NASHVILLE, Tenn. (Feb. 2)—The independent expert hired by Bridgestone/Firestone Inc. to investigate the causes of tread separation in the company´s recalled ATX and Wilderness AT tires has determined there was no single cause for the failures. Instead, Dr. Sanjay Govindjee of the University of California at Berkeley concluded that a number of factors, including climate, tire design, manufacturing differences at Bridgestone/Firestone´s Decatur, Ill., plant and usage factors, combined to cause cracking in the tires´ interbelt rubber layer. "In all cases, failure rates for the recalled tires are fractions of a percent and thus determining a single cause for the tire failures is an unrealistic expectation," Govindjee said in a prepared statement. Among specifics released in the report, Govindjee said his analysis showed that the interbelt material properties of Decatur-sourced tires were substantially different from those at other plants and were more subject to fatigue. He said he also found design differences in the Radial ATX that could lead to a higher propensity to fatigue than the Wilderness AT.