NASHVILLE, Tenn.—An internal probe by experts at Bridgestone/Firestone Inc. has concluded there was no single factor that caused the abnormal rate of tread separations in the millions of P235/75R15 Firestone Radial ATX, ATX II and Wilderness AT tires the company voluntarily recalled.
"Instead, the company has concluded that a combination of design factors, external factors such as low inflation pressure, and certain manufacturing factors at the company's Decatur, Ill., plant—in extreme cases and working together—have contributed to the phenomenon," BFS said in a report issued Dec. 19.
About 70 percent of the recalled tires were original equipment on Ford Motor Co.'s Explorer sport-utility vehicle and other light trucks.
The National Highway Traffic Safety Administration (NHTSA) said it is investigating hundreds of complaints of tread separations, including allegations of 148 deaths and more than 525 injuries.
BFS and Ford also are defendants in dozens of product liability lawsuits resulting from crashes involving Ford Explorers shod with Firestone tires.
In the report, BFS said it examined 2,500 recalled tires and cut apart 850 of them for further analysis.
Tire inspectors at the company's technical and research facilities in Akron examined the recalled tires at 40 different "measurement points" and performed "real world field tests" on them at the BFS technical proving grounds in Fort Stockton, Texas, the report stated.
The investigation focused on the physical properties of the tires' materials and manufacturing processes, as well as vehicle and external conditions that can affect tire performance.
The report said the increased rate of tread separations was traceable to the following factors:
A shoulder pocket design problem on ATX tires that could lead to cracking at the shoulder pocket bottom and "become the starting point of a failure;"
Inadequate inflation pressure leading to high internal tire temperatures that decreased the adhesion level between the belts;
Rubber processing techniques unique to the company's Decatur plant that resulted in a lower level of belt adhesion than in tires of the same size and lines manufactured in other BFS plants; and
Ford Motor Co.'s recommended air pressure for the Ford Explorer leading to "a decreased margin of safety for tire performance," i.e. tire overloading.
The BFS report also noted that punctures, impact damage and improper repairs also contributed to tread separations and were observed in 35 percent of the tires returned for tread separation claims.
At the time of the recall, BFS recommended that drivers increase the air pressure of P235/75R15 Firestone tires on Ford Explorers to 30 psi from Ford's recommended 26 psi.
BFS said ATX and ATX II tires no longer are being produced and the Wilderness AT was introduced with a new tread design for last year's redesigned Ford Explorer.
The report said rubber skim stock, used to coat the tire's steel cords, was mixed differently at Decatur than at other BFS plants. It contained "more inorganic material" which—although it was within Firestone specifications—made Decatur-produced skim stock weaker than that produced at other plants.
BFS said it is changing the skim stock production process at Decatur to that used at other plants.
The report team used two outside laboratories to produce independent results and also consulted with experts from Ford, BFS said.
"I can say that I am satisfied that our root cause team has conducted an exhaustive and comprehensive study and has examined all factors that could have caused the tire problems," said John Lampe, BFS president, chairman and CEO, in a statement.
Tread separations were not the exclusive cause of the serious accidents, Mr. Lampe added, and the investigation did not deal with "why a vehicle may roll over in the event of a tire failure."
Mr. Lampe said the report confirms "that our recall, initiated in August, was more than adequate to protect the public."
In a statement, also issued Dec. 19, Ford said: "Our ongoing statistical analysis and test data seem in agreement with many of the preliminary conclusions of Bridgestone/ Firestone, as we understand them."
However, the auto maker took issue with BFS' contention that the weight of the Explorer—which Ford noted was well within the weight range for the tires—was one cause of the problems.
"Our experience with more than 2.9 million competitive make tires that performed almost flawlessly in the field leads us to conclude that the vehicle is not a significant contributor to tread separation," Ford said.
Alex Barnett, an attorney for the Washington, D.C.-based law firm Cohen, Milstein, Hausfeld and Toll P.L.L.C., said the tire industry should be well aware that drivers don't check their air pressure frequently enough, and "there should be a greater margin of safety" built into tires.
Cohen Milstein is involved in the consolidated legal proceedings against BFS and Ford in a federal district court in Indianapolis.
"They (BFS) appear to be laying the blame at the feet of consumers for not maintaining tires properly," Mr. Barnett said. "The motoring public should be leery of these types of reports issued by companies."
Mr. Barnett said the report didn't "represent the full extent of the problem." There also needs to be a more extensive recall and more independent testing, he said.
Sanjay Govindjee, a professor of engineering at the University of California at Berkeley who was hired by BFS to analyze the tread separation problem, was scheduled to issue his report by Jan 1.
On Dec. 19, BFS also announced a series of actions based on recommendations of a specially appointed review team comprising company technical, engineering and safety specialists.
Beginning in January, a quality subcommittee composed of the top managers from the sales technical service and the quality assurance departments will meet regularly and report directly to Mr. Lampe, the company said.
The group will gather, review and analyze quality data. Meanwhile, BFS will establish an enhanced safety "early warning" system derived from internal adjustment and claims data and public data from NHTSA.
BFS also will set up a system to facilitate quick detection of anomalies in each type of data and establish a quick response structure.
BFS said it is tracking original equipment fitments for exported vehicles and is creating a global quality information network to develop a system for early problem detection.
Tire development specialists also are establishing global standards for production processes and systems, so that enhancements and improvements can be implemented and maintained throughout the Bridgestone Group worldwide, BFS said.
The company said it is implementing changes in a number of aspects of tire construction and design, giving top priority to sport-utility-vehicle and light-truck tires, which are subject to heavy loads and are increasingly used at higher speeds.
BFS said it will continue to upgrade production facilities and processes, and has earmarked approximately $50 million in additional capital outlays in 2001 for further quality improvements and changes in product specifications.