II. BACKGROUND and ALTERNATIVES

There are two basic types of Tire Pressure Monitoring Systems (TPMS) currently available that can alert the driver while driving that the tire pressure is low:  direct measurement systems and indirect measurement systems.  A direct measurement system measures tire pressure directly.  A variation of the direct measurement system (a direct measurement system with a pump) will soon be available that can inflate the tire when it gets low, relieving the driver of that responsibility. 

An indirect measurement system measures wheel speed or something other than tire pressure.   The current ABS-based systems are an indirect measurements system.  They measure wheel speed and then compare the variance in wheel speed from one wheel to another.

Direct measurement systems

Most direct measurement systems have pressure and temperature sensors in each tire, usually attached to the inflation valve.  They broadcast their data to a central receiver, or in some cases to individual antennae that transmit the data to the control module, which analyzes them and sends appropriate signals to a display.  This display can be as simple as a single telltale, or as complex as pressure and temperature displays for all four tires (or five including the spare).  Direct measurement systems advantages include: (1) much more sensitivity to small pressure losses, with claims ranging from +/- 0.1 psi to 1 psi; (2) the ability to directly measure pressure in any tire at any time, including before starting the vehicle, and including the spare tire.  The disadvantages include: (1) the higher cost; (2) possible maintenance problems when tires are taken on and off the rim (sensors have been broken off).  These systems have not been installed on many vehicles, although they have been used on cars with run-flat tires and as accessories on high-end luxury vehicles. 

Direct measurement system with a pump

A direct measurement system with a pump has the same qualities as a pressure-sensor-based system, except that it also has the ability to pump the tire back up to the placard tire pressure.  Each tire has a sensor and a pump.  The system display is designed to give a warning when a particular tire needs to be continuously inflated and if the tire pressure gets too low, indicating that a particular tire has a problem and needs servicing. Unless there is a catastrophic failure or a rapid loss of pressure due to a nail or puncture, the pump can keep the tire inflated to get the vehicle to its destination.  However, once the vehicle stops, the pump stops, and the tire may deflate.  The advantages of these systems include: (1) driver convenience, they only need to worry about their tire inflation when they get a warning of a continuing problem that the pump has to continue working to control; (2) better fuel economy, tread wear, and safety by keeping tires up to correct pressure.  The disadvantages include: (1) the higher cost; (2) maintenance considerations - when rotating the tires, the pumps must stay on the same side of the car or taken off and put back on the rotated tire.  These systems have not been installed on any light vehicles, although they have been used on a number of heavy trucks for several years. 

Indirect measurement systems

The current indirect measurement systems are based on Anti-lock Brakes (ABS).  They take information from the ABS wheel-speed sensors and look for small changes in wheel speed that occur when a tire loses pressure.  Low pressure results in a smaller wheel radius, which increases the speed of that wheel relative to the others.  The systems work by comparing the relative speed of one tire to the other tires on the same vehicle. 

The advantages for these systems include low cost and minor changes to the vehicle that has an ABS system, including a new dashboard telltale and upgraded software in the electrical system.  Disadvantages include:  (1) not all vehicles have ABS, so costs are significantly higher for vehicles without ABS; (2) the indirect system cannot tell which tire is underinflated; (3) if all tires lose pressure evenly, it cannot detect it, since it works on the relative wheel speed; (4) in some current systems, some combinations of two tires being underinflated cannot be detected.  Regarding #3 and 4, current ABS-based systems cannot detect certain conditions of low tire pressure.  To meet Alternative 2 requirements, the ABS-based systems would need to be upgraded.  (5) they cannot check the spare tire; (6) the vehicle must be moving; (7) they require significant time, sometimes hours, to calibrate the system and several minutes, sometimes tens of minutes, to detect a pressure loss; and (8) they cannot detect small pressure losses.  Regarding #8, the best claim is that they can detect a 20 percent relative pressure loss differential between tires, but others state they can only detect a 30 percent loss, e.g. a tire properly inflated to 30 pounds per square inch (psi) would have to deflate to 21 psi before the system would detect it.  (9)  some systems cannot detect a pressure loss at vehicle speeds of 70 mph or higher. 

Based on these technologies, NHTSA proposed two alternatives (Alternative 1 and 2 below).  After considering comments to the docket, the agency decided to examine another alternative (Alternative 3 below).  After comments from the Office of Management and Budget, the agency decided to examine an alternative that allowed the current indirect systems to meet the standard (Alternative 4 below).

ALTERNATIVES

Alternative 1:  Require activation of the tire pressure monitor system (TPMS) when one to four tires fall 20 percent or more below the recommended placard pressure, or as shown in Table II-1 below a minimum pressure activation floor (140 kPa or roughly 20 psi for p-metric tires), whichever is higher. 

Alternative 2:  Require activation of the TPMS when one, two, or three tires fall 25 percent or more below the recommended placard pressure, or as shown in Table II-1 below a minimum pressure activation floor (140 kPa or roughly 20 psi for p-metric tires), whichever is higher. 

Alternative 3:  Require activation of the TPMS when one to four tires fall 25 percent or more below the recommended placard pressure, or as shown in Table II-1 below a minimum pressure activation floor (140 kPa or roughly 20 psi for p-metric tires), whichever is higher. 

Alternative 4:  Require activation of the TPMS when one tire falls 30 percent or more below the recommended placard pressure, or as shown in Table II-1 below a minimum pressure activation floor (140 kPa or roughly 20 psi for p-metric tires), whichever is higher. 

Table II-1
TPMS lamp activation floor

The activation floor shown in Table II-1 shows the level below or at which the warning must be activated.  The floor is different depending upon the tire type.  All tires are required to have a single maximum inflation pressure labeled on the sidewall and that pressure must be one of the values above.  If a vehicle has p-metric tires marked 240, 300, or 350 kPa, it is a standard load tire that will be tested at 20, 25, or 30 percent below placard, or 140 kPa, whichever is higher.  If a vehicle has a p-metric tire marked 280 or 340 kPa, it is an extra load tire that will be tested at 20, 25, or 30 percent below placard, or 160 kPa, whichever is higher.  (Extra load tires are marked XL or extra load on the sidewall).  LT-tires on light trucks have higher maximum inflation pressures and therefore have been assigned a higher floor below which the warning has to be activated.  The values in Table II-1 are the only values that can be used for maximum inflation pressure.    

Currently, the lowest P-metric tire recommended placard pressure is 26 psi; thus, in all cases systems meeting Alternative 1’s 20 percent below placard requirement would be activated above the 20 psi floor. However, for Alternative 2 and Alternative 3, the 20 psi floor would come into play for vehicles with a 26 psi placard (26 psi x 0.75 = 19.5 psi).  For Alternative 4, the 20 psi floor would come into play for vehicles with a 28 psi placard or lower (28 psi x 0.7 = 19.6 psi). 

Rationales

The rationales for these alternatives are:

1. A 140 kPa floor for p-metric tires is proposed because the agency believes that below that level, safety in terms of vehicle handling, stability performance, and tire failure is an issue.  The agency ran a variety of p-metric tires in what it calls a “low pressure endurance test” at 20 psi with a 100 percent load at 75 mph for 90 minutes on a dynamometer.  None of these tires failed.  In a second set of test it calls a “low pressure high speed test” at 20 psi with a 67 percent load for 90 minutes, in 30 minutes steps at 140, 150, and 160 km/h (87, 93, and 99 mph), about 30 percent of the tires failed.  This leads the agency to believe that there will be a safety margin, in terms of tire failures, if a TPMS warning is provided by 20 psi, unless the vehicle is driven at high speeds (above 140 km/h) that will allow consumers to fill their tires back up before the tire fails. 

The lowest inflation pressure used in the 2000 Tire & Rim Association Yearbook is 140 kPa for P-metric tires.  In the 2001 Tire & Rim Association Yearbook, the 140 kPA pressures have been deleted, apparently because the Association believes they are too low for P-metric tires.  The agency agrees that 140 kPA is too low and believes a floor is needed to assure that drivers are warned when tire pressure gets to or below that level.  For the LT tires, we used the 2000 JATMA yearbook for the lower limits for Load Range C, D, and E tires.  For most cases, the floor is about 58 percent of the maximum inflation pressure.

2. For Alternative 1, 20 percent below placard was chosen after considering several factors.  First, there was no bright line at which the agency could declare that loss of air pressure definitely becomes a safety issue.  The agency did not want to set the level so that the warnings became a nuisance (the agency believes consumers would consider the warning at a nuisance level at about 10 percent below placard).  The nuisance level comes in when consumers are warned too often.  For example, a tire may lose air pressure due to cold weather overnight. But this does not necessarily indicate a need to inflate the tire.  Frequent notifications for trivial reasons would lead consumers to disregard the warning.  Our assessment of current TPMSs leads us to conclude that direct TPMSs can detect 20 percent under-inflation while indirect TPMSs cannot.   

3. For Alternative 2, the agency considered whether it should propose a level that is 30 percent below placard.  The agency looked at the available technology and found that the current indirect measurement systems could not detect 30 percent below placard for all combinations of one to four tires.  Many current ABS-based systems can determine when one or three tires are 30 percent below the other tires, and can determine certain combinations (but not all combinations) of two tires being low.  None can detect when all four tires are at equal under-inflation levels.  The agency then used its judgment to estimate how good an indirect ABS-based system could perform.  We wanted the system to do better, and decided that one, two, or three tires that are 25 percent or more below the placard starting point in our tests was a reasonable goal for these systems. This level was proposed as Alternative 2 in the NPRM.  Comments from many sources indicated that the indirect systems alone could not meet 25 percent for one to three tires.   

4. For Alternative 3, the agency examined a comment from TRW (docket no. 8572-110), which indicated that the proposed Alternative 2 could be met with an indirect system if two direct tire pressure sensors and a radio frequency receiver were added to the system.  The agency considered this hybrid system configuration (direct and indirect) and believes that it could also detect when all four tires are 25 percent below placard and the 140 kPa floor.   

5. For Alternative 4, this is the level that indirect systems on the market can currently all detect, when one tire is 30 percent below placard.  However, the 140 kPa floor might be difficult for some systems to detect if the placard pressure is 180 kPa (26 psi). 

Analytical Assumptions

1. We assume that a direct measurement system would be required to meet Alternative 1 that requires the TPMS to activate at 20 percent below placard pressure for one to four tires.  The current indirect measurement system could not meet this criterion for all four tires since it compares the relative wheel speed of one tire to the other tires.

2. None of the four current indirect measurement systems tested by NHTSA (see Chapter III) could meet Alternative 2.  Not all the systems activated the warning when the pressure in one tire was reduced by 25 to 30 percent, nor did they activate the warning when all of the different groups of two tires were low compared to the other two tires.   In addition, some pickup truck rear axle configurations have both rear tires using one ABS sensor and cannot individually sense wheel speed.  Thus, these pickup trucks are not candidates for meeting the TPMS by using an ABS sensor, without changes that would allow individual wheel sensing.  In essence, at this time the agency believes that Alternative 2 will require either a direct system or an improvement in the indirect measurement systems like the hybrid system that TRW discussed in its docket submission.  In the future it is possible that an indirect system with improvements could meet the Alternative 2 criteria. 

3. The agency believes that Alternative 3 will require either a direct system or an improvement in the indirect measurement systems like the hybrid (direct and indirect) system TRW discussed in its docket submission.   

4. For both Alternatives 2 and 3, we assume a hybrid system would be provided for vehicles that have ABS-systems currently (about two-thirds of the fleet).  For vehicles that don’t have an ABS-type system, we assume that a direct measurement system would be supplied.  A direct measurement system costs less than adding ABS to the vehicle.  A manufacturer could add ABS to the vehicle, but that is a marketing decision not brought on by the TPMS requirements. 

5. For Alternative 4, the agency assumes that indirect systems would be used by all vehicles that have 4 wheel ABS systems.  For many light trucks this will mean changes to the way the ABS is connected to the rear axle (a change from a 3 channel ABS system to a 4 channel ABS system) so that individual wheel speeds can be detected. For vehicles without 4 wheel ABS, the agency assumes that direct systems would be used.