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Eye on electronics

Motor, Sep 2002 by Dale, Mike

Tire pressure monitoring systems are designed to solve the problem of underinflated tires. But current monitoring equipment might not be up to speed.

It's no secret to shop owners and technicians: The average motorist doesn't check the air pressure in his car's tires nearly often enough. Even for those who do, the number of service stations that have free air and accurate gauges has been dwindling as the number of gasoline/convenience stores has expanded. Underinflation gets most of the blame for the serious problem of last year, which led to the recall of 6 million Firestone tires. It's believed that those tires failed due to the high heat generated by excessive sidewall flexing caused by underinflation.

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This is not just a Firestone problem, though. A survey conducted by Tire Business showed that of 766 vehicles surveyed, 72% were operating with at least one tire underinflated by an average of 6 psi; 40% of the vehicles had all four tires underinflated by an average 7 psi. The Rubber Manufacturers Association reported that in 1999, there were 647 tire-related fatalities in the U.S.

The Cycloid Corp. (Cranberry, Pennsylvania) makes tire pressure monitoring systems (TPMS). By their numbers, underinflated tires waste 1.25 billion gallons of gasoline each year. According to the company, low tire pressure is the leading cause of early wear-out of some 4.5 million tires a year. They say tire pressure monitoring could add as much as half a mile per gallon to the CAFE rating of a vehicle. The point is that underinflated tires are costly, dangerous and, unfortunately, plentiful.

Well, the government has decided to do something about this. Starting in November 2003, the National Highway Traffic Safety Administration (NHTSA) will require 10% of 2004 model vehicles to have built-in tire pressure monitoring systems. This figure rises to 35% of the 2005s, 65% of 2006 models and all of the 2007s. These new requirements are the result of a mandate from Congress stemming from the Firestone fiasco.

First of all, what's really new here is that the whole fleet is going to get tire pressure monitoring systems eventually. A number of manufacturers have had these systems on high-end vehicles for a while. GM, for example, has sold more than 2 million vehicles with these systems over the last 15 years. High-end European and Japanese cars also have variations of these systems. This NHTSA regulation will force them into lower-price vehicles, as well.

Before deciding what sort of system carmakers will have to adopt, NHTSA did an extensive study to determine what the capabilities and flaws were in existing systems. Here is some of what they found out:

There are two basic kinds of TPMSs-pressure sensor-based (PSB) and wheel speed-based (WSB). In the PSB direct-reading land, a sensor and radio transmitter are mounted in the tire, usually as a part of the valve stem. A small battery inside the sensor powers the transmitter. Tire pressure and temperature are broadcast to a receiver that then relays the information to the vehicle's body computer. Because the transmitters operate at around 500 mHz, they act like a cell phone and are not bothered by the rotation of the wheel or the presence of metal suspension and body parts. Because the distance the signal from each transmitter has to cross is small, a lithium battery can provide the power.

The wheel-speed-based (WSB) or indirect method of monitoring tire pressure uses the wheel signal from the ABS sensor. As the tire's inflation decreases, so, too, does its radius. The tire then has to speed up to keep up with the other tires. Since most late-model vehicles already have ABS and a body computer, the main cost of adding this system is the need for a display device on the dash to give the warnings. NHTSA estimates the manufacturing cost to be in the area of $13. In contrast, the direct-reading system is expected to add as much as $200 to the cost of a vehicle.

NHTSA tested four WSB systems. All could detect a tire that was 50% low on inflation, provided the car was on a winding road course. On a 7.5-mile oval track, a situation where there's very little turning input from the driver, only three of the four systems could detect a tire with only 14 psi in it! NHTSA concluded that the wheels on the same axle needed to be traveling at different rotational speeds for the WSB software to make a clean detection.

In the NHTSA test, none of the four WSB systems was able to warn if all four tires were underinflated. Remember, there has to be a speed difference between any single tire vs. the other three for the WSB concept to work.

There are other issues with WSB-- type systems, as well. NHTSAs report says that "The wheel-speed-based systems must be reset if the inflation in one or more tires is changed, if one or more tires is replaced or repaired or if the tires are rotated. The reset procedure allows the WSB TPMS to 'relearn' the rotational signature of each tire."

According to NHTSA, based on expected tire life and service intervals, these reset procedures could need to be done as many as a hundred times during the life of a vehicle. The kicker is that with the systems tested, it was possible for someone to reset the system without having had the tire inflations corrected.

Automotive Industry

Eye on electronics

Motor, Sep 2002 by Dale, Mike

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