By Bruce Davis, Tire Business staff©
AKRON (Dec. 11, 2000) -- Months after the initial investigation of the Firestone
ATX and Wilderness AT P-metric light truck tires, independent experts continue
to disagree over the root cause of the problem.
Some tire failure analysis experts believe the problem is a design defect. But others are just as convinced the problem is the result primarily of operating conditions--including underinflation, overloading and high-speed driving--under the control of the driver.
Those claiming a design defect speak of an insufficient safety margin. Tires typically are designed to bear loads far beyond those for which they are intended. In the case of the Firestone ATX and Wilderness AT on the Ford Explorer, the recommended 26 psi operating pressure left the tire with too narrow of a safety margin when run underinflated, overloaded or both, these experts say.
One aspect of the tire that both sides point to is that Bridgestone/Firestone Inc. inadvertently may have built the tire with a tread that was too robust. At a 14/32-inch tread depth when new, the tires were designed as suitable for all-terrain use. In reality, though, the vast majority of owners use their sport-utility vehicles on the road nearly all the time, subjecting them mostly to high-speed running.
When new, the deep tread depth is a potential source of extra heat, especially in warmer climates, according to William "Max" Nonnamaker, president, owner and founder of Nonnamaker & Associates Inc., a tire forensics firm in Akron.
At the other end of the scale, the robust tread lasted too long, leaving the tire in service beyond what should have been its normal expected lifespan, some experts said. Tread life of 65,000 miles is not uncommon in this tire, and at least one case in litigation in Florida involves a tire with more than 85,000 miles on it, according to dealers and analysts. In the event such a tire was run even slightly underinflated, or slightly overloaded, the tire´s safety margin was eroded even more.
One person convinced of a design defect is Dick Baumgardner, a plaintiffs´ expert witness and former Firestone Tire & Rubber Co. employee. He bases his opinion of what he calls the "Firestone disease" on his examination of 85 failed Firestone ATX and Wilderness AT tires.
"I´m convinced this is a design defect because all the failures occur in the same manner," he said. "The tread separation initiates at the belt edge of the top belt, migrates into the junction of the skim coat and the belt edge wedge. The root cause is a heat reversion over time. The rubber between the belts fatigues, and a separation follows."
Mr. Baumgardner´s assessment bears some similarity to the preliminary investigation summary issued by the independent expert hired by Bridgestone/Firestone to examine the tires and their manufacture to determine what the cause--if any--of the failures could be.
"All evidence to date points to a slowly developing fatigue crack that propagates through the belt wedge material and then subsequently into the belt skim between the steel belts," Sanjay Govindjee, the University of California-Berkeley associate professor of civil engineering hired to conduct the investigation, wrote in a letter to BFS. "At some stage, the cracks reach a critical size and the tires subsequently fail."
On the other hand, Charles Gold, a Texas-based consultant who testifies almost exclusively in defense of tire and car companies, said Mr. Govindjee´s statements depict the classic case of a tire that failed after running underinflated for an extended period of time.
Mr. Gold said nearly every failed tire he´s examined showed signs of unrepaired or poorly repaired punctures, or clear evidence of having been run underinflated.
Mr. Nonnamaker concurs with Mr. Baumgardner´s assessment that the tire wasn´t built robust enough for its intended use. But he also believes BFS had some problems with dry or improperly compounded skim stock rubber--the rubber calendered around the steel cords that make up the tire belts. Mr. Nonnamaker, who worked for Firestone in the 1950s, Mohawk Rubber Co. in the ´60s and Smithers Scientific Services Inc. in the ´70s, said he has seen signs of adhesion problems between the steel belts in the Wilderness AT tires he´s examined.
Bridgestone/Firestone has been accused of having workers at its plants use calendered materials that have aged several days. If this was the case, Mr. Nonnamaker said, the surfaces of the belts could have dried out, and if such materials were used they would have lacked the tack necessary to create the kind of bond that would crosslink properly during vulcanization.
Skilled workers can, to an extent, overcome this problem by using solvents to refresh the surfaces, Mr. Nonnamaker said. But the surface solvents must then be dried completely before the belts are assembled or bonding problems can result.
Mr. Govindjee also wrote that the problem is a "quite complex interaction of the effects of tire design (geometric layout and material selection), the manufacturability of the tire (geometric and material variations), and loading conditions (dynamic loads associated with the Ford Explorer, running inflation pressures and temperatures)."
Mr. Govindjee´s preliminary report also spoke of measurable differences in failure rates between the ATX and Wilderness AT designs, and singled out the Decatur, Ill., plant as the source of most of the questionable ATX tires.
Mr. Nonnamaker agreed his findings also point to a problem at Decatur. Mr. Baumgardner called the Decatur connection a red herring, and that tires from other plants were equally prone to failure.