The Auto Channel
The Largest Independent Automotive Research Resource
Official Website of the New Car Buyer

Firestone Announces Findings of Root Cause Analysis

19 December 2000

Firestone Announces Findings of Root Cause Analysis: Increased Rate of Tread Separation Claims Caused by Combination of Factors
            - August Recall Was More than Adequate, Company Says -

    NASHVILLE, Tenn., Dec. 19 Firestone announced today the
findings of the company's intensive four-month analysis into why a certain
group of its tires, primarily on Ford Explorers, may have been more likely to
experience tread separations.  The company found that in extreme cases several
factors acting in combination could cause an increase in tread separations.
    A combination of the company's Radial ATX tread design in the P235/75R15
size, the effect of external factors relating to the tires on Ford Explorers,
including low tire inflation and the loading of the vehicle, and certain
manufacturing factors relating to Firestone's Decatur, Ill. plant were
identified as principal reasons why a small percentage of P235/75R15 ATX and
Wilderness AT tires produced at the Decatur plant experienced an increased
rate of tread separation claims.
    "Our team's findings confirm what the initial statistical claims
information demonstrated from the outset -- that a small number of tires
generated higher rates of tread separation claims when used on Ford Explorers
and that our recall initiated in August was more than adequate to protect the
public," said John Lampe, Bridgestone/Firestone President, Chairman and CEO.
"Our engineers and analysts found through their review of the data a higher
claims rate for those tires we recalled, but this same data reinforced our
belief that other Firestone tires perform at world class levels.  This
superior performance includes Firestone's line of 15" and 16" Wilderness AT
tires produced at Joliette, Quebec, Canada, and Wilson, North Carolina, as
well as the 16" Wilderness AT tires produced at Decatur.
    "We committed to our customers and the motoring public to find out why
some Firestone tires were more likely to experience tread separations on Ford
Explorers.  That has been our principal focus since we announced our recall
effort in August," said Lampe.  "Today, I can say I am satisfied that our root
cause team has conducted an exhaustive and comprehensive study and has
examined all factors that could have caused the tire problems."
    Firestone began its review and analysis nearly four months ago, bringing
together a joint team of engineers, technical experts and statisticians.  The
review involved testing tires in the company's technical and research centers,
analyzing recalled tires including failed tires, and using finite element
    In addition, the company analyzed reported claims information, tire
performance information and manufacturing information from all of its plants
in the U.S. and used two outside laboratories to provide an independent source
of analysis.  The team worked together with and separately from Ford Motor
Company specialists, sharing information and comparing results with them.
Additionally, the company brought in an independent expert, Dr. Sanjay
Govindjee, to conduct his own review, the results of which are expected later
this month.
    Firestone has been working closely with the National Highway Traffic and
Safety Administration (NHTSA) and Ford Motor Company as it conducted its
investigation and provided them with the company's findings last week.
Firestone believes that once NHTSA and Ford review the materials they will
agree with the company's findings.
    Lampe added that Firestone is certain that tread separations alone are not
the exclusive cause of serious accidents, including those that involve
rollovers.  The investigation Firestone conducted did not deal with why a
vehicle may roll over in the event of a tire failure.

    Firestone's Analysis
    In order to find the cause or causes of why some tires had higher claims
rates, Firestone reviewed data from millions of tires produced from 1991 to
2000.  The company examined more than 2,500 recalled tires, cutting apart more
than 850 tires to analyze tire components.  In addition, the root cause team
conducted the same investigation on new production Wilderness AT tires.
The cut tire inspections included looking at 40 measurement points per tire
and analyzing the following tire components:

    *  #2 Belt Width
    *  Belt Step
    *  Inter Belt Gauge
    *  Wedge Gauge/Location
    *  Buttress Gauge
    *  Base Gauge
    *  W7 Gauge (Distance from the bottom of shoulder pocket to the belt edge)

    Extensive testing was undertaken at the company's technical and research
centers in Akron, Ohio, and the company performed real world field tests on
the tires at its technical proving grounds in Fort Stockton, Texas, to
determine what unique factors in the recalled tires could result in a higher
rate of tread separation claims.
    These tests focused on three key areas:  physical properties of the tires;
materials and manufacturing processes; and vehicle and external conditions
that can affect tire performance.  Specifically, the team analyzed design
specifications changes between the P235/75R15 size tires in the ATX line and
the Wilderness AT tires of the same size and manufacturing practices among the
plants that could lead to variations in interbelt materials.  External
conditions such as vehicle load and pressure differences, ambient temperature,
low inflation and high speed were also studied.
    The results were then compared with the statistical claims data for the
recalled ATX, ATXII and Wilderness AT tires and with other tires produced by
the company to determine how the technical analysis compared to the company's
claims experience.
    The investigation and finite element modeling analysis identified a number
of individual aspects in the design and manufacturing process and individual
vehicle and external factors that could contribute to a higher percentage of
tread separations.  The investigation found that each of these individual
factors would not alone lead to the increased tread separation claims rates.
Instead, all of the following factors acting in combination appeared to
explain the increased rates:

    *  The shoulder pocket design of P235/75R15 ATX tires could lead to
cracking at the shoulder pocket bottom.  This could become the starting point
of a failure, which when combined with the other factors result in a reduction
of resistance against belt detachment.

    *  Low inflation pressure in the recalled ATX, ATXII and Wilderness AT
tires increased the running temperature of tires and would contribute to a
decreased belt adhesion level.

    *  Vehicle load levels and low standard tire pressure initially specified
for the primary vehicle for which the tires were designed led to a decreased
margin of safety for tire performance.

    *  ATX P235/75R15 tires and Wilderness AT tires of the same size produced
at Decatur exhibited different belt adhesion characteristics, including lower
initial adhesion, than those same size and line tires produced at other BFS

    "These factors the team has identified, when working in concert, are the
major causes for why some P235/75R15 ATX, ATXII and Decatur produced
Wilderness AT tires experienced an increased rate of tread separation," said
Lampe.  He emphasized that the root cause findings do not affect all of the
recalled tires.  "Tire industry experience demonstrates that many tread
separations can be caused by various forms of tire damage encountered in daily
use, such as punctures, improper repairs, severe impacts or being misapplied.
Many of the reported incidents are attributable to these factors, and not
those identified in the company's root cause investigation.  This was one of
the difficulties the team encountered in conducting our analysis."

    Recommendations, Enhancements Made Regarding P235/75R15 Tires
    "As soon as we began to understand our findings, we announced
recommendations regarding the recalled tires and implemented enhancements to
our manufacturing processes based on our ongoing root cause review," Lampe
said.  "Although we have reported the results of our analysis today, as soon
as we began to better understand the factors that might have been involved, we
made recommendations regarding the use of the tires.  We also instituted
manufacturing enhancements as part of our commitment to continuous
    At the time of the recall, Firestone recommended that the inflation
pressure on Firestone P235/75R15 tires mounted on Ford Explorers be increased.
Based upon Firestone's recommendation, Ford implemented a change in the
recommended inflation pressure from 26 pounds per square inch (psi) to 30 psi
in September.
    The P235/75R15 ATX and ATXII tires are no longer in production.  The tread
of the Wilderness AT line of tires, which replaced the ATX II as original
equipment on the Ford Explorer in 1996, is significantly different from the
ATX and ATXII tires.  The Wilderness AT was introduced with a new tread design
as part of that year's redesigned Ford Explorer.
    Although the skim stock, a rubber coating surrounding the steel belts,
from Decatur was within specifications, Firestone is implementing changes to
the processing of skim stock at Decatur so that its physical properties will
be equivalent to that of the other Firestone plants before it begins using
that material again.
    The company is changing how it will collect, analyze, and manage the data
it receives regarding tire claims and adjustments.  This will allow the
company to assimilate data and respond as soon as practicable if any anomalies
in tire performance are observed.
    "I said we would work diligently to examine all known facts that were
relevant to the recalled tires and then immediately make all changes necessary
to ensure public safety," said Lampe.  "I believe this team has done just
that.  While no company can ever make a tire that is indestructible, I can say
with overwhelming confidence that the review conducted by this group and the
resulting enhancements Firestone has made has refocused our company on
manufacturing the highest quality tires found anywhere in the world."
    In addition Lampe said, the company is implementing the findings of a
special internal Comprehensive Quality Review team, comprised of technical,
engineering and safety professionals.  This team reviewed the overall tire
manufacturing and data collection systems within BFS, focusing on production
operations, tire enhancements and quality assurance.  The team members have
completed their review and Bridgestone/Firestone is putting their
recommendations into practice.
    "We are a proud company with a long history and a tradition of customer
service and satisfaction.  The fact that some of our customers have questioned
our commitment to them and our commitment to their safety has been a soul-
wrenching experience," Lampe concluded.  "By identifying the sources of the
increased claims rate and confirming that those factors are unique to the
recalled tires, by instituting the enhancements identified during this
investigation along with improvements in how we collect and analyze tire
performance and claims data, I am convinced that Firestone will continue to
regain our customers' confidence and trust in our products."