Engineering Analysis Report and Initial Decision
EA00-023: Firestone Wilderness AT Tires
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The National Highway Traffic Safety Administration (NHTSA) has made an initial decision that a defect related to motor vehicle safety exists in certain P235/75R15 and P255/70R16 Firestone Wilderness AT tires manufactured before May 1998 that are installed on sport utility vehicles (SUV). This Engineering Analysis Report provides the basis for that decision.
Belt-leaving-belt tread separation failures of ATX and Wilderness AT tires manufactured by BridgestoneFirestone, Inc. (Firestone), have led to numerous crashes, injuries, and fatalities. In August 2000, Firestone determined that a safety-related defect existed in all Firestone P235/75R15 ATX tires and in Firestone Wilderness AT tires of that size manufactured at its Decatur, Illinois plant, and commenced a recall to replace those tires. Wilderness AT tires were the successor to ATX tires and are similar to them in many respects. NHTSA's Office of Defects Investigation (ODI) has conducted an extensive investigation to determine whether any other Wilderness tires contain such a defect, and whether they should be recalled as well.
The focus of ODI's investigation was on those non-recalled tires that are similar to the recalled tires; i.e., Wilderness AT tires of the size P235/75R15 and P255/70R16 manufactured by Firestone for supply to Ford Motor Company (Ford) as original equipment, as well as replacement tires manufactured to the same specifications ("focus tires"). Most of the focus tires were manufactured at Firestone's Wilson, North Carolina and Joliette, Quebec plants, beginning in 1994. In late 1998, Firestone began producing P255/70R16 Wilderness AT tires at Decatur, and in mid-1999, it began producing P235/75R15 Wilderness AT tires at a new plant in Aiken, South Carolina. Also, fewer than 100,000 P235/75R15 Wilderness AT tires were produced at Firestone's Oklahoma City, Oklahoma plant. The focus tires were predominantly used as original equipment on Ford Explorer SUVs and, to a lesser extent, on Ford Ranger compact pickup trucks, and as replacement tires for use on these and other SUVs and pickups.
ODI's investigation included, with respect to both Firestone tires and peer tires, thorough analyses of available data regarding the performance of tires in the field; shearography analysis to evaluate crack initiation and growth patterns and their severity in tires obtained from areas of the country where most of the failures have occurred; and observations, physical measurements, and chemical analyses. ODI also reviewed numerous documents and extensive test data submitted by Firestone and others.
Belt-leaving-belt tread separations, whether or not accompanied by a loss of air from the tire, reduce the ability of a driver to control the vehicle, particularly when the failure occurs on a rear tire and at high speeds. Such a loss of control can lead to a crash. The likelihood of a crash, and of injuries or fatalities from such a crash, is far greater when the tread separation occurs on a SUV than when it occurs on a pickup truck.
Tread separation claims included in the Firestone claims database involving the recalled and focus tires have been associated with numerous crashes that have led to 74 deaths and over 350 injuries (as of March 2001). Tread separation complaints from all sources included in the ODI consumer complaint database (including the Firestone claims data) that can be identified as involving these tires have reportedly led to 192 deaths and over 500 injuries (as of September 2001).
The belt-leaving-belt tread separations in the recalled and focus tires generally occur only after several years of operation. Thus, since the focus tires have not been on the road as long as the recalled ATX tires, the absolute number of failures of those tires, and the unadjusted failure rate of those tires, are less than those of comparable ATX tires. Claims in the Firestone claims database involving the focus tires have been associated with 17 deaths and 41 injuries, with additional crashes and casualties reported in the ODI complaint database, including reports of six additional fatalities. However, on a plant-by-plant basis, the focus tires manufactured at the Wilson and Joliette plants have exhibited tread separation failure trends that are similar to those experienced by the recalled ATX tires at similar service intervals.
These failure trends indicate that it is likely that, if they are not removed from service, the focus tires - at least those manufactured before May 1998 - will experience a similar increase in tread separation failures over the next few years, leading to a substantial number of future crashes, injuries, and deaths. The tread separation failure experience of the focus tires is far worse than that of their peers, especially that of the Goodyear Wrangler RT/S tires used as original equipment on many Ford Explorers.
The belt?leaving?belt tread separations that have occurred and are continuing to occur in the recalled and focus tires begin as belt?edge separation at the edge of the second, or top, belt. This is the area of highest strain in a steel belted radial tire and is a region with relatively poor cord?to?rubber adhesion because bare steel is exposed at the cut ends of the cords. Once belt?edge separations have initiated, they can grow circumferentially and laterally along the edge of the second belt and develop into cracks between the belts. If they grow large enough, they can result in catastrophic tread detachment, particularly at high speeds, when the centrifugal forces acting on the tire are greatest.
ODI conducted a non-destructive analysis of numerous randomly collected focus tires and peer tires from southern states, where most of the failures have occurred, using shearography, which can detect separations inside a tire. This shearography analysis demonstrated that the patterns and levels of cracks and separations between the belts were far more severe in the focus tires than in peer tires. Many of the focus tires that were examined were in the later stages of failure progression prior to complete separation of the upper belt. The shearography results for tires manufactured at Wilson were similar to those manufactured at Joliette.
A critical design feature used by tire manufacturers to suppress the initiation and growth of belt-edge cracks is the "belt wedge," a strip of rubber located between the two belts near the belt edges on each side of the tire. The belt wedge thickness, or gauge, in the ATX tires and the Wilderness AT tires produced prior to May 1998 is generally narrower than the wedge gauge in peer tires, and the wedge gauge in cured tires was often less than Firestone's target for this dimension. The tires with this wedge did not adequately resist the initiation and propagation of belt-edge cracks between the steel belts. During March and April 1998, Firestone changed the material composition and increased the gauge of the wedge in its Wilderness AT tires (and some other tire models).
Another important feature of radial tires related to the prevention of belt-leaving-belt separations is the gauge of the rubber between the two steel belts, or "inter-belt gauge." The inter-belt gauge initially specified by Firestone for the focus tires is generally narrower than the inter-belt gauges in peer tires and is narrower than Firestone's original specification for the ATX tires in the early 1990s. Moreover, the actual measured gauge under the tread grooves in several of the focus tires measured by ODI was far less than Firestone's minimum design specification. Since an inadequate inter-belt gauge reduces the tire's resistance to crack growth and its belt adhesion capabilities, this narrow inter-belt gauge may be partially responsible for the relatively low peel adhesion properties of the focus tires compared to peer tires. In August 1999, after becoming concerned about the adequacy of the inter-belt gauge in the cured Wilderness AT tires, especially in the regions directly under the tread grooves, Firestone changed the inter-belt gauge specification back to the original dimension.
Another relevant feature is the design of the shoulder pocket of the focus tires, which can cause higher stresses at the belt edge and lead to a narrowing, or "pinching," of the wedge gauge at the pocket. The focus tires exhibit a series of weak spots around the tire's circumference, leading to the initiation and growth of cracks earlier than in competitor tires and in other Firestone tires produced for light trucks and SUVs. In addition, many of the focus tires exhibited shoulder pocket cracking similar to that which Firestone identified as a significant contributor to the risk of tread detachment in the recalled ATX tires.
Because the tread separations at issue in this investigation occur only after several years of exposure, almost all of the failures on which ODI's analysis of field experience was based involved tires manufactured before the May 1998, when Firestone increased the dimensions and improved the material of the belt wedge. In theory, these modifications to the wedge would tend to inhibit the initiation and propagation of the belt-edge cracks that lead to tread separations. If these modifications actually improved the resistance of the focus tires to belt-edge separations, the historical failure trends described above may not predict the future performance of the newer tires. However, because tread separation failures rarely occur in the focus tires until at least three years of use, it is not now possible to ascertain from field experience whether their actual performance has improved significantly.
The rate of tread separation failures on Ranger pickups is lower that the rate of such failures on Explorers for a variety of reasons, including the fact that the Explorer generally carries higher loads and is a more demanding application, and the tires on the Explorer had a significantly lower recommended inflation pressure (especially on the rear wheels). The risk of such a separation on Rangers remains a cause for possible concern. Nevertheless, because the likelihood of a crash due to a tread separation, and of deaths and injuries resulting from such a crash, is substantially lower when the separation occurs on a pickup than on a SUV, NHTSA's initial defect decision does not apply to focus tires installed on pickup trucks.
Under the National Traffic and Motor Vehicle Safety Act, in order to compel a manufacturer to conduct a recall, NHTSA has the burden of proving that a safety-related defect exists in the manufacturer's products. The record of this investigation supports a determination that a safety-related defect exists in the focus tires manufactured by Firestone prior to its 1998 modifications to the belt wedge that are installed on SUVs. Although the agency has concerns about the possibility of future tread separations in focus tires manufactured after the wedge change, the available evidence at this time does not clearly demonstrate that a safety-related defect exists in those focus tires. NHTSA will, however, continue to closely monitor the performance of these tires.
Therefore, on the basis of the information developed during the ODI investigation, NHTSA has made an initial decision that a safety-related defect exists in Firestone Wilderness AT P235/75R15 and P255/70R16 tires manufactured to the Ford specifications prior to May 1998 that are installed on SUVs. These tires were manufactured primarily at Wilson and Joliette and, to a lesser extent, at Oklahoma City. The initial decision does not apply to the P255/70R16 tires produced at Decatur or any of the Wilderness AT tires produced at Aiken, since these tires were all manufactured after May 1998.