NASHVILLE, Tenn.—The outside engineer hired by Bridge-stone/Firestone Inc. to investigate the causes of belt separation in the company's recalled ATX and Wilderness AT tires has determined there was no single cause for the failures, although he cited load as the overriding mitigating factor in the failures.
The report, conducted by Sanjay Govindjee of the University of California at Berkeley and funded by Bridgestone/Firestone, did little to quell calls from plantiffs' attorneys and consumer protection advocates for a more thorough investigation of the problem and a wider recall.
A panel of tire consultants, convened by Florida personal injury attorney Bruce Kaster, criticized the study because it does not address key issues such as design and manufacturing flaws that they feel play critical roles in the belt separation problem.
Additionally, the consumer watchdog group Public Citizen and the plaintiffs' attorneys research group SafetyForum.com reiterated their call for a complete recall of all Wilderness AT tires, regardless of place of manufacture, based on adjustment rate data made public in Mr. Govindjee's report.
Mr. Govindjee, an associate professor of civil engineering, concluded that a number of factors including climate, design of the tire, manufacturing differences at BFS' 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," Mr. Govindjee said.
Among specifics released in the report, Mr. 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.
One possible source of the Decatur plant's problems was that plant's practice of pelletizing rubber after compounding instead of calendering it, he said. In order to keep the pellets from sticking to one another before being used in the extrusion process they were coated with a lubricant. Residue from this lubricant may have weakened the bonding strength of the rubber, although Mr. Govindjee said his research was not extensive enough to draw absolute conclusions.
He said he also found design differences in the Radial ATX that could lead to a higher propensity to fatigue than the Wilderness AT.
"Of the factors of inflation pressure, speed, vehicle load and belt design," Mr. Govindjee wrote, "vehicle load plays the most important role in controlling crack growth rates."
In his report, Mr. Govindjee focused on a mechanical and materials engineering analysis, examining recalled tires to determine what could cause a fatigue crack or belt separation to grow between the two steel belt layers of the tires.
To determine how these factors affect tire life, Mr. Govindjee used information provided from BFS and its research center and laboratory, independent laboratories and a survey of material properties from returned tires. His work also included an assessment of tire loading on the Ford Explorer, analysis of heating factors, and finite element modeling, BFS said.
The report also clarifies that the failure mode of the tires in question technically is a belt separation, not a tread separation. The failures occur in nearly all cases between the two steel belts and is caused by cracking of the natural rubber interbelt layer.
In responses to written questions, Mr. Govindjee said his analysis did not reveal any rubber-steel adhesion problems, as was the case in the notorious Firestone 500 case 30 years earlier. In this case, the rupture occurs within the interbelt rubber.
Mr. Govindjee's analysis is flawed, the consultants' panel concluded, because it looked only at Firestone tires and not at other Bridgestone tires or competitors' models, and he did not address manufacturing anomalies among various BFS plants.
The panel—consisting of Alan Milner, an engineering consultant in the metallurgicals field, former Firestone tire engineer Dick Baumgardner, and former Michelin Tire engineer Dennis Carlson—drew conclusions from comparing Japanese-built and U.S.-built ATX P-metric light truck tires, according to Bruce Kaster, the Ocala, Fla., attorney who is involved in dozens of lawsuits against BFS.
Among specific charges leveled at BFS were the discovery of contaminants such as solvents, wax residue and moisture in the steel belts of tires examined by the consultants.