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 factors other than tire pressure. Most current ABS-based systems are indirect measurement systems. They measure wheel speed and then compare the variance in wheel speed from one wheel to another to determine whether a tire is under-inflated.

Although not currently in production, we believe that it would be possible to produce hybrid TPMSs with performance characteristics of both direct and indirect TPMSs.

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 if the spare is included).

Direct measurement systems’ advantages include: (1) much greater 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 separate 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, (only need to worry about tire inflation when 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. These systems have not been installed on any light vehicles, although they have been used on a number of heavy trucks for several years. Because of cost issues, a direct measurement system with a pump has not been considered in further analyses.

Indirect measurement systems

The current indirect measurement systems utilize the wheel speed sensors of Anti-lock Brake Systems (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:

  • (1) low cost [JW1] and
  • (2) the need for only 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 under-inflated;
  • (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 under-inflated cannot be detected (e.g., two tires on the same axle or the same side of the vehicle). (Regarding #3 and 4, current ABS-based systems cannot detect certain conditions of low tire pressure. To meet the proposal, the ABS-based systems would need to be improved.)
  • (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.

Hybrid measurement systems

The agency believes that an indirect measurement system supplemented with direct tire pressure measurement in two wheels and a radio frequency receiver, a “hybrid” system, could meet the proposal. This system was first discussed by TRW in its docket comment [6]. To date, no such systems have been produced.

ALTERNATIVES

In contrast to the June 5, 2002, final rule the agency is not proposing alternative levels of stringency. The proposal is that the driver must be given a warning when tire pressure is 25 percent or more below the placard pressure for one to four tires, or when tire pressure is at or below the defined minimum activation pressure (MAP).

The MAP presented in Table II-1 shows the level at or below 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 indicated in the table. If a vehicle has p-metric tires marked 240, 300, or 350 kPa, it is a standard load tire that will be tested at 25 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 25 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.

 

Table II-1
TPMS Lamp Minimum Activation Pressure
Tire type Maximum or
Rated Inflation
Pressure (kPa)*
Maximum or
Rated Inflation
Pressure (psi)
Activation
Floor (kPa)
Activation
Floor (psi)
P-metric – Standard Load 240, 300, or 350 35, 44, or 51 140 20
P-metric – Extra Load 280 or 340 41 or 49 160 23
Load Range C (LT) 350 51 200 29
Load Range D (LT) 450 65 260 38
Load Range E (LT) 550 80 320 46

* The standard is based on kPa, the psi values have been rounded to the nearest whole number.

Currently, the lowest P-metric tire recommended placard pressure is 26 psi. At 26 psi recommended placard pressure, the 20-psi floor would come into play.

The rationales for the minimum activation pressure are:

A 20 psi 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 dynamo-meter. 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. Since tires could pass the “low pressure high speed test” at 20 psi, this leads the agency to believe that there will be a safety margin, in terms of tire failures, if a TPMS warning is provided at or above 20 psi, that will allow consumers to fill their tires back up before the tire fails, unless the vehicle is driven at very high speeds (above 140 km/h or 87 mph).

The lowest inflation pressure used in the 2000 Tire & Rim Association Yearbook is 140 kPa (20 psi) 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.

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