Firestone announced on Friday the completion of Dr. Sanjay Govindjee’s analysis
and technical report detailing why the recalled ATX and Decatur-produced Wilderness
AT tyres, primarily on Ford Explorers, experienced a higher rate of tread separation
claims.
This is the company’s statement in full:
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Dr.Govindjee’s findings and analysis independently confirm that there was no
single causal factor.
"In all cases, failure rates for the recalled tyres are fractions of a
percent and thus determining a single cause for the tyre 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 tyres 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 tyre, manufacturing differences at Firestone’s Decatur
plant and usage factors."
Dr. Govindjee examined recalled tyres to determine what could cause a fatigue
crack or belt separation to grow between the two steel belt layers of the tyres.
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By GlobalDataHis 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 tyres.
For example, tyres from warmer climates showed a substantial degradation in
their material properties; further, tyres 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 tyres from extra loading were
important in pushing a fatigue crack to grow. Further, his analysis showed that
the interbelt material properties of the tyres 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 tyres 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 tyre life, Dr. Govindjee used information
provided from Firestone and its research centre and laboratory, independent
laboratories and a survey of material properties from returned tyres.
His analytical work included the assessment of tyre loading on the Ford Explorer,
analysis of heating factors – which included an analysis of tyre 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 tyres 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 tyres. 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 tyres 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 tyre 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 tyres for which this company
has been known over the past 100 years."
