the equivalent of a 2-occupant load, like the
light load condition of the 25 vehicle test. The 4Runner did not tip up
at TRC and it did not tip up at the other facilities. The Blazer did
not tip up in the J-Turn at TRC, but it did at the other facilities. We
do not think that this is a result of the surface coefficient of
friction (due to the similarities of the ranges) but rather due to the
greater degree of vertical irregularities and pavement cracks at DPG
and APG than at TRC. Tip-up is often triggered by vertical oscillations
of the vehicle suspension during high cornering forces in maneuver
tests. DPG had the most vertical surface irregularities that caused the
Blazer to tip up most easily. The Blazer tipped up in the Fishhook at
TRC, and it also tipped up in the Fishhook at the other facilities.
Again, the tip-up speeds were lower at APG and DPG, which would be
expected due to the greater surface irregularities.
Table 4.--Results From NHTSA J-Turn and Fishhook Tests
--------------------------------------------------------------------------------------------------------------------------------------------------------
Initial steer left Initial steer right
-----------------------------------------------------------------
Commanded Moderate or major Maneuver Moderate or major Maneuver
Test vehicle and configuration Test maneuver Test facility handwheel lift entrance lift entrance
angle, deg ---------------------- speed, ---------------------- speed,
Yes/No mph Yes/No mph
--------------------------------------------------------------------------------------------------------------------------------------------------------
Toyota 4Runner, VSC enabled....... NHTSA............... TRC 354 No.................. 58.21 No.................. 59.29
J-Turn \1\..........
DPG 402 No.................. 61.56 No.................. 61.21
APG 362 No.................. 61.68 No.................. 62.11
Fishhook \2\........ TRC 287 No.................. 48.75 No.................. 50.13
DPG 327 No.................. 53.05 No.................. 50.94
APG 294 No.................. 52.63 No.................. 51.44
Toyota 4Runner, VSC disabled...... NHTSA............... TRC 354 No.................. 60.4 No.................. 60.00
J-Turn \1\..........
DPG 402 No.................. 60.97 No.................. 61.63
APG 362 No.................. 62.38 No.................. 62.27
Fishhook \2\........ TRC 287 No.................. 49.84 No.................. 49.79
DPG 327 No.................. 52.20 No.................. 51.93
APG 294 No.................. 51.04 No.................. 51.14
Chevrolet Blazer.................. NHTSA............... TRC 401 No.................. 60.90 No.................. 62.27
J-Turn \1\..........
DPG 382 Yes................. 49.80 Yes................. 44.90
APG 395 Yes................. 57.36 Yes................. 58.68
Fishhook \2\........ TRC 326 Yes................. 40.32 Yes................. 40.09
DPG 311 Yes................. 37.80 Yes................. 38.01
APG 321 Yes................. 35.52 Yes................. 38.54
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ NHTSA J-Turn maximum nominal entrance speed is 60 mph.
\2\ Fishhook maximum nominal entrance speed is 50 mph.
We recognize the potential difficulties caused by changes in
surface friction coefficient, and we have tried to minimize them. We
have observed the Fishhook maneuver to be less sensitive to surface
conditions than the J-Turn, and we have used changes in vehicle load
condition rather than changes in tip-up speed to signify degrees of
test severity in a way least likely to be influenced by surface
coefficient. None of the changes of pavement and temperature in our
test experience has caused a change in the Fishhook result (tip-up or
no tip-up) for a vehicle. We believe the comments based on computer
simulation overstate the sensitivity observed in our actual tests.
G. Steering Reversal
Honda commented that using a roll rate measurement within 1.5
degrees/sec of a zero crossing as shown in Figure 2 to trigger the
reverse steering in a fishhook maneuver occasionally leads to an
unusually long dwell time (T1) for certain vehicles at
certain load conditions. It suggested setting a default value for dwell
time to force a reverse
[[Page 59262]]
steering action if the absolute value of the vehicle roll rate stayed
too long at a value that was very low but not low enough to trigger
reversal. It explained that tests in which excessive dwell times
occurred would be less severe and possibly not cause a tip-up that
would have occurred with a shorter dwell.
Automotive Testing Inc. commented at length on the same phenomenon.
It observed that the low but steady roll rate above 1.5 degrees/sec
that can delay the triggering of steering reversal is a result of tire
deflections continuing the roll motion of the whole vehicle after the
point of maximum roll of the suspension system. It believes that a
default trigger negates the design of the maneuver to let the vehicle
motions select the steering response, but describes some ways of using
filtering of the roll rate signal to cause the steering to trigger
earlier in these cases. But it acknowledges that letting the vehicle
react to the actual roll motion of the whole vehicle rather than to a
roll signal distorted by signal processing may be preferable.
At this point we are preserving the consistent application of the
fishhook steering algorithm. We do not believe that commenters have
presented us a substantive reason to depart from this application. If
the vehicle tips up despite a long dwell time, there is no change in
test result. If the vehicle does not tip, it will be retested with a
reduced steering angle according to the current procedure, which may
change the roll frequency harmonics and dwell time. We will observe the
steering reversal dwell times during the first group of tests and, if
necessary, reconsider the commenter's observations on this issue.
H. Fifteen-Passenger Vans
The National Transportation Safety Board, Public Citizen and others
commented on the rollover issues surrounding fifteen-passenger vans.
NHTSA agrees that it is important to investigate the commenters'
concerns about the rollover susceptibility of fifteen-passenger vans.
To do this, we will conduct an evaluation of fifteen-passenger vans'
rollover susceptibility at different loading conditions and evaluate
available electronic stability control systems on these vehicles.
I. Tip-up Criterion
Mechanical Systems Analysis, Inc. and several other commenters
suggested that the tip-up criterion of 2 inches simultaneous wheel lift
is too conservative. It recommended a criterion of 20 degrees body roll
instead because suspension bouncing on test surface irregularities
could influence performance under our criterion. Other similar
recommendations were given for body roll angles between 15 and 20
degrees. The 2 inch wheel lift criterion is met at about 11 degrees of
body roll on average.
NHTSA's tests were performed on a very smooth test area at TRC of
Ohio. The tip-up criterion maximized driver safety and minimized tire
wear by allowing us to increase speed in 5 mph increments with a
reasonable expectation of avoiding sudden violent tip-ups that could
``pole-vault'' the vehicle on its outriggers. However, we observed tip-
ups at lower than expected speeds during tests at other facilities (DPG
and APG as described above) that were probably influenced by surface
irregularity as described by the commenter. We believe that our tip-up
criterion is appropriate for an excellent facility like TRC, but we
agree that the criterion should be revisited if NCAP tests were to take
place at a facility with a more irregular surface.
J. Testing of Passenger Cars v. Light Trucks
Consumers Union and IIHS recommended that we not test passenger
cars in order to devote all the available time and resources for
maneuver tests to light trucks. We agree that it is very unlikely that
passenger cars will tip up in the maneuver test. We have tested
passenger cars at the low end of the SSF range for passenger cars
without observing any tip-ups. It seems reasonable to rate passenger
cars using the ``no tip-up'' curve of the risk model along with SSF
measurements. However, we prefer to track whether this continues to be
true. Hence, we will continue to test a few passenger cars each year at
the low end of the SSF range to reinforce the ``no tip-up'' assumption.
Therefore, two passenger cars are listed in Table 5.
K. Testing With Stability Control Systems
Toyota suggested that NHTSA should selectively choose vehicles with
optional equipment that assists the driver in controlling the vehicle
such as electronic yaw stability control, while in a previous comment
Honda suggested the opposite policy. Honda believed that even a vehicle
with standard stability control should be tested with it turned off if
the vehicle has an ``off'' switch. It has been NHTSA's policy for
rollover resistance ratings that we test vehicles most representative
of those sold. Also, we are interested in the potential safety benefits
of electronic yaw stability control and have alerted consumers to its
purpose and availability on individual models in our present consumer
information. Therefore, when it is standard equipment or optional
equipment found on the majority of vehicles of a particular model, we
will test with stability control turned on and report that the test
vehicle was so equipped. Also, if the market penetration of a stability
control option is too low for NHTSA to choose it for inclusion on our
test vehicle, we will consider optional NCAP tests at the
manufacturer's expense.
VIII. Final Form for Rollover Resistance Ratings--Alternative I
A. Combined Ratings
NHTSA will use the statistical model shown in Figure 4 to combine
the vehicle's SSF measurement and its performance in the Fishhook
maneuver with 5-occupant loading as a prediction of its rollover rate
per single-vehicle crash. The predicted rollover rate will be
translated into a star rating in the same way used in the present
rollover resistance ratings: one star for a rollover rate greater than
40 percent; two stars, greater than 30 percent; three stars, greater
than 20 percent; four stars, greater than 10 percent; five stars, less
than or equal to 10 percent.
The decision to combine the static (SSF) and the dynamic (maneuver
test) vehicle measurements in a single rollover resistance rating is
consistent with the view of most commenters that separate ratings would
be confusing to consumers. It is also the best way of achieving NHTSA's
goal of presenting risk-based ratings because it maximizes the vehicle
information used to make the prediction of the rate of rollovers per
single-vehicle crash. Those who favored separate static and dynamic
ratings expressed concern that the influence of electronic stability
control would be small in the combined rating. It is true that
electronic stability control will not have a great influence on
rollover resistance ratings because the dynamic test result has less
predictive power than the static measurement on rollover rate and the
effect of electronic (yaw) stability control on the dynamic test is
also modest. We believe that the potential benefit of electronic
stability control lies in helping drivers to stay on the road and away
from tripping devices rather than providing much increase in rollover
resistance, especially regarding tripped rollovers. Rather than reduce
the rate of rollovers in single-vehicle crashes, electronic stability
control may reduce the number of single-vehicle crashes in the first
place. However, its effectiveness in reducing single-vehicle
[[Page 59263]]
crashes remains to be demonstrated by crash statistics.
For the present time, we will retain the use of five stars to
express rollover resistance ratings. Focus groups consistently find
that presentation understandable. However, the NAS and a number of
commenters were in favor of presentations that are able to show smaller
differences between vehicles, contrast the range of ratings between
types of vehicles and show the relative position of a vehicle's rating
among other vehicles of the same type. NHTSA is performing additional
consumer research to determine the best approach to providing consumers
with more detailed information to supplement the star ratings. Several
presentation methods are being tested, and we will consider those test
results and propose appropriate changes to how we present rollover
information to consumers.
B. Dynamic Testing
The Fishhook maneuver test will be conducted according to the
procedure in Appendix I, and we will discontinue the J-Turn maneuver
test. This decision is a consequence of the logistic regression
analysis of the crash data, SSF and results of the J-Turn and Fishhook
tests at two load conditions for 25 vehicles. From a statistical point
of view, the J-Turn test results were redundant in the presence of the
Fishhook test results. The J-Turn test also seems to be more sensitive
to irregularities in pavement surface and friction and changes in
ambient temperature than the Fishhook test. It also causes more concern
about tire wear effects than the Fishhook, and it was criticized by
some commenters as less representative of ``real-world'' driving
situations.
We have decided to change the heavy load condition from an
anthropomorphic dummy (water dummy) in every rear seating position
(along with the test driver and instruments of approximately a
passenger weight in the front) to a standard load representing five
occupants in all vehicles capable of at least that loading. During the
test of the 25 vehicles, it became obvious that heavy load tests were
being run at very unequal conditions especially between vans and other
vehicles (two water dummies in some vehicles but six water dummies in
others). While very heavy passenger loads can certainly reduce rollover
resistance and potentially cause special problems, crashes at those
loads are too few to greatly influence the overall rollover rate of
vehicles. Over 94% of van rollovers in our 293,000 crash database
occurred with five or fewer occupants, and over 99% of rollovers of
other vehicles occurred with five or fewer occupants. The average
passenger load of vehicles in our crash database was less than two:
1.81 for vans; 1.54 for SUVs; 1.48 for cars; and 1.35 for pickup
trucks. In order to use the maneuver tests to predict real-world
rollover rates rather than investigate possible poor performance at
high occupancy levels, it is not useful to test the vehicles under
widely differing loadings while there is much less loading variation
represented in the crash statistics. Consequently, the maneuver test
data used in the logistic regression analysis involving the 25 dynamic
test vehicles in the heavy load condition represented performance with
a 5-occupant loading (obtained using three water dummies in the rear
seating positions) for all vehicles capable of carrying at least that
load.
The use of dynamic maneuver tests creates the need for a policy
regarding tire de-beading. The tests are conducted using the tire
pressure recommended by the vehicle manufacturer and labeled on the
vehicle. We have experienced a number of instances in which the tire
bead became unseated from the rim, resulting in total air loss and rim
contact with the paved surface. This causes damage to the test facility
and the possibility of a rollover of the test vehicle. For at least a
year, we have been using inner tubes in all tires placed on rollover
test vehicles. This action reduces the instances of total de-beading,
but does not eliminate them entirely. In some instances, a tire with a
tube that is not pinched during the process can experience a partial
de-bead in which the rim makes contact with the pavement surface and
then the tire becomes remounted on the rim by the pressure of the tube.
It has been NHTSA's experience on the test track that if a maneuver
results in rim contact without destroying the tube, the next run at a
higher speed will destroy the tube and cause a complete de-beading of
the tire and hard contact of the rim with risk to the driver, test
surface and vehicle.
In the case of rim contact without total de-beading, it is a near
certainty that total de-beading would have occurred without the tube,
and total de-beading despite the tube is highly likely at the next
speed increment. Thus, we consider rim contact to indicate de-beading,
and it will be NHTSA's policy to terminate the test if rim contact with
the pavement is observed even if the tube prevents total de-beading.
The vehicle did not actually tip up in the maneuver if the test is
terminated as a result of rim contact indicating tire de-beading.
However, debeading is a bad outcome for the test because tire de-
beading is associated with on-road tripped rollovers that actually
outnumber on-road untripped rollovers. Therefore, it would be improper
to ignore tire debeading and predict the vehicle's rollover rate as if
it had completed the test without tip-up or de-beading. The only
alternative in the case of rim contact is to simply not compute a
rollover resistance rating of the vehicle because the test was not
completed. It will be reported that the dynamic test could not be
completed because of tire debeading, but the SSF measurement will be
retained in the detailed consumer information.
C. Demonstration Program
In April 2003, NHTSA's VRTC began the Demonstration Test program at
TRC of Ohio using the test protocol of Appendix I for Fishhook maneuver
tests of 18 new vehicles. Table 5 lists the vehicles in this group. We
will verify tip-ups using new tires as explained in our answer to
Ford's comments in Section VII. Unless we discover serious procedural
problems, these vehicles will be given 2004 NCAP rollover resistance
ratings according to the system established in this final notice.
Table 5.--Vehicles Included in Demonstration Test
----------------------------------------------------------------------------------------------------------------
Make Model Bodystyle
----------------------------------------------------------------------------------------------------------------
1............... Chevrolet...................... Silverado 4x2............................. PU ext. cab.
2............... Chevrolet...................... Silverado 4x4............................. PU ext. cab.
3............... Chevrolet...................... Trailblazer 4x2........................... 4-dr Utility.
4............... Chevrolet...................... Trailblazer 4x4........................... 4-dr Utility.
5............... Ford........................... Explorer 4x2.............................. 4-dr Utility.
6............... Ford........................... Explorer 4x4.............................. 4-dr Utility.
7............... Ford........................... Explorer SportTrac 4x2.................... 4-dr Utility.
8............... Ford........................... Explorer SportTrac 4x4.................... 4-dr Utility.
[[Page 59264]]
9............... Ford........................... Focus..................................... 4-dr wagon.
10.............. Jeep........................... Liberty 4x2............................... 4-dr Utility.
11.............. Jeep........................... Liberty 4x4............................... 4-dr Utility.
12.............. Subaru Outback (4x4)........... 4-dr wagon................................
13.............. Toyota......................... Echo...................................... 4-dr sedan.
14.............. Toyota......................... 4Runner 4x2............................... 4-dr Utility.
15.............. Toyota......................... 4Runner 4x4............................... 4-dr Utility.
16.............. Toyota......................... Tacoma 4x2................................ PU ExCab.
17.............. Toyota......................... Tacoma 4x4................................ PU ExCab.
18.............. Volvo.......................... XC90 (4x4)................................ 4-dr Utility.
----------------------------------------------------------------------------------------------------------------
X. Assessment of Costs and Benefits
Since this is a consumer information program, no Regulatory
Evaluation was developed for this notice. Adding the dynamic maneuver
tests to the Rollover NCAP will not require vehicle manufacturers to
take any action. The costs are Federal Government costs for developing
the test protocol and rating system, conducting the tests, and
disseminating the information. The benefits are information to
consumers. Consumers want additional information. It is impossible for
us to quantify the effect on consumer behavior or on manufacturer
behavior.
XI. Rulemaking Analyses and Notices
A. Executive Order 12866
Executive Order 12866, ``Regulatory Planning and Review'' (58 FR
51735, October 4, 1993), provides for making determinations whether a
regulatory action is ``significant'' and therefore subject to Office of
Management and Budget (OMB) review and to the requirements of the
Executive Order. The Order defines a ``significant regulatory action''
as one that is likely to result in a rule that may:
(1) Have an annual effect on the economy of $100 million or more or
adversely affect in a material way the economy, a sector of the
economy, productivity, competition, jobs, the environment, public
health or safety, or State, local, or Tribal governments or
communities;
(2) Create a serious inconsistency or otherwise interfere with an
action taken or planned by another agency;
(3) Materially alter the budgetary impact of entitlements, grants,
user fees, or loan programs or the rights and obligations of recipients
thereof; or
(4) Raise novel legal or policy issues arising out of legal
mandates, the President's priorities, or the principles set forth in
the Executive Order.
NHTSA has considered the impact of this action under Executive
Order 12866 and the Department of Transportation's regulatory policies
and procedures. This action has been determined to be economically not
significant. However, because it is a subject of Congressional
interest, this rulemaking document was reviewed by the Office of
Management and Budget under Executive Order 12866, ``Regulatory
Planning and Review.''
B. Regulatory Flexibility Act
The Regulatory Flexibility Act of 1980 (5 U.S.C. Sec. 601 et seq.)
requires agencies to evaluate the potential effects of their proposed
and final rules on small business, small organizations and small
governmental jurisdictions. I hereby certify that the amendment will
not have a significant economic impact on a substantial number of small
entities. The proposed action does not impose regulatory requirements
on any manufacturer or other party.
C. National Environmental Policy Act
NHTSA has analyzed this proposal for the purposes of the National
Environmental Policy Act. The agency has determined that implementation
of this action will not have any significant impact on the quality of
the human environment.
D. Executive Order 13132 (Federalism)
The agency has analyzed this rulemaking in accordance with the
principles and criteria contained in Executive Order 13132 and has
determined that it does not have sufficient federal implications to
warrant consultation with State and local officials or the preparation
of a federalism summary impact statement. The action will not have any
substantial impact on the States, or on the current Federal-State
relationship, or on the current distribution of power and
responsibilities among the various local officials.
E. Unfunded Mandates Act
The Unfunded Mandates Reform Act of 1995 requires agencies to
prepare a written assessment of the costs, benefits and other effects
of proposed or final rules that include a Federal mandate likely to
result in the expenditure by State, local or tribal governments, in the
aggregate, or by the private sector, of more than $100 million annually
(adjusted annually for inflation with base year of 1995). Adjusting
this amount by the implicit gross domestic product price deflator for
the year 2002 results in $113 million (110.66/98.11 = 1.13). The
assessment may be included in conjunction with other assessments, as it
is here.
The action does not impose regulatory requirements on any
manufacturer or other party.
F. Civil Justice Reform
This action will not have any retroactive effect. Under 49 U.S.C.
21403, whenever a Federal motor vehicle safety standard is in effect, a
State may not adopt or maintain a safety standard applicable to the
same aspect of performance which is not identical to the Federal
standard, except to the extent that the state requirement imposes a
higher level of performance and applies only to vehicles procured for
the State's use. 49 U.S.C. 21461 sets forth a procedure for judicial
review of final rules establishing, amending or revoking Federal motor
vehicle safety standards. That section does not require submission of a
petition for reconsideration or other administrative proceedings before
parties may file suit in court.
G. Paperwork Reduction Act
This document does not contain ``collections of information,'' as
that term is defined in 5 CFR Part 1320 Controlling Paperwork Burdens
on the Public.
H. Plain Language
Executive Order 12866 requires each agency to write all rules in
plain language. This action will not result in regulatory language.
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Issued on: October 2, 2003.
Jeffrey W. Runge,
Administrator.
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Appendix I. Fishhook Maneuver Test Procedure
1.0 Introduction
1.1 General
This document describes the test procedure used by the National
Highway Traffic Safety Administration's (NHTSA) New Car Assessment
Program (NCAP) to evaluate light vehicle dynamic rollover
propensity. The procedure is comprised of one characterization
maneuver and one rollover resistance maneuver.
1.2 Rollover Resistance Requirements of the TREAD Act
Section 12 of the ``Transportation Recall, Enhancement,
Accountability and Documentation (TREAD) Act of November 2000''
reflects the desire of Congress to supplement SSF [Static Stability
Factor] with a dynamic stability test using vehicle maneuvers.
Congress directed NHTSA to ``develop a dynamic test on rollovers by
motor vehicles for a consumer information program; and carry out a
program conducting such tests.'' NHTSA's NCAP Light Vehicle Dynamic
Rollover Propensity Test Procedure described in this document was
developed as part of NHTSA's effort to fulfill the requirements of
the TREAD Act.
1.3 Recent NHTSA Light Vehicle Dynamic Rollover Propensity Research
During the spring through fall of 2001 NHTSA performed an
extensive assessment of many test track maneuvers potentially
capable of quantifying on-road, untripped rollover propensity. In
brief, five vehicle characterization and nine dynamic rollover
propensity maneuvers were studied. Each maneuver was either
discarded or retained for subsequent program phases. The 2001
research project is documented in [1].
During the spring through fall of 2002 NHTSA performed a
comprehensive evaluation of rollover resistance for a broad spectrum
of twenty-six light vehicles. The test vehicles were evaluated with
one Characterization maneuver and two Rollover Resistance maneuvers.
Up to two load configurations per vehicle were used. The 2002
research project is documented in [2].
2.0 Test Equipment
2.1 Vehicle Load Configurations
NHTSA's dynamic rollover propensity test procedure uses one of
two loading configurations: Nominal or Multi-Passenger. A
description of each configuration is provided below.
Both vehicle load configurations include instrumentation, a
steering machine, and outriggers.
Test vehicle bumper assemblies are removed for outrigger
installation. The reduction in vehicle weight due to the removal of
the bumpers is offset by the additional weight of the outriggers and
their mounting system. The outrigger system typically outweighs the
bumper assemblies.
2.1.1 Nominal Load Configuration
The Nominal Load Configuration consists of the driver,
instrumentation, steering machine, outriggers, and full tank of
fuel. Weight and location specifications for the data acquisition
system and steering machine are presented in Table I.1 and Figure
I.1.
Table I.1.--Equipment Location and Weight
----------------------------------------------------------------------------------------------------------------
Weight, typical
Equipment Location (lbs)
----------------------------------------------------------------------------------------------------------------
Data Acquisition System...................... Front passenger seat......................... 58
Steering Machine............................. Handwheel.................................... 31
Steering Machine Electronics Box............. Passenger row foot well behind the front 39
passenger seat. If vehicle does not have a
rear passenger row foot well, the
Electronics Box should be placed in the
front passenger seat foot well.
----------------------------------------------------------------------------------------------------------------
Non-pickup truck vehicles with only front designated seating
positions use the Nominal Load Configuration.
2.1.2 Multi-Passenger Configuration
The Multi-Passenger Configuration includes all elements of the
Nominal Load Configuration plus ballast in the form of water
dummies. Water dummies are installed as follows:
For vehicles with three or more designated rear seating
positions, three 175 lb water dummies are used. The water dummies
shall be positioned on the rear seats (second seating row) closest
to driver and front passenger seats (first seating row). If there
are only two seating positions in the second seating row, the third
water dummy shall be placed in the center of the third seating row,
provided it is a designated seating position. Refer to Figure I.2.
For vehicles with two designated rear seating positions, two 175
lb water dummies shall be positioned in the rear seats. Refer to
Figure I.3.
For pickups with only front designated seating positions, three
175 lb water dummies will be used. The water dummies shall be
positioned behind the cab in a manner that emulates a second seating
row. If it is not possible to fit three water dummies directly
behind the cab, the third water dummy shall be placed in the center
of a simulated third seating row. Refer to Figure I.4.
For pickups with two seating rows, three 175 lb water dummies
will be used. If the second seating row includes three designated
seating positions, each water dummy shall be placed in these
positions. If the second seating row includes two designated seating
positions, two 175 lb water dummies shall be positioned in the
second seating row of the cab, and the third water dummy shall be
positioned behind the cab in a manner that emulates the center
seating position of a third seating row. Refer to Figure I.5.
For all vehicles, if the Multi-Passenger Configuration results
in the vehicle exceeding its Gross Vehicle Weight Rating (GVWR) and/
or rear Gross Axle Weight Rating (GAWR), the weight of each dummy
will be equally reduced until the GVWR and/or rear GAWR are no
longer exceeded. The weight of the water dummies shall not be
reduced if only the front GAWR is exceeded and the front axle weight
does not exceed the front GAWR by more that 50 pounds, i.e., if the
Multi-Passenger Configuration results in the vehicle exceeding its
front GAWR, and its GVWR and/or rear GAWR, the weight of each dummy
will be equally reduced until the GVWR and rear GAWR are no longer
exceeded and the front GAWR is not exceeded by more that 50 pounds.
For non-pickup truck vehicles with only front designated seating
positions, the Multi-Passenger Configuration is omitted from the
test matrix.
2.2 Safety Outriggers
Safety outriggers are installed on all test vehicles during all
test maneuvers. NHTSA uses outriggers machined from 6Al-4V titanium.
NHTSA's ``short'' outriggers are used for vehicles with baseline
weights under 3,500 pounds in a baseline condition (as delivered);
``standard'' outriggers are used for vehicles with baseline weights
from 3,500 and 7,000 pounds; and ``long'' outriggers are used for
vehicles with baseline weights from 7,001 to 10,000 pounds.
Information on NHTSA's titanium outrigger system is documented in
[3].
2.3 Tires
All tires must be new, and of the same make, model, size, and
DOT specification of those installed on vehicles when purchased new.
Tire inflation pressures are to be in accordance with the
recommendations indicated on each vehicle's identification placard.
2.3.1 Tire Mounting Technique
When mounting tires to the rims used for testing, no tire
mounting lubricant should be used. Lubricant is not used due to
uncertainty surrounding the occurrences of tire debeading observed
during NHTSA's rollover research. To eliminate the possibility of
tire lubricant contributing to this phenomenon, it should not be
used. Because no lubricant is used, care must be taken to confirm
that the tire is fully seated on the
[[Page 59270]]
wheel rim at the completion of the mounting procedure.
2.3.2 Frequency of Tire Changes
To minimize the effects of tire wear on vehicle response and
rollover propensity, rollover research requires frequent tire
changes. For each loading condition, the following guidelines must
be followed:
[sbull] One set of tires is to be used for each Slowly
Increasing Steer test series. Each series is comprised of left and
right steer tests.
[sbull] Up to two tire sets are to be used for the Fishhook
maneuver test series. The actual number of tire sets used is
dependent on the response of each vehicle. The tire change protocol
is presented in the Fishhook maneuver test procedure (Section 3.2).
Note: A tire change between the completion of the Slowly Increasing
Steer maneuver and initiation of Fishhook testing is not required
provided the abbreviated Slowly Increasing Steer procedure described
in Section 3.1.2 is used. If the abbreviated procedure is not used
(i.e., the maneuver is performed such that maximum lateral
acceleration is achieved), a tire change between the completion of
the Slowly Increasing Steer maneuver and initiation of Fishhook
testing is required, as tire wear associated with these tests may
potentially confound Fishhook test outcome.
2.3.3 Use of Inner Tubes
Fishhook maneuvers have been shown to produce debeading of the
outside front and rear tires. The occurrence of debeads can result
in significant damage to the test surface. NHTSA research has
concluded the easiest, most cost effective way to minimize debeading
is the use of inner tubes designed for radial tires. Inner tubes
must be installed prior to any Fishhook test `` one inner tube for
each of the vehicle's tires. Inner tubes should be appropriately
sized for the test vehicle's tires.
Installation of inner tubes is not required prior to Slowly
Increasing Steer tests, regardless of vehicle or load condition.
2.4 Data Collection
All data is to be sampled at 200 Hz. NHTSA's signal conditioning
consists of amplification, anti-alias filtering, and digitizing.
Amplifier gains are selected to maximize the signal-to-nois