NASHVILLE, Tenn., Feb. 2 /PRNewswire/ -- 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 usage
factors."
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 findings.
"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 modeling.
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 retained 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."
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