08 January 2001
Investigation of Firestone tires focusing on fatigue
by Bob Felton
Contaminated rubber and too little cover caused separations, company says.
The primary cause of the Firestone ATX-model tread separations that led to the recall of 6.5 million tires was found in a process at the Decatur, Ill., plant that allowed the rubber binding the steel belts to be contaminated with lubricants, along with too little cover over the outermost belt, according to a 19 December company report. Other causes remain under investigation, the company said.
Analyses of failure data indicate the following:
- Almost 40% of ATX-model failures are in tires manufactured at the Decatur plant, suggesting fabrication problems.
- The number of Decatur-produced ATX tires that have failed is more than 10 times greater than the number of failed Wilderness AT tires manufactured at the same plant-an incongruity that indicates the ATX design, rather than fabrication, might be the problem.
- An overwhelming majority of all the failed tires were mounted on the rear axle, and a
- majority of those tires were mounted on the driver's side of the vehicle, indicating the problem might arise from the interaction between the tire and the vehicle.
In addition to conducting an internal study, Firestone has retained Dr. Sanjay Govindjee, a materials expert and a member of the civil engineering faculty at the University of California at Berkeley, to carry out a separate, independent analysis of the failures. The following is a summary of Dr. Govindjee's investigation:
InTech: Does the data point toward design, fabrication, or tire-vehicle interaction as the primary cause of the tire failures?
Govindjee: The investigation is looking into all those possibilities and has drawn no conclusions.
InTech: Why are you focusing on fatigue as the failure mechanism?
Govindjee: That's based upon visual inspection of failed tires and interpretation of the reports about failed tires elsewhere.
InTech: There have been newspaper reports that plant workers said "stale" materials were sometimes used to manufacture tires and that inspections were often cursory. Is that so?
Govindjee: I spent a day in the Wilson, N.C., plant and more than that in the Decatur plant and didn't see anything like that.
InTech: Temperature and inflation pressure have been widely speculated on as important contributors to the tire failures. What is the role of those factors in your theory of the failures?
Govindjee: Certainly those things contribute to failures, but there is no theory yet. We're still looking at everything.
InTech: A 16 October press release from Firestone said a preliminary report would be issued in early November, and the final report would probably be issued early next year. Is that still your schedule?
Govindjee: There probably won't be a preliminary report. The final report might be delayed slightly.
Dr. Govindjee said he will issue his final report this month. He was not immediately available for comment on the 19 December Firestone report.
The company's research established that adhesion strength declines with increases of internal tire temperature, and the company continues to point to underinflation as a likely contributor to tire failures. The company also noted that 16% of all recalled tires had been repaired for punctures, though 35% of failed tires had such repairs. The company concluded: "Improper repairs that did not effectively stop air leaks made up about two-thirds of these repairs. It appears that improper repairs are a factor in tread separations." The company further noted that most "tread belt separations are caused by service-related tire damage."
Further, noting the dramatic difference between failures of Radial ATX and Wilderness AT tires produced at the Decatur plant, Firestone has speculated in press releases published on its own Web site that the likely cause of the difference lies in the tire design. They've gone on to note, however, that a clear preponderance of all failures occurs at the left rear tire, raising the possibility that some as-yet-unidentified interaction between the Ford Explorer and Firestone tire designs has played a role.
Complicating matters is the confusion over tire pressure: Firestone recommends tires be inflated to 30 pounds per square inch (psi), and Ford recommends 26 psi—a difference that, according to Firestone tests, significantly increases operating temperature and tire stresses.
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Bob Felton is technical editor of InTech.