Firestone's long-awaited announcement delineating the root cause of tread separation on its Wilderness AT and ATX I and II tires used on the Ford Explorer might have been titled, "We told you so."
The results of the company's findings revealed little more than what had been leaked out during the last two months: A combination of the tread design, manufacturing defects and external factors resulted in tread separation. The external factors were, according to Firestone, vehicles that were improperly loaded with cargo and the tire pressure recommended by Ford Motor Co.
Some 148 deaths are now attributed to the tread separations, according to the company.
Firestone submitted the results of inquiry to the National Highway Traffic Safety Administration and Ford, saying it was confident that after each had time to review the report, that they would both concur with the findings.
In addition to submitting the findings for review, the company restated its beliefs that it acted appropriately and effectively in recalling the tires involved.
"Our team's findings confirm what the initial statistical claims information demonstrated from the outset: That a small number of tires generated higher rates of separation claims when used on Ford Explorers and that our recall initiated in August was more than adequate to protect the public," said John Lampe, Bridgestone/Firestone chairman and CEO.
In addition to its own internal study, the company contracted an outside investigator, Dr. Sanjay Govindjee of the University of California-Berkeley, to conduct an independent investigation. Govindjee's results are expected by the end of this month.
The company outlined the four primary factors that combined to cause problems with the tires:
· The shoulder pocket design of the tires could lead to cracking at the shoulder pocket bottom. This could become the starting point of a failure;
· Low inflation pressure in the recalled tires increased the running temperature of the tires;
· Vehicle load levels and low standard tire pressure initially specified for the primary vehicle for which the tires were designed led to a decreased margin of safety for tire performance
· Tires produced at the Decatur, Ill., plant exhibited different belt adhesion characteristics, including initial adhesion, than those same size and line tires produced at other plants.
According to Firestone officials, it has taken steps to correct some of the problems. It has instituted a change in the manufacturing processes at the Decatur plant.
Additionally, it said Ford has changed its recommended tire from 26 pounds per square inch to 30 psi. Ford released a statement suggesting it was reaching similar conclusions about the tires, but not accepting any role regarding the impact the recommended tire pressure or design of the Explorer may have had in the failure of the tires.