IV. Tire Pressure Monitoring Systems
There are currently two types of TPMSs: direct and indirect. Other types, including hybrid TPMSs that combine aspects of both direct and indirect systems, may be developed in the future. Direct TPMSs directly measure the pressure in a vehicle''s tires, while indirect TPMSs estimate differences in pressure by comparing the rotational speed of the wheels. To varying degrees, both types can inform the driver when the pressure in one or more tires falls below a pre-determined level. Unless the TPMS is connected to an automatic inflation system, the driver must stop the vehicle and inflate the under-inflated tire(s), preferably to the pressure recommended by the vehicle manufacturer. Currently, TPMSs are available as original equipment on a few vehicle models. They are available also as after-market equipment, but few are sold. At this time, NHTSA does not have any information indicating that a hybrid TPMS is being planned for production. However, the agency received comments from TRW, a TPMS manufacturer, stating its belief that such a system could be produced.
The VRTC evaluated six direct and four indirect TPMSs that are currently available. (27) The VRTC found that the direct TPMSs were accurate to within an average of ± 1.0 psi. (28) This leads the agency to believe that those current TPMSs are more accurate than the systems that were available at the time of the agency''s 1981 rulemaking on TPMSs.
Following is a description of the two currently available types of TPMSs and their capabilities.
A. Indirect TPMSs
Current indirect TPMSs work with a vehicle''s ABS. The ABS employs wheel speed sensors to measure the rotational speed of each of the four wheels. As a tire''s pressure decreases, the rolling radius decreases, and the rotational speed of that wheel increases correspondingly. Most current indirect TPMSs compare the sums of the wheel speeds on each diagonal (i.e., the sum of the speeds of the right front and left rear wheels as compared to the sum of the speeds of the left front and right rear wheels). Dividing the difference of the sums by the average of the four wheels speeds allows the indirect TPMS to have a ratio that is independent of vehicle speed. This ratio is best expressed by the following equation: [(RF + LR) -- (LF + RR)/Average Speed]. If this ratio deviates from a set tolerance, one or more tires must be over- or under-inflated. A telltale then indicates to the driver that a tire is under-inflated. However, the telltale cannot identify which tire is under-inflated. Current vehicles that have indirect TPMSs include the Toyota Sienna, Ford Windstar, and Oldsmobile Alero.
Current indirect TPMSs must compare the average of the speeds of the diagonal wheels for several reasons. First, current indirect TPMSs cannot compare the speed of one wheel to the speeds of the other three wheels individually or to the average speed of the four wheels. During any degree of turning, the outside tires must rotate faster than the inside tires. Thus, all four wheel speeds deviate significantly when the vehicle is in a curve or turn. If a current indirect TPMS compared each individual wheel speed to the average of all four wheels speeds, the system would provide a false alarm each time the vehicle rounded a curve or made a turn. The same would be true if the indirect TPMS compared each individual wheel speed to the speed of the other three wheels individually. Since the outside wheels would rotate much faster than the inside wheels in a curve or turn, each outside tire would appear to be under-inflated when compared to an inside tire.
Current indirect TPMSs also cannot compare the speeds of the front wheels to the speeds of the rear wheels because in curves, the front and rear wheels (on both sides of the vehicle) rotate at different speeds. This is primarily due to the fact that the front axle is steerable and follows a different trajectory than the rear axle. As a result, current indirect TPMS must compare a tire from each side and a tire from the front and rear axles to factor out the speed difference caused by curves and turns. Thus, current indirect TPMSs must compare the average speed of the diagonal wheels.
The VRTC tested four current ABS-based indirect TPMSs. None met all the requirements of either alternative proposed in the NPRM. All but one did not illuminate the low tire pressure warning telltale when the pressure in the vehicle''s tires decreased to 20 or 25 percent below the placard pressure. (29) The VRTC determined that since reductions in tire diameter with reductions in pressure are very slight in the 15-40 psi range, most current indirect TPMSs require a 20 to 30 percent drop in pressure before they are able to detect under-inflation. The VRTC also concluded that those thresholds were highly dependent on tire and loading factors.
The VRTC also found that none of the tested indirect TPMSs were able to detect significant under-inflation when all four of the vehicle''s tires were equally under-inflated, or when two tires on the same axle or two tires on the same side of the vehicle were equally under-inflated. However, the VRTC did find that indirect TPMSs could detect when two tires located diagonally from each other (e.g., the front left and back right tires) became significantly under-inflated.
B. Direct TPMSs
Direct TPMSs use pressure sensors, located in each wheel, to directly measure the pressure in each tire. These sensors broadcast pressure data via a wireless radio frequency transmitter to a central receiver. The data are then analyzed and the results sent to a display mounted inside the vehicle. The type of display varies from a simple telltale, which is how most vehicles are currently equipped, to a display showing the pressure in each tire, sometimes including the spare tire. Thus, direct TPMSs can be linked to a display that tells the driver which tire is under-inflated. An example of a vehicle equipped with a direct system is the Chevrolet Corvette.
Since direct TPMSs actually measure the pressure in each tire, they are able to detect when any tire or when each tire in any combination of tires is under-inflated, including when all four of the vehicle''s tires are equally under-inflated. Direct TPMSs also can detect small pressure losses. Some systems can detect a drop in pressure as small as 1 psi.
C. Hybrid TPMSs
In their comments on the NPRM, TRW, a manufacturer of both direct and indirect TPMSs, stated that in order to meet the proposed requirements of the 3-tire, 25 percent alternative, current indirect TPMSs would need the equivalent of the addition of two tire pressure sensors and a radio frequency receiver. The tire pressure sensors would be installed on wheels located diagonally from each other.
For the following reasons, the agency believes that such a ""hybrid"" TPMS would be able to overcome the limitations of current indirect TPMSs, i.e., the inability to detect when all four tires, or two tires on the same axle or same side of the vehicle are under-inflated. First, a hybrid TPMS would be able to detect when two tires on the same axle or the same side of the vehicle were under-inflated because one of those tires necessarily would contain a direct pressure sensor. Second, a hybrid TPMS would be able to detect when the two tires without a direct pressure sensor were under-inflated because they would be located diagonally from each other, and, as the VRTC found in its review of current TPMSs, current indirect TPMSs are able to detect when two tires located diagonally from each other are under-inflated. Third, a hybrid TPMS would be able to detect when three or four tires were under-inflated because one of those tires necessarily would contain a direct pressure sensor.
However, since the agency does not have any information indicating that a hybrid TPMS is currently being planned for production, the agency does not know when such a system could be produced.
28 This is not to say that the systems were able to detect a 1.0 psi drop in pressure. The systems were accurate within ± 1.0 psi once tire pressure had fallen by a certain percentage.
29 The Continental Teves indirect TPMS on the BMW M3 activated the warning telltale at pressures between 9 and 21 percent below the placard pressure.