By CLARE ANSBERRY
Staff Reporter of THE WALL STREET JOURNAL
Bridgestone/Firestone Inc.'s analysis of failed tires, though not final,
shows significantly more problems with one line of its recalled tires and
that failures occur most often on the left rear, indicating some
connection with the design of the vehicle.
Determining what went wrong with the tires is
crucial for Firestone in trying to assure
consumers that the problem can and will be
corrected. In August, Firestone, a unit of
Japan's Bridgestone Corp., recalled 6.5 million
tires, linked to 119 deaths in the U.S. and more than 40 overseas,
involving accidents mainly on Ford Motor Co.'s Explorer sports-utility
vehicle. Firestone has been trying to pinpoint the cause ever since,
conducting its own internal analysis as well as monitoring the work of
an
independent expert, Sanjay Govindjee, an associate professor at the
University of California, Berkeley.
"Based on the company's review to date, our technical teams believe the
performance issue with the tire ultimately will be the cumulative effect
of
tire design, tire components and the interaction of the tire with the
vehicle," said Firestone Chief Executive Officer John T. Lampe. He said
4.8 million of the 6.5 million recalled tires have been replaced.
Firestone said it has determined that claims for Wilderness tires number
one-tenth of those for equal-size ATX tires made at the company's
Decatur, Ill., plant. The Decatur plant has been responsible for a
disproportionate number of tire complaints and is also the subject of
ongoing investigation. The disparity is leading the company to examine
how the two tires differ, specifically the respective tread design and
the
bonding material between the two steel belts.
Dick Baumgardner, a tire expert who often testifies against tire makers
in lawsuits, said the treads on certain of the recalled tires are too wide
and the corners too squared off. With a wider tread, more rubber
comes in contact with the pavement, which tends to heat up the tire. If
the corners are squared off, there is more stress -- and again heat --
on the edges of the steel belts. The depth and width of the tread
grooves also affect how heat is dissipated and, thus, temperature as
well.
Heat accelerates the breakdown of the bond between the steel belts,
particularly if that bond already seems to be vulnerable to heat, as in
the case of the recalled tires. Indeed, along with examining design,
Firestone is focusing on the material between the two steel belts and
whether that material can resist the heat generated by a tire during
normal operation without losing strength. The interbelt area, which
includes the steel cords that are made into two belts coated with rubber
and a strip of rubber between the edges of the two belts, is critical
because that is where tread separations tend to occur.
"That particular layer between two belts seems to almost be explosive in
terms of its breaking down," says Mr. Baumgardner. "We don't have the
same breakdown around the tires, but just in the one thin layer
between the two belts."
Firestone has also determined that the vast majority of claims on the
recalled tires involve a left rear tire, leading to questions about how
a
vehicle's design puts more load or stress on that particular tire. In
testing on Explorers, Firestone's Greer Tidwell said that even when
sitting still, the left rear tire on the Explorer experienced a greater
weight load. The load on the tires can affect the temperature of the
materials inside the tire, which Firestone measured with an internal
thermometer.
During those same tests, the company found that lower tire inflation
generated excessive heat. The issue of tire inflation has been raised
repeatedly since the recall, especially since Ford had recommended that
drivers inflate their tires to 26 pounds a square inch, which Firestone
believed didn't provide enough of a safety margin. Firestone has said
the tires should be inflated to 30 pounds a square inch, which Ford is
now recommending.
Ford spokesman Jon Harmon said left rear tires have been involved in
tread separations on all vehicles and not just the Explorer. "That's a
consistent pattern and shows nothing unusual about the Explorer." He
said Ford hasn't completed its own analysis. "We're not really ready to
get into what we have found until we have the complete analysis done,"
he said.
Write to Clare Ansberry at clare.ansberry@wsj.com