Critics Blast Tire Pressure Warning Systems
September 23, 2006
Ralph Vartabedian / Los Angeles Times
Nothing but compressed air, certainly not the rubber tread on the tires, keeps your vehicle suspended over the road.
So, the amount of air in tires is vitally important, a fact relentlessly driven home by safety experts but ignored by many motorists.
After more than 80 people died in Ford Explorers equipped with poorly inflated Firestone tires in the late 1990s, Congress passed the Tread Act, mandating, among other things, that automakers install tire pressure monitoring systems on future vehicles. The act, being phased in, will require all new cars to have such a monitor by September 2008.
But as implemented by the National Highway Traffic Safety Administration, the rules were quickly assailed by critics for a weak and ineffective approach to the problem.
Since 2000, the issue of tire pressure has become even more critical, as motorists optto increasing numbers for low aspect ratio tires that depend on closely monitored air pressure to avoid blowouts caused by potholes and other road hazards.
A close look at tire pressure monitoring systems shows that, in general, they will fail to relieve car owners of the burden of closely watching tire pressures. While they may help to alert people of dangerously low air pressure, they generally will not warn drivers of air pressure low enough to cause slow tire damage.
Two basic types of technology are used in tire pressure monitoring systems, or TPMS: direct and indirect. The less expensive approach adopted by some auto manufacturers is called "indirect" and uses anti-lock braking systems to provide signals for the system.
When a tire has low pressure, it tends to rotate at a slower speed than the other tires, which the anti-lock system sensors and computers can detect. All automakers have to do is create some software and put a warning light on the dashboard.
But Clarence Ditlow, executive director of the Center for Auto Safety, said these systems "are wrong 50 percent of the time." Ditlow, along with the Washington-based consumer group PublicCitizen, sued the NHTSA and won a judgment that the agency's rule did not satisfy the intent of the Tread Act. But ultimately, the agency issued a rule that still left safety advocates and some members of the tire industry fuming.
Even the system's core mission -- to warn of dangerously low air pressure -- has loopholes under the NHTSA's final revised rule issued last year. The system does not have to detect low air pressure for a full 20 minutes after a tire begins to lose air, for example. The loophole was put in to accommodate the limits of technology in the indirect system, according to Gerald Donaldson, a safety expert at Advocates for Highway Safety in Washington, D.C.
Another key shortcoming of NHTSA's rule is that the systems are not required to tell the driver which tire is low, only that one of the four tires has a problem. Donaldson, among others, said the lack of specific information could lead some motorists to ignore the warning.
The second type of technology is called the "direct" system and uses independent pressure sensors inside each tire stem. The sensors have electronic transmitters that can send more reliable air pressure data to the car's computer. While an indirect system might cost only a few dollars, a direct system could cost $50 or more for all four tires, according tire experts. Most, if not all, automakers will opt for direct systems by 2008, experts say.
With both types of technology, however, the NHTSA set up rules that significantly compromised the effectiveness of the systems, critics say. The standard calls for warning the driver when the tire pressure drops 25 percent below the level recommended by the vehicle manufacturer.
"One of our concerns is that it gives the motorist a false sense of security," said Matt Edmonds, vice president at Tire Rack, an Internet tire retailer that has long conducted independent testing of tires. "The unfortunate part is that with indirect systems you can get a false warning. Then there is no confidence in the system."
Tire manufacturers set maximum inflation ratings, but the vehicle manufacturer decides the recommended inflation for a specific vehicle.
Many vehicles, for example, have a recommended pressure of 32 pounds per square inch (psi), meaning the TPMS would not notify the driver until the pressure dropped by 8 pounds per square inch to 24 psi. In some extreme cases, such as the older Ford Explorers, tires carry recommended pressure of only 26 psi, meaning the warning system would not alert the driver until the pressure was down to 19.5 psi.
Donaldson said the NHTSA should have required a system that notifies a driver when pressure drops 15 percent or 20 percent below the recommended level. A NHTSA official said the agency was concerned that if drivers were notified of low pressure too often, they would begin to ignore the warnings, terming it a "nuisance." The 25 percent level was selected because it provided a warning of "extreme danger," according to an agency spokesman.
But by the time pressure drops 25 percent, the tire is undergoing damage to its internal construction, according to experts.
Bill VandeWater, consumer tire products manager for Bridgestone, said tire damage begins when pressure drops 5 pounds per square inch below the vehicle manufacturer's recommendation.
"We would never like to see it go less than 5 pounds below the level recommended by the vehicle maker," he said.
After the NHTSA issued its final rule, the tire industry sent a letter explaining bluntly what it thought: The Tire Industry Association, a trade group, said it believed the final rule was "fatally flawed" and added, "This final rule will not keep the motoring public safe."
The upshot is that the consumer is still responsible for crouching down at least once a month and measuring the pressure in each of the tires on his or her vehicle.