Firestone's Request for a Defect Investigation on the Handling
of Ford Explorers After a Rear Tire Tread Separation
Kenneth N. Weinstein
Associate Administrator for Safety Assurance
Jeffrey W. Runge, M.D.


On May 31, 2001, Bridgestone/Firestone, Inc. (Firestone) submitted a “request” that NHTSA open a safety defect investigation regarding the handling and control characteristics of Ford Explorer sport utility vehicles (SUV) following a tread separation on a rear tire. The request was filed shortly after Ford Motor Company (Ford) announced on May 22, 2001 that it would provide free replacements for all Firestone Wilderness AT tires on Ford vehicles. It was part of Firestone's effort to argue that the crashes (many of which involved rollovers) that occurred in Explorers following tread separations of Firestone ATX and Wilderness AT tires were to a large extent due to the design of the Explorer rather than a defect in the tires.

Under 49 U.S.C. 30162 and 49 CFR Part 552, any person may “petition” NHTSA to open a defect investigation. Firestone purposely elected not to file such a “petition” and characterized its document as a “request.” Nevertheless, the agency promptly stated that it would analyze the issues raised by Firestone as though they had been raised in a petition. The analysis of those issues (copy attached) was primarily prepared by the Office of Defects Investigation (ODI) with input from the Office of Research and Development (both at NHTSA headquarters and at NHTSA's Vehicle Research and Test Center (VRTC) in East Liberty, Ohio), the Office of Safety Performance Standards, and independent experts on vehicle dynamics issues hired by ODI.

The Firestone request is based almost totally on test data compiled and analyzed by Dr. Dennis A. Guenther, a vehicle dynamics consultant hired by Firestone. At the time of the original request, Dr. Guenther had tested two Explorer models and two other SUVs. Firestone advised ODI that Dr. Guenther had not completed his testing and that he intended to complete that testing by the end of July 2001. In fact, however, Firestone submitted supplemental test data on August 22 and September 24, and it did not submit its completed test data until December 4, 2001. Although ODI began to review the available test data and to consider the issues raised by the Firestone request during the summer of 2001, it had to wait to complete its analysis until all of the Firestone data was received.


The Firestone request involves numerous complex issues in the field of vehicle dynamics. ODI's analysis of that request addresses those issues and contains an Appendix that provides a discussion of the most relevant terms and concepts, which will not be repeated here.

There are two primary sets of “findings” underlying the Firestone request. The first set involves the levels of “linear range understeer” exhibited by various vehicle models. Firestone asserts that (1) at least some Explorer models have a significantly lower amount of “linear range understeer” than the “other SUVs [Dr. Guenther] has evaluated;” (2) the Explorers lose much of their “understeer” when fully loaded, while the other tested SUVs do not; and (3) at least some Explorer models, unlike the other SUVs tested, lose all of their “understeer” and become “oversteer” following a tread separation on a rear tire. The second set of “findings” relates to the controllability of an “oversteer” vehicle. Firestone asserts that a vehicle that exhibits “linear range oversteer” is “extremely difficult for most drivers to control, particularly at interstate highway speeds” and that at or above “critical speed,” such a vehicle is “not controllable by any driver."

Firestone concludes, based on these “findings,” “…that the Explorer is defectively designed in that it has an inadequate margin of control, due to insufficient understeer, in the foreseeable circumstance of tread separation during normal highway driving in most load and turning circumstances.” While Firestone initially contended that this conclusion applied to almost all Explorer models, in its final December 4, 2001 submission, it limited its applicability to the “1995 model year and later 2WD Explorers tested,” noting that the “pre-1994 Explorers tested and the 4WD Explorer tested appear to have understeer characteristics closer to” the other SUV models that Dr. Guenther tested.

In evaluating these Firestone findings and conclusions, ODI reviewed Dr. Guenther's test results, as well as relevant linear range understeer data developed by VRTC in the late 1990s during its evaluation of various dynamic rollover test procedures, as well as data submitted by Ford based on tests it performed on numerous SUVs and a few other light duty vehicles. For some of these vehicles, Ford also provided understeer data with respect to vehicle performance with a rear tire whose outer tread had been removed. ODI did not perform any new testing to evaluate the Firestone request.

As explained in detail in the analysis, based upon its consideration of all of this data, ODI found that the tests conducted by Dr. Guenther were objective and performed under acceptably controlled conditions, that his measurements were made with an acceptable level of accuracy, and that his results were objectively reduced and analyzed. However, ODI also found that, due to the many sources of variability in measuring understeer, unless differences of at least 1°/g are found when comparing the measured level of understeer of different vehicle models, one cannot confidently state that the vehicles truly have different levels of understeer.

With respect to Firestone's “findings,” ODI found that Dr. Guenther's test data indicate that the model year (MY) 1995 and later 2WD Explorers have less understeer than the other Explorers he tested and than the peer SUVs he tested. Thus, Firestone's first “finding” is literally accurate. However, data reflecting tests of a more representative sample of SUVs by VRTC and Ford show that the level of understeer of these Explorer models is greater than or similar to those of many other contemporaneous SUVs.

With respect to Firestone's second “finding,” ODI concluded that the available data do not support Firestone's assertion that the Explorer loses much of its understeer when fully loaded. Moreover, the data indicate that the reductions in understeer for the Explorer models tested by Dr. Guenther are not as great as those for the other SUVs he tested, as well as those of many other SUVs tested by Ford.

With respect to Firestone's third “finding,” the data indicate that Explorers are not unique among SUVs in exhibiting linear range oversteer characteristics following a rear tire tread separation. Twenty of the 25 SUVs tested by Ford, as well as one passenger car and two minivans tested by Ford, exhibit linear range oversteer characteristics following a rear tire tread separation.

With respect to Firestone's “findings” about the controllability of vehicles with linear range oversteer, NHTSA's analysis found that such vehicles are more difficult for drivers to control than understeer vehicles. However, they are generally not impossible to control. Whether or not a driver is able to adequately control an oversteer vehicle depends not only on the level of oversteer, but also on factors such as the vehicle's speed, the complexity of the tasks facing the driver, and the driver's experience, knowledge, and alertness.

Moreover, there are other vehicle-related factors in addition to linear range oversteer that can affect controllability following a tread separation. This is reflected by the fact that a substantial number of MY 1991-1994 Explorers (which Dr. Guenther found do not become oversteer following a rear tire tread separation) were involved in loss of control crashes following rear tire tread separations of Firestone ATX or Wilderness AT tires. Conversely, Ford has presented videotapes of tests that it conducted of Explorers and other SUVs with a rear tire tread separation being driven at high speeds, in some cases significantly higher than their critical speeds, without a loss of control or stability. Moreover, data developed during the agency's defect investigation of Firestone tires demonstrate that the vast majority of tread separations, including many that occurred at highway speeds, did not result in crashes. For these reasons, ODI believes that whether a vehicle becomes oversteer in the linear range does not provide a sufficient measure for evaluating the risk of a loss of control in the event of a rear tire tread separation.

Finally, the analysis references work done by ODI during its defect investigation of Firestone tires, noting that there is no significant difference between the rate of crashes per 100 tread separation claims for Explorers and the rate for other SUVs.


Based upon the extensive analysis summarized above, ODI has decided not to grant Firestone's request that it open a defect investigation into the handling and controllability of Explorers following a tread separation on a rear tire. We concur that a vehicle that exhibits linear range oversteer is more difficult for a driver to control than an understeer vehicle. In addition, the many crashes following tread separations of tires on SUVs that are documented in the Firestone claims database and that have been reported to ODI by consumers and others demonstrate that a tread separation on such a vehicle can lead to loss of control, particularly when it is a rear tire that fails and the vehicle is being driven at high speed. However, the data does not support Firestone's contention that Explorers in general, or even MY 1995 and later 2WD Explorers in particular, are more likely to exhibit linear range oversteer characteristics following a rear tire tread separation than many of their peers. Moreover, a vehicle cannot be found to contain a safety-related defect under our statute solely because it has not been designed to preclude linear range oversteer following an unexpected, catastrophic event such as a tread separation.