- The Washington Times - Friday, November 9, 2001

The Firestone tire controversy may be dying down, but one bit of fallout from that problem will be with us for years to come. The intense publicity associated with the investigation of tire failures created a new reminder that underinflation can cause tires to fail, with catastrophic results.
In late 2000, reacting to public pressure, Congress passed a law called the Transportation Recall Enhancement, Accountability, and Documentation Act or TREAD. A part of the new law requires the National Highway Traffic Safety Administration the federal agency in Washington that oversees vehicle safety issue to take steps to establish a new federal safety standard to require tire pressure monitoring systems in new passenger cars and light trucks.
The importance of tire-pressure monitoring was further demonstrated when NHTSA released the results of a February 2001 tire-inflation-pressure survey. During the survey, investigators measured tire pressure on 11,530 vehicles at 24 locations across the country and found that 36 percent of passenger cars and 40 percent of light trucks had one or more tires that were 20 percent below recommended pressure.
Federal regulators tried to require inflation-pressure monitors as far back as 1970, but the attempt failed, mainly because of immature technology and high cost. During the 1970s, the price of in-vehicle warning devices began to drop. So, NHTSA regulators tried once again in 1981 to mandate pressure monitors. And once again, the initiative failed for the same reasons as in 1970.
Now, regulators have better, less-expensive technology to support their proposed requirements, but the dilemma is deciding what type of technology to specify in the regulation. There are two types of tire pressure monitoring systems. Direct systems actually measure the pressure in the tire, while indirect systems estimate the pressure. Systems of both types are available on an increasing number of vehicle models.
Indirect systems work with the vehicle's anti-lock brake system. ABS uses wheel speed sensors to measure the rotational speed of all four wheels. As a tire's pressure decreases, the rolling radius decreases, and the rotational speed of that wheel increases correspondingly. If one tire becomes significantly underinflated while the others remain at the proper pressure, the system can detect the difference in rotational speed and send a warning to the driver. Several Toyota, Ford and General Motors vehicles have indirect systems.
Direct pressure systems use pressure sensors, located in each wheel, to measure pressure. These sensors include transmitters that broadcast data via a wireless radio frequency to a central receiver where it is analyzed. The receiver is connected to a display mounted inside the vehicle. Some displays can show pressure and temperature in each tire, including the spare. The Chevrolet Corvette uses such a system.
Indirect systems offer the advantages of simplicity and low cost. Vehicles already equipped with ABS need only the capability to monitor wheel-speed sensors, a low-pressure warning light, a reset button, and some software changes. NHTSA estimates the manufacturing cost at less than $13, but there are several disadvantages. Four indirect systems tested by NHTSA had problems detecting two significantly underinflated tires on the same axle and on the same side of the vehicle. Further, none could warn the driver when all of the tires are underinflated by the same amount.
Direct systems offer an array of important advantages. They can detect when any tire or combination of tires is low, including when all four are equally underinflated. Direct systems also can operate while the vehicle is stationary, so the driver can be alerted as soon as the vehicle is started. Cost estimates for direct systems in mass production range from about $50 to $100.

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