By TIMOTHY AEPPEL
Staff Reporter of THE
WALL STREET
JOURNAL
Dec. 18, 2000
The outside expert hired by Bridgestone/Firestone Inc. to probe the company's recent tire problems said he has begun studying a series of tests the company conducted using Ford Motor Co.'s Explorer sport-utility vehicle.
"I have the data that characterize the dynamic loading that the Explorer puts on the tires," said Sanjay Govindjee, an associate professor in the Department of Civil and Environmental Engineering at the University of California, Berkeley. Mr. Govindjee said he hasn't completed analyzing the data yet, so he hasn't been able to determine whether loading factors played a role in the tire failures.
Firestone, the U.S. unit of Japan's Bridgestone Corp., recalled 6.5 million tires in August. Most of the tires were sold as original equipment on Ford Explorers.
Mr. Govindjee says the tests were conducted by Firestone on a test track and were designed to mimic the performance of the tires on the Explorer under different loading conditions. The loads ranged from the lightest possible -- with just a driver in the vehicle -- up to clearly overloaded conditions. The tires were instrumented to measure the forces applied to them.
Mr. Govindjee said he also is studying results of Firestone tests done on the tire materials themselves.
Tests of the so-called skim stock, the rubber that bonds together the steel belts inside the tires, show that skim stock in tires made at Firestone's Decatur, Ill., plant is chemically different from the material used in similar tires made elsewhere. Most of the recalled tires were produced in Decatur. "The differences are there, they exist," said Mr. Govindjee. "But it's still not clear to me whether those differences are the cause of the situation."
Mr. Govindjee said his current focus is fatigue cracks that appear along the edges of the steel belts inside the recalled tires. Such cracks often occur in tires, he noted, but don't normally spread beyond the edges of the belts. In the recent spate of tire failures, he said, the cracks appeared to grow steadily until the two steel belts came completely apart.
"Cracks are common to all tires," he said. "The question is why they keep spreading."
Mr. Govindjee hopes to complete his investigation by year end. Firestone and Ford both met separately with investigators from the National Highway Traffic Safety Administration this week to discuss the status of their probes.
Write to Timothy Aeppel at timothy.aeppel@wsj.com