Size 235/75R15 Firestone Wilderness AT, ATX and ATX II tread separations involved in fatal Ford Explorer roll-overs "resulted from a crack that grew in the rubber between (the tires') two belts," independent tire expert Dr. Sanjay Govindjee said in his final report today (click here for full text).
Govindjee, an associate professor of civil engineering at the University of California at Berkeley, was brought in last September by Bridgestone/Firestone (BFS) Inc. to determine why the tires had failed.
"Cracking is influenced by a number of factors, including climate, design of the tire, manufacturing differences at BFS' Decatur, Ill., plant and other factors." Overloading, excessive heat build-up due to underinflation and hot weather conditions hastened crack growth, he says. In addition, "interbelt material properties" of tires made in Decatur "were substantially different from those at other plants and were more subject to fatigue."
BFS officials are reviewing Govindjee's report, which has been sent to the National Highway Traffic Safety Administration (NHTSA) and other government agencies.
