Tire Design May Have Led to Blowouts
Investigators trying to isolate the underlying causes of Firestone tire failures on Ford Explorers are looking at a design characteristic that may have made the tires run hotter and at the manufacturing process at a Firestone plant.
The effect of heat on the tires remains the central focus of separate probes by the government and the companies, according to several sources familiar with details of briefings last week by Ford Motor Co. and Bridgestone/Firestone to federal investigators at the National Highway Traffic Safety Administration.
In another development Monday, Firestone officials told congressional investigators that Ford did not change the specifications for the tires even as it built heavier Explorers in the late 1990s. Overloading also builds up heat in tires.
“Firestone told our investigators that the margin of safety decreased as the weight of the Explorer increased,” said Ken Johnson, a spokesman for Rep. W.J. (Billy) Tauzin (R-La.), who led a congressional investigation of tire safety this fall.
Ford spokesman Mike Vaughn said the weight increase was “insignificant.” Vaughn said a 1991 four-door 4x2 Explorer had a curb weight of 3,841 pounds, compared with 3,939 pounds for a similarly equipped 1997 model. The difference of 98 pounds was “not substantial enough to change the specifications for the tires,” he said.
Experts say that it is fairly likely that no single, simple failure will be identified, but instead the problem probably resulted from a combination of factors involving tires, vehicles and driving conditions.
A source familiar with the briefings said the design issue has to do with an area of the tire called the “shoulder,” where the sidewall and tread come together. On the recalled 15-inch Firestone ATX tires, the shoulder drops more steeply than on other models.
“In layperson’s terms, these sidewalls are chunky,” said the source. The “chunkiness” is believed to explain new test results that show the recalled tires run hotter than competitors’ models.
“Heat is the enemy of tires,” said the source. “The higher the temperature, the faster it accelerates the aging of the rubber.”
The briefings in Dearborn, Mich., and Akron, Ohio, also identified a potential problem in the manufacturing process at Firestone’s Decatur, Ill., plant: A lubricant used in mixing a rubber compound may have changed the chemical properties of the end product, which was used to build tires. The result: “less peel strength,” according to one of the sources. Firestone claims data has previously shown that Decatur-built tires were more prone to problems.
The briefings represented an important milestone in trying to determine what caused the tire failures. NHTSA officials said agency investigators listened with interest to the arcane details of tire construction, but will reach their own conclusions.
The Firestone tire failures have been linked to at least 148 highway deaths in the United States and more abroad. In the worst cases, the treads peeled off tires on Ford Explorers traveling at highway speeds, precipitating deadly rollover crashes.
Firestone has replaced more than 5.6 million of an estimated 6.5 million ATX, ATX II and 15-inch Wilderness tires, installed mainly on Explorers, that it recalled Aug. 9.
The problem with the tires was first recognized overseas in the hot climates of the Middle East, Southeast Asia and Latin America. Likewise, in the United States, problems emerged earlier in Texas, Arizona and Florida.
The Times reported in September that the Firestone tires Ford selected for the Explorer had a lower rating for heat resistance than the tires on virtually all sport-utility vehicles made by rival companies. Ford said at the time that Firestone was responsible for the tires’ performance in all areas, including heat resistance.
But Firestone had recommended that its tires be inflated to 30 pounds per square inch, while Ford recommended a lower inflation pressure of 26 psi to Explorer owners. The lower pressure recommended by Ford would have caused the tires to run hotter. Ford rescinded its recommendation in September and adopted the higher tire pressure Firestone called for.
“When you add it all up, it just became a bad mix,” said a source familiar with the briefings. “The 26 psi alone might not have made a difference, but all of these things come together. They made a lot of mistakes in terms of the tire for this vehicle.”
Citing Firestone, congressional spokesman Johnson said that on the heavier, late-model Explorers, the tires’ margin of safety was 23 psi. That meant a loss of three pounds of pressure--from the recommended 26 psi--could make the tires susceptible to failure.
A Ford spokesman said Monday that the company still firmly believes the failures were the result of “a tire problem, not a vehicle problem.”
