Appendix B to the Preamble - Glossary
Air Bags--In General
Air bags are inflatable restraints. Enough gas must be
pumped into them to cushion occupants. Otherwise, occupants,
especially large ones, could "bottom out" the air bag and hit the
vehicle interior in a crash. Thus, the amount of pressure within
air bags must be carefully controlled. This is done by
controlling both the rate at which gas is pumped into the air bag
and the rate at which the gas is released from the air bag
through vents or microscopic holes in the fabric itself.
Categories of Frontal Air Bags
Advanced air bags. Advanced air bags are air bags that
minimize the risk of serious injury to out-of-position occupants
and provide improved protection to occupants in high speed
crashes. They accomplish this either by incorporating various
technologies that enable the air bags to adapt their performance
to a wider range of occupant sizes and crash conditions and/or by
being designed to both inflate in a manner that does not pose
such risk as well as to provide improved protection. Some of
these technologies are multi-stage inflators, occupant position
sensors, occupant weight and pattern sensors, and new air bag
fold patterns. (The inflators and sensors are explained below.)
Redesigned air bags.(1)
Redesigned air bags are bag systems
used in vehicles that have been certified to the unbelted sled
test option instead of the unbelted crash test option in Standard
No. 208. Typically, a redesigned air bag in a MY 1998 or 1999
vehicle model has less power than the air bags in earlier model
years of that vehicle model. However, the power levels of
current air bags vary widely. For example, the redesigned air
bags in some current vehicles are more powerful than the
unredesigned air bags in some earlier vehicles.
Inflators
Inflators are the devices which pump the gas into air bags
to inflate them in a crash.
Single stage inflators. Single stage inflators fill air
bags with the same level of power in all crashes, regardless of
whether the crash is a relatively low or high speed crash.
Multi-stage inflators. Multi-stage inflators (also known as
multi-level inflators) operate at different levels of power,
depending on which stage is activated. The activation of the
different stages can be linked to crash severity sensors. In a
vehicle with dual-stage inflators, only the first stage (lowest
level of power) will be activated in relatively low speed
crashes, while the first and second stages (highest level of
power) will be activated in higher speed crashes. As crash
severity increases, so must the pressure inside the air bag in
order to cushion the occupants.
Sensors
Many advanced air bag systems utilize various sensors to
obtain information about crashes, vehicles and their occupants.
This information is used to adapt the performance of the air bag
to the particular circumstances of the crash. It is used in
determining whether an air bag should deploy and, if it should,
and if the air bag has multiple inflation levels, at what level.
Examples of these sensors include the following:
Crash severity sensors. Crash severity sensors measure the
severity of a crash, i.e., the rate of reduction in velocity when
a vehicle strikes another object. If a relatively low severity
crash is sensed, only the lowest stage of a dual-stage inflator
will fill the air bag; if a more severe crash is sensed, both
stages will fill the air bag, inflating it at a higher level.
Belt use sensors. Belt use sensors determine whether an
occupant is belted or not. An advanced air bag system in
vehicles with crash severity sensors and dual-stage inflators
might use belt use information to adjust deployment thresholds
for unbelted and belted occupants. Since an unbelted occupant
needs the protection of an air bag at lower speeds than a belted
occupant does, the air bag would deploy at a lower threshold for
an unbelted occupant. (Deployment thresholds are explained
below.)
Seat position sensors. Seat position sensors determine how
far forward or back a seat is adjusted on its seat track. An
advanced air bag system could be designed so a dual-stage air bag
deploys at a lower level when the seat is all the way forward
than it does when the seat is farther back. This would benefit
those short-statured drivers who move their seats all the way
forward.
Occupant weight sensors. Occupant weight sensors measure
the weight of an occupant. An advanced air bag system might use
this information to prevent the air bag from deploying at all in
the presence of children.
Pattern sensors. Pattern sensors evaluate the impression
made by an occupant or object on the seat cushion to make
determinations about occupant presence and the overall size and
position of the occupant. They could also sense the presence of
a particular object like a child seat. An advanced air bag
system might use this information to prevent the air bag from
deploying in the presence of children. An advanced air bag
system might utilize both an occupant weight sensor and an
occupant pattern sensor.
Deployment Thresholds
The term "deployment threshold" is typically used to refer
to the lowest rate of reduction in vehicle velocity in a crash at
which a particular air bag is designed to deploy.
No-fire threshold. The no-fire threshold is the crash speed
below which the air bag is designed to never deploy.
All-fire threshold. The all-fire threshold is the crash
speed at or above which the air bag is designed to always deploy.
Gray zone. The gray zone is the range of speeds between the
no-fire and all-fire thresholds in which the air bag may or may
not deploy.
Vehicles with advanced air bags may have different
deployment thresholds for belted and unbelted occupants, e.g.,
the deployment threshold may be higher if an occupant is belted.
(See belt use sensors above.)
Crash Tests vs. Sled Tests
In crash tests, instrumented test dummies are placed in a
production vehicle which is then crashed into a barrier.
Measurements from the test dummies are used to determine the
forces, and estimate the risk of serious injury, that people
would have experienced in the crash.
In sled tests, no crash takes place. The vehicle is placed
on a sled-on-rails, and instrumented test dummies are placed in
the vehicle. The sled and vehicle are accelerated very rapidly
backward. As the vehicle moves backward, the dummies move
forward inside the vehicle in much the same way that people would
in a frontal crash. The air bags are manually deployed at a
pre-selected time during the sled test. Measurements from the
dummies are used to determine the forces, and estimate the risk
of serious injury, that people would have experienced in the
crash.
Fixed Barrier Crash Tests
All of the crash tests proposed in this SNPRM are fixed
barrier crash tests, i.e., the test vehicle is crashed into a
barrier that is fixed in place (as opposed to moving). The types
of proposed fixed barrier crash tests are shown in Figure B1.
Rigid barrier test, perpendicular impact. In a rigid
barrier, perpendicular impact test, the vehicle is crashed
straight into a rigid barrier that does not absorb any crash
energy. The full width of the vehicle's front end hits the
barrier.
Rigid barrier, oblique impact test. In a rigid barrier,
oblique impact test, the vehicle is crashed at an angle into a
rigid barrier.
Offset deformable barrier test. In an offset deformable
barrier test, one side of a vehicle's front end, not the full
width, is crashed into a barrier with a deformable face that
absorbs some of the crash energy.
Crash Pulses
A crash pulse is the graph or picture of how quickly the
vehicle occupant compartment is decelerating at different times
during a crash.
Stiff crash pulses. In crashes with stiff pulses, the
occupant compartment decelerates very abruptly. An example of a
crash with a stiff pulse would be a full head-on crash of a
vehicle into a like vehicle. The perpendicular rigid barrier
crash test produces a stiff crash pulse.
Soft crash pulses. In crashes with soft pulses, the
occupant compartment decelerates less abruptly, compared to
crashes with hard pulses. An example of a crash with a soft
pulse would be the crash of a vehicle into sand-filled barrels
such as those seen at toll booths or at the leading edge of a
concrete median barrier. The offset deformable barrier crash
test and the 30 degree oblique rigid barrier crash test produce
soft crash pulses.
In crashes involving comparable reductions in velocity, an
unrestrained occupant would hit the vehicle interior (i.e.,
steering wheel, instrument panel and windshield) at a much higher
speed in a crash with a stiff pulse than in a crash with a soft
pulse.
Belted and Unbelted Tests
Belted tests use belted dummies, while unbelted tests use
unbelted dummies. Despite increases in seat belt use, nearly 50
percent of all occupants in potentially fatal crashes are
unbelted. Unbelted tests are intended to evaluate the protection
provided these persons, many of whom are teenagers and young
adults.
Static Out-of-Position Tests
Static out-of-position tests are called "static" because the
vehicle does not move during the test. These tests are used to
measure the risk that an air bag poses to out-of-position
occupants. Test dummies are placed in specified positions that
are extremely close to the air bag, typically with some portion
of the dummy touching the air bag cover. The air bag is
deployed. Measurements from the test dummy are used to determine
the forces, and estimate the risk of serious injury, that people
would have experienced in the crash.
Injury Criteria and Performance Limits--In general
In a crash test, sled test, or static out-of-position test,
measurements are taken from the test dummy instruments that
indicate the forces that a person would have experienced under
the same conditions. Standard No. 208 specifies several injury
criteria. For each criterion, the Standard also specifies a
performance limit, based on the level of forces that create a
significant risk of producing serious injury.
Injury Criteria
This SNPRM proposes performance limits for various injury
criteria to address the risk of several types of injuries. Among
these injury criteria are:
Head Injury Criterion or HIC. Head Injury Criterion or HIC
address the risk of head injury;
Nij. Nij addresses the risk of neck injury; and
Chest Acceleration and Chest Deflection. Chest Acceleration
and Chest Deflection address the risk of chest injury.
Test Dummies
This SNPRM proposes to use several test dummies to represent
children and adults of different sizes. These dummies are:
12-month old Crash Restraints Air Bag Interaction (CRABI)
dummy, representing an infant;
Hybrid III 3-year-old and 6-year-old child dummies,
representing young children;
Hybrid III 5th percentile adult female dummy, representing a
small woman;
Hybrid III 50th percentile adult male dummy, representing an
average-size man.
1. These air bags are also
sometimes called depowered air bags, second generation air bags or next generation air
bags.