Firestone Confirms No Single Factor In Tire Failure
NASHVILLE, Tenn., Feb. 2 Firestone today announced the
completion of Dr. Sanjay Govindjee's analysis and technical report detailing
why the recalled ATX and Decatur-produced Wilderness AT tires, primarily on
Ford Explorers, experienced a higher rate of tread separation claims. Dr.
Govindjee's findings and analysis independently confirm that there was no
single causal factor.
"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," said Dr. Govindjee, an associate professor of civil
engineering at the University of California at Berkeley. "From a mechanical
and materials engineering perspective, the phenomena of belt separation in
these tires resulted from a crack that grew in the rubber between the two
belts. This cracking is influenced by a number of factors, including climate,
design of the tire, manufacturing differences at Firestone's Decatur plant and
Dr. Govindjee examined recalled tires to determine what could cause a
fatigue crack or belt separation to grow between the two steel belt layers of
the tires. His analysis, which included standard testing and new lab and
field testing, found that climate played a critical role in reducing the
material properties or capacity of the tires. For example, tires from warmer
climates showed a substantial degradation in their material properties;
further, tires that were run at high speeds, in highly loaded vehicles or at
low inflation displayed pronounced heat build-up. While the effect of
temperature on the capacity of the interbelt materials was confirmed, he also
found the demands on the tires from extra loading were important in pushing a
fatigue crack to grow. Further, his analysis showed that the interbelt
material properties of the tires in question produced in Decatur, were
substantially different from those at other plants and were more subject to
fatigue. His analysis also found design differences in the Radial ATX could
lead to a higher propensity to fatigue than the Wilderness AT.
Dr. Govindjee focused on a mechanical and materials engineering analysis
and, as a result, his report has some differences in scope and focus from
Firestone's internal investigation, released December 19, 2000. His overall
conclusions provide additional information consistent with the company's
"Firestone's technical staff is now thoroughly reviewing Dr. Govindjee's
report as part of our quality assurance and continuous improvement program,"
said John Lampe, Bridgestone/Firestone Chairman, CEO and President. "We thank
Dr. Govindjee for his hard work. I am confident that with our own internal
review, combined with Dr. Govindjee's analysis, we have determined why some of
our tires experienced a higher rate of tread separation claims and that our
recall in August was more than adequate to protect the public."
To determine how these factors affect tire life, Dr. Govindjee used
information provided from Firestone and its research center and laboratory,
independent laboratories and a survey of material properties from returned
tires. His analytical work included the assessment of tire loading on the
Ford Explorer, analysis of heating factors - which included an analysis of
tire inflation pressure, loading and speed - and extensive finite element
In addition, Dr. Govindjee used field data on Ford Explorers regarding
actual dynamic loading or forces placed on the tires by the vehicle. This
data was collected at Firestone's test track in Acuna, Mexico on a 1998 Ford
XLT Explorer four-wheel drive. Measurements were performed at four different
inflation pressures, four different cargo loads and at different speeds.
Dr. Govindjee was hired by Firestone in September to provide an
independent analysis of its recalled tires. While Firestone provided
information and data requested by Dr. Govindjee, his work was conducted
independently from the company's own internal review and analysis.
Firestone has provided Dr. Govindjee's report and findings to the National
Highway Traffic Safety Administration and other governmental groups.
"I am grateful to Dr. Govindjee for his efforts in this analysis," said
Lampe. "Finding the root cause of why a small percentage of our P235/75R15
ATX and Decatur-produced Wilderness AT tires suffered tread separations was a
priority for this company, and we know it was extremely important to our
customers. Dr. Govindjee's report is another step forward in assuring that
the difficulties we encountered last year with this one particular tire size
and type won't happen again. His report confirms and reinforces our earlier
internal findings. It assures me that our company has taken the necessary and
appropriate corrective steps to deal with this difficult situation. And it
supports my firm belief that our company today is building the world class
tires for which this company has been known over the past 100 years."
A full copy of Dr. Govindjee's report is available at http://www.firestone.com