After testing tires returned by customers, the company found that the steel belt could become exposed after being driven for several thousand miles, she said.
"The request was pretty unusual," said the manager, Dan Edwards, who in his eight years at the speedway has provided special access to people testing tires, engines and racecars.
The government tests tires for endurance by running them for 34 hours at 50 miles per hour, and for speed by running them for 90 minutes at 70 to 85 miles per hour.
On Fridays, in locations as far-flung as Qatar and Malaysia, the racers practice and experiment with their bike setups, testing tires and tinkering with things like traction control until the rider feels the bike is optimized for that particular track.
Michael Vaughn, a Ford spokesman, said that the company had tested the tires at 26 pounds per square inch by driving Explorers with the tires at 100 miles per hour for a distance of 200 miles on a test track in Arizona in 1989.
Jacques Nasser, the president and chief executive of Ford Motor, said the automaker first suspected problems with Firestone tires in January 1999, after reports of accidents, some fatal, in the Middle East and South America, but that Firestone tested the tires and said they were not at fault.
Mr. Tauzin accused Ford in an interview earlier this week of not having tested the tires at 26 pounds per square inch, the pressure at which Ford recommends that they be inflated.
The principal topics of this article are, factors contributing to the friction mechanism at the tire ice contact; experimental studies of pneumatic tires on ice and indoor and outdoor test facilities for testing tires on ice; and tire ice models that predict the tractive and braking performance on ice.
Bridgestone/Firestone Inc. tested a tire model made for two Toyota Motor vehicles after small cracks were found on the sidewalls, but both companies said today that the problems appeared to be only cosmetic.
This table allows characterization of the tire and tests the tire under varying loads at different slip and camber angles.
