Analysis of a Request for a Defect Investigation Submitted by Bridgestone/Firestone, Inc.

I. Introduction:

On May 31, 2001, Bridgestone/Firestone, Inc. (Firestone) submitted a request to the National Highway Traffic Safety Administration (NHTSA) to initiate a safety defect investigation regarding the handling and control characteristics of Ford Explorer[1] sport utility vehicles (SUVs) following a tread separation of a rear tire. Although Firestone’s letter to NHTSA was not in the form of a formal petition for defect investigation, the agency is evaluating the request as if it were a petition for a defect investigation.

Specifically, Firestone alleges that “…certain of the Explorer models will experience an ‘oversteer’ condition in most circumstances following a tread separation of a left rear tire, an event that is clearly foreseeable” and that “…Explorers as tested are defectively designed in that they have an inadequate margin of control (due to insufficient understeer) to permit control by average drivers in the foreseeable events of tread separation during normal highway driving in most load conditions and turning circumstances.” The amount of “understeer” of a vehicle is characterized by its understeer gradient, and a brief description of understeer gradient appears later in this document. Firestone also contends that this “inadequate margin of control…can make the Explorer’s handling imprecise and unpredictable in foreseeable circumstances, such as tread separation, where precise and predictable handling is essential to safe vehicle control.”

With its request, Firestone submitted a document titled, “Firestone Statement concerning Dr. Dennis A. Guenther’s Engineering Analysis of Ford Explorer Margin of Control in the Event of Belt/Tread Separation” (Firestone Statement), which refers to a study conducted by Dr. Dennis A. Guenther of FTI/SEA Consulting. The study included dynamic tests of four 4-door, rear two-wheel drive (4X2) SUVs: two models of Ford Explorer and two peer SUVs.

In a meeting with NHTSA staff on June 15, 2001, Dr. Guenther indicated that additional tests of previously tested vehicles and tests of additional peer SUVs were planned and that complete results and analyses of all testing would be made available to NHTSA's Office of Defects Investigation (ODI) by the end of July 2001. However, additional information was not submitted until August 22, 2001, when Firestone submitted a document titled, “Engineering Evaluation of Explorer Directional Control (Updated 8/16/01)” (Firestone Update), authored by Dr. Guenther. The Firestone Update includes results of testing of a two additional models of Ford Explorer and of additional tests of one of the Ford Explorers tested in the earlier program, but does not report results for any additional peer SUVs. It also included various observations related to the circumstances under which Explorer crashes occurred. In making these observations Dr. Guenther “examined several crashes involving Explorers and reviewed police accident reports relating to Explorer accidents.”

On September 24, 2001, Firestone submitted a document titled, “Supplement to Engineering Evaluation of Explorer Directional Control” (Firestone Supplement), which includes results of testing of two additional models of Ford Explorers and of one additional peer SUV. The Firestone Supplement does not include any discussion of these new test results, but states “…the test results from one additional vehicle are being analyzed at this time and will be included in a final report for this initial evaluation program.”

In a meeting with NHTSA staff on December 4, 2001, Firestone submitted a final version of Dr. Guenther’s report titled “Engineering Evaluation of Explorer Directional Control,” dated November 30, 2001 (Firestone Final Report). This document included the results of all of the testing previously submitted to ODI, included the results of tests of one additional peer SUV, and revised some of the conclusions and observations in the August 16, 2001 submission. At the meeting, Firestone also presented additional information to NHTSA (Firestone Presentation). This presentation included a video tape showing a demonstration of 40 mph "step steer" tests of three SUVs (including one Explorer), a review of five fatal crashes involving MY 1995 and later Explorers that experienced rear tire tread separations, a list of crashes of Explorers equipped with non-Firestone tires that experienced crashes following rear tire tread separations, a discussion of various causes of tire tread separations, statements related to the "undesirable nature" of vehicles that exhibit oversteer characteristics, and a statistical analysis of data from NHTSA’s Fatality Analysis Reporting System (FARS) comparing the fatal crash experience of various drivetrain and body style Ford Explorers to that of other SUVs.


II. The Firestone Statement:

The Firestone Statement presents the "preliminary" results of dynamic testing of certain SUVs that was conducted by Dr. Guenther at the request of Firestone’s counsel in connection with “…the preparation of Firestone’s defense in personal injury litigation…” related to crashes involving Explorers. A “Statement of Dr. Dennis A. Guenther,” which accompanied the Firestone Statement, indicates that Firestone’s counsel requested that Dr. Guenther “…carry out an investigation of the directional control of the Ford Explorer following a tire tread separation” that included accident reconstruction, review of police accident reports, and vehicle measurements. The Firestone Statement includes information related to the dynamic testing of two MYs of 4-door, 4X2 Ford Explorers, a MY 1996 and a MY 2000, and two peer 4-door, 4X2 SUVs, a MY 1996 Chevrolet Blazer, and a MY 2001 Jeep Cherokee. Also, the MY 1996 Ford Explorer was tested with both Firestone and Goodyear original equipment manufacturer (OEM) tires fitted to the vehicle.

The Firestone Statement only includes the results of a portion of the dynamic testing conducted by Dr. Guenther, the “constant radius circle” tests to measure the understeer-oversteer characteristics of the vehicles tested. (The understeer gradient results from these tests, as well as those for the other vehicles tested by Dr. Guenther, are summarized in Table 1.)

    The other tests performed, for which results were not included in the Firestone Statement, are:
  1. “step steer” tests to measure vehicle response times as related to lateral acceleration and yaw velocity response and to measure the gain of these responses in relation to steering wheel input; and
  2. “frequency response” tests to measure lateral acceleration gain, yaw velocity response gain and the phase angles at the frequencies tested (up to 3 to 4 hertz (hz), cycles per second).

The following are the descriptions of these three types of tests included in the Firestone Final Report:

  1. "Constant Radius Circle – The vehicle is driven around a 200-foot constant radius circle with very slowly increasing speed beginning at 0.0 mph. The driver adjusts the steering angle (by turning the steering wheel) as necessary to keep the vehicle on the path of the circle. Test runs were done in both directions, clockwise and counter-clockwise…";
  2. "Step Steer – The vehicle is driven on the test pad area in a straight line at a constant speed. The driver then rapidly turns the steering wheel until it hits a mechanical stop. Steering wheel stops are set to attain a desired lateral acceleration at the test speeds. This steer angle is held until steady-state response is established. Tests were run in both directions (right turn/left turn) and at two speeds (60 mph and 40 mph)…"; and
  3. "Frequency Response – Sinusoidal sweep steering tests are frequently used to determine the linear response of vehicles. The vehicle is driven on a long straightaway with the driver steering with slowly increasing frequency up to approximately 3 to 4 hz followed by decreasing frequency. The test was run at a nominal speed of 66 mph."

The “step steer” and “constant radius circle” tests were performed both with “four good tires” and with “the left rear tire detreaded.” The “frequency response” tests were only performed in the “four good tires” condition.

All of the tests were conducted in the "lightly loaded" condition (vehicle curb weight plus the driver and necessary instrumentation), and all but the “frequency response” tests were also conducted in the gross vehicle weight rating (GVWR) load condition. In the June 15, 2001 meeting with NHTSA, Dr. Guenther stated that the GVWR load condition is one in which the total vehicle weight is equal to the vehicle’s GVWR and front and rear axle loads are close to being the same percentage of their respective gross axle weight ratings. Dr. Guenther also stated that the "detreaded" tires were supplied to him by Firestone and that it was his understanding that the tread and "outer belt" were removed by slicing into the shoulder area of the tire and then carefully cutting and peeling the tread and "outer belt" from the rest of the tire.

Based on Dr. Guenther’s analysis of the results included in the Firestone Statement, Firestone made the following “findings” (quoted from pages 1 and 2 of the Firestone Statement):

  1. the Explorer models that he tested, as designed, have a significantly lower amount of understeer than the other SUVs he has evaluated, less than half as much as the Jeep Cherokee and Chevrolet Blazer;
  2. the Explorer loses much of its small margin of understeer when it is loaded to gross vehicle weight rating; the Cherokee and the Blazer do not;
  3. the Explorer models tested, unlike the Cherokee and the Blazer, lose all of their understeer and become oversteer vehicles in most circumstances following tread separation on a left rear tire, the predominant tire position in Explorer tread separation crashes; the only exception in Dr. Guenther’s investigation is a light load configuration in a counter-clockwise turn, with the separated tire mounted on the left rear, a circumstance where the vehicle retains a very small amount of understeer; and
  4. an oversteer vehicle is extremely difficult for most drivers to control, particularly at interstate highway speeds where it can become directionally unstable.

The Firestone Statement contends, 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.”


III. The Firestone Update:

The Firestone Update includes the results of testing for two additional models of Explorer, a MY 1999 2-door, 4X2 Ford Explorer, and a MY 1997 4-door, four-wheel drive (4X4) Ford Explorer under the same test conditions used in the tests for which results were included in the Firestone Statement. It also includes the results of two additional tests of the MY 1996 Ford Explorer tested in the earlier program. One of these additional tests involved equipping the MY 1996 Ford Explorer with a non-OEM “Ford recommended replacement” Goodyear tire, and the other test condition involved conducting tests with the “detreaded” tire being fitted to the right rear wheel of the vehicle rather than the left rear wheel. It also includes the results for the “step steer” and “frequency response” tests for the vehicles for which the understeer gradient results from the “constant radius circle” tests were included in the Firestone Statement.

The Firestone Update includes additional “conclusions.” One of these is of primary importance since it effectively revises the fourth "findings" included in the Firestone Statement. That "finding" states that "an oversteer vehicle is extremely difficult for most drivers to control, particularly at interstate highway speeds where it can become directionally unstable" and is based on a statement in the "Conclusion" section of the Firestone Statement. That is that "[o]versteer can make a vehicle directionally instable and subject to loss of control in the hands of most drivers." The Firestone Update includes an additional sentence in its "Conclusion" section that states that "[a]t or above critical speed the Explorers are not controllable by any driver." This is a much stronger statement regarding the safety consequences of a rear tire tread separations on Explorers.

With respect to the results of the “step steer” and “frequency response” tests, the Firestone Update concludes (1) that the step steer test results show that the Explorers exhibit longer response times and high yaw velocity gains with a “detreaded” rear tire when compared to the other SUVs tested; and (2) that Explorers exhibit a second “peak” in their roll angle frequency response gain characteristics that is not found in the other SUVs that were tested and that “[g]enerally such large frequency responses are considered undesirable.”


IV. The Firestone Supplement:

The Firestone Supplement includes the results of tests of three additional vehicles: two MY 1993 Ford Explorers and a MY 1996 Jeep Grand Cherokee. All of these tests were performed in the same test conditions used in the tests for which results were previously submitted to ODI. The information submitted with this document on these additional test vehicles did not include information on the body style (2-door vs. 4-door), drivetrain configuration (4X2 vs. 4X4), or the tires fitted on the vehicles during the tests. Information related to the body style, drivetrain configuration, and tires used during testing of these vehicles is included in the Firestone Final Report.

The Firestone Supplement does not include any additional “findings” and “conclusions,” or, for that matter, any discussion of the test results included in it.


V. The Firestone Final Report:

The Firestone Final Report includes the test results of all the vehicles that Dr. Guenther tested as part of his work for Firestone, including the results of tests of a MY 1992 Nissan Pathfinder. This document revises several of the observations and conclusions included in the Firestone Update. Of most significance, the conclusions drawn by Dr. Guenther in the Firestone Update as they relate to the particular models of Explorers that exhibit "undesirable" performance are revised. The conclusions in the Firestone Update related to the understeer gradients, frequency response characteristics, and gain and response times apply to the “Explorers tested (except the 1997 4 wheel drive).” However, in the Firestone Final Report, they are applied only to the “1995 model year and later 2WD Explorers tested.” The Final Report states that the “pre-1994 Explorers tested and the 4WD Explorer tested appear to have understeer characteristics closer to the Blazer, Grand Cherokee and the Cherokee.” The report further states that, as regards the understeer characteristics of the vehicles tested, the “1992 Pathfinder tested appears to fall between these groups.”


VI. The Firestone Presentation:

The entire Firestone Presentation has been placed in the docket (public file) for this proceeding. Some of the items presented at the December 4, 2001 meeting with Firestone and Dr. Guenther are discussed below.

A video tape included in the presentation shows 40 mph step steer tests being conducted with a MY 1996 Chevrolet Blazer, a MY 1996 Ford Explorer, and a MY 2001 Jeep Cherokee, all of which are 4X2, 4-door vehicles. In each case, the vehicles were fitted with a "detreaded" left rear tire, and tests were performed in a right turn. The video shows that for similar steering wheel inputs, the Blazer and Cherokee both exhibited stable responses. However, the Explorer's response was unstable, with the vehicle spinning out of control.

In presenting the list of 16 crashes of Explorers equipped with non-Firestone tires that experienced rear tire tread separations, Firestone argued that the performance of Explorers under circumstances of a rear tire tread separation is not dependent upon the brand of tire involved.

In discussing the various causes of tire tread separations, Firestone stated that beyond issues related to the design and manufacture of tires, such as those that resulted in the recalls of Firestone ATX and Wilderness AT tires, there are factors related to the use of tires that can result in tread separation, and that “tread separations will continue” to occur even in the absence of design and manufacturing faults in tires. These factors include punctures, improper repairs, overloading, underinflation, and impact damage.

In its statistical analysis of FARS data, Firestone compared the rates of fatal crashes per 100,000 registered vehicle-years for Explorers to those of the other SUVs tested by Dr. Guenther for various crash categories and for different combinations of drivetrain and body-style.


VII. ODI’s Analysis of the Firestone Request:

To assist those who are not familiar with the basic concept of dynamic systems and automotive vehicle dynamics, ODI has prepared an appendix to this document entitled, "Discussion of Some Basic Concepts of Dynamic Systems and Vehicle Dynamics."

A. Analysis of Test Results Submitted in Support of Firestone’s Request:

The Firestone Statement lists four “findings” (listed above) upon which it based its conclusion that the Explorer is “defectively designed.” These purport to be based on the results of the tests conducted by Dr. Guenther that were included in the Firestone Statement. ODI believes that the vehicle performance measures that resulted from these tests are among the appropriate measures that could be used to examine the influence of a vehicle’s control and stability characteristics on the likelihood of a driver’s losing control of a vehicle resulting in a crash following a rear tire tread separation. However, to evaluate the Firestone findings that are based on these test results, it is necessary to examine the data reported by Dr. Guenther from two perspectives. The first is whether the reported results accurately characterize the actual response characteristics of the vehicles tested. The second is whether the sample of vehicles tested is adequately diverse and representative of the SUVs being driven on U.S. highways.

1. Accuracy of the Reported Test Results:

ODI has examined whether the tests were objective and performed under acceptably controlled conditions, whether the measurements were made with an acceptable level of accuracy, and whether the results were objectively reduced and analyzed.

ODI performed this evaluation only for the “constant radius circle” tests from which the understeer gradients were determined. A similar evaluation for the other tests performed by Dr. Guenther was not performed, primarily because the results of the “step steer” and frequency response” tests were not primary components of any of Firestone’s findings or contentions.

To evaluate the results from the “constant radius circle” tests, ODI compared the results of Dr. Guenther’s tests to the results of comparable tests performed by others and independently analyzed Dr. Guenther’s raw test data.

There were two other sources of relevant understeer gradient data. First, recent and ongoing research at NHTSA’s Vehicle Research and Test Center (VRTC) evaluating various dynamic rollover test procedures included vehicle characterization tests, one of which measured understeer characteristics. VRTC tested seven SUVs, including a MY 1998 4-door, 4X4 Ford Explorer. VRTC used a different test procedure, a constant speed, slowly increasing steering wheel angle test, than that used by Dr. Guenther, and VRTC did not test any vehicles with a rear tire tread separation.

Second, Ford Motor Company (Ford) derived understeer gradients from tests performed on a variety of SUVs, including various model year, drivetrain, and body style Explorers, as well as a few other light duty vehicles. For some of these makes and models, Ford also provided understeer gradients of the vehicles with a rear tire tread separation.

The lightly loaded load condition used in both VRTC’s and Ford’s tests was the same as that used in Dr. Guenther’s testing. However, the GVWR test condition used by Dr. Guenther was different from that used by Ford and VRTC. Ford and VRTC used a loading procedure that results in the total vehicle weight being equal to the vehicle’s GVWR and its rear axle load being equal to the vehicle’s rear axle’s gross axle weight rating. Due to the more rearward center of gravity location that would result, the GVWR load condition used by Ford and VRTC would tend to result in slightly lower understeer gradients than the loading used by Dr. Guenther.

Table 2 includes the understeer gradients reported by Dr. Guenther for vehicles in the lightly loaded condition without a rear tire tread separation along with understeer gradients for comparable vehicles with intact tires that were tested by VRTC and Ford. Table 3A includes the understeer gradients in the lightly loaded condition for all vehicle models for which test results are available from more than one source and duplicates some of the results presented in Table 2. Table 3B includes the understeer gradients in the lightly loaded condition for vehicles for which results were only available from one source.

a. Objectivity of Procedures Used by Dr. Guenther:

Based on the information provided by Firestone with its request and made available to ODI during the June 15, 2001 meeting, the tests performed by Dr. Guenther used a procedure recognized by both U.S. and international automotive engineering standards organizations to measure a vehicle’s understeer/oversteer characteristics and with test conditions appropriate and adequately controlled for those procedures.

b. Accuracy of Measurements Made by Dr. Guenther:

c. Objectivity of Data Reduction and Analysis Used by Dr. Guenther:

No precise information on the data reduction techniques used in Dr. Guenther’s analyses to determine the understeer gradient from each individual test run was provided to ODI. However, since Firestone provided ODI with the raw data from the original tests performed by Dr. Guenther and reported in the Firestone Statement, VRTC was able to compare his results with those that result from using the data reduction techniques normally used by VRTC for analyzing similar test data.

Using the results of VRTC's reduction of Dr. Guenther "raw" data, ODI separately analyzed the results of the three clockwise tests and the three counter-clockwise tests.

There were 40 “sets” of test runs performed in Dr. Guenther’s original test program, five test vehicles, two load conditions (lightly loaded and GVWR), two tire conditions (“4 normal tires” and “left rear tire detreaded”), and two steering directions (clockwise and counter-clockwise). For those 40 sets of test runs, 34 of the understeer gradient values reported by Dr. Guenther were lower than those that resulted from VRTC’s analysis, and 6 of the values were higher. In 10 of the 34 cases where Dr. Guenther’s values were lower than those calculated by VRTC, the value reported by Dr. Guenther was outside the 95% confidence limits calculated by the VRTC analysis. In none of the 6 cases where Dr. Guenther’s values were higher than those calculated by VRTC were the values reported by Dr. Guenther outside the 95% confidence limits calculated by the VRTC analysis.

Table 4A includes the understeer gradient values reported by Dr. Guenther for all five of these vehicles for the “4 normal tires” condition for both the clockwise and counter-clockwise tests and for both the lightly loaded and GVWR condition. Table 4A also includes those understeer gradients determined in VRTC’s analysis of Dr. Guenther’s test data. A review of the data in Table 4A shows that 18 of the 20 values reported by Dr. Guenther are lower than those determined from VRTC’s analysis, and two are higher. In six of the 18 cases where Dr. Guenther’s values were lower than those calculated from VRTC values, the values reported by Dr. Guenther were outside the 95% confidence limits calculated by the VRTC analysis. In the two cases where Dr. Guenther’s values were higher than that calculated from the VRTC values, Dr. Guenther’s values were not outside the 95% confidence limits calculated by the VRTC analysis.

Ford also analyzed the raw test data from Dr. Guenther’s tests for the MY 1996 Chevrolet Blazer and the MY 2001 Jeep Cherokee, and provided ODI with the average of the understeer gradient values for the clockwise and counter-clockwise directions for the “4 normal tires” conditions. In both cases, the average understeer gradient values resulting from Ford’s analysis were lower than those reported by both Dr. Guenther and by VRTC, and in the case of the Cherokee, the Ford value fell outside the 95% confidence limits calculated by VRTC.

ODI attempted to ascertain whether the understeer gradient values determined from any one of the techniques would always be higher or lower than those determined from either of the other techniques. For both of the cases where results were available from all three sources, the results using the VRTC method were higher than Dr. Guenther’s and Ford’s, and Dr. Guenther’s results were higher than Ford’s. However, based on only two examples, it is not possible to determine with any confidence that there are any systematic differences among the three data reduction techniques.

2. Variability of Understeer Gradient Test Results:

In determining whether the differences between the results obtained by Dr. Guenther, Ford and VRTC have any practical significance, it is necessary to examine them in the context of all of the sources of variability that exist in measuring the understeer gradients of vehicles.

First, the tests used to determine the understeer gradients for vehicles have an inherent level of variability from one test run to another. VRTC’s testing included six test runs per vehicle, three in the clockwise direction and three in the counter-clockwise direction; this same test protocol was used by Dr. Guenther. In VRTC’s data reduction and analysis procedure, the test data for each run were analyzed using a linear regression to produce a straight line fit of the data, and the slope of that line represents the understeer gradient for that test. Then, in order to quantify the test-run-to-test-run variability, a statistical analysis of the individual understeer gradient values for each of the six tests runs of a vehicle was performed, and the mean (average) and the standard deviation for the six test runs was calculated. As an example, VRTC’s tests of the MY 1998 Ford Explorer resulted in an average understeer gradient of 2.58°(degrees)/g and a standard deviation of 0.31°/g for the six test runs upon which the result in Tables 1 and 2 is based. Using standard statistical techniques, the 95% confidence interval for these test results is from 2.25°/g to 2.91°/g. This characterizes the test-run-to-test-run variability and is a measure of the confidence with which the results of a series of tests of one vehicle being tested at one test site conducted at one time can be judged as representing the “true” understeer gradient of that vehicle. Dr. Guenther only reported the average of the understeer gradient values determined from the six individual test runs that he performed.

Some of the variability comes from sources other than the variations in understeer gradient values that result from the test-run-to-test-run differences and from the differing data reduction techniques discussed above. These other sources include variations in testing the same vehicle at different times at the same facility, variations in testing at different test facilities with different road surfaces, variations in test procedures (including conducting constant radius circle tests at different radii) and variations in testing different samples of the same vehicle model.

Ford has indicated that it has evaluated some of these sources of variability for the "constant radius circle" test procedure. These evaluations involved testing the same vehicle at different times at the same test site, at different test radii at the same test site, and at different test sites. Based on these tests, Ford estimates these sources of variability result in a 95% confidence interval of +/-0.25°/g. This variability should be added to the test-run-to-test-run variability discussed above when comparing understeer gradients that were measured in different test series.

Moreover, variation in test results between different samples of the “same” vehicle model may be as large as, and possibly larger than, the other sources of variability discussed above, due to normal production tolerances and tolerance stack-up during the manufacturing process. Based on the above discussion, ODI believes that unless differences of at least 1°/g are found when comparing the measured understeer gradients of different vehicle models, one cannot confidently state that the vehicles truly have different understeer gradients.

3. Comparison of Dr. Guenther’s Measurements of Understeer Values to Those Measured by Others:

To further evaluate whether the results reported by Dr. Guenther accurately characterize the responses of the vehicles tested, those results were compared to the results of tests performed by others. This comparison is possible since, as discussed above, understeer gradients of vehicles that are the same as, or similar to, those tested by Dr. Guenther are available from other sources.

As noted earlier, the test procedure used by VRTC is different from that used by Dr. Guenther and Ford. For rear-wheel drive vehicles, understeer gradient test results using this test procedure will tend to be higher than that of the "constant radius circle" test procedure. ODI is not aware of any studies that have attempted to quantify the differences in the understeer gradients measured using the two test procedures.

Table 3A includes results from VRTC tests of five vehicles for which understeer gradient results are also available from Dr. Guenther and/or Ford. In three of those five cases, the understeer gradients reported by Dr. Guenther and/or Ford are less than those from VRTC and are also outside the 95% confidence limits derived for the VRTC tests, and in the other two cases, the results from Dr. Guenther and/or Ford are close to those from VRTC and are within the 95% confidence limits derived for the VRTC tests.

The understeer gradient results from Firestone, Ford, and VRTC shown in Tables 2 and 3A are nearly all within the +/-1°/g range of test variability discussed above. There are only two exceptions. One is the difference between the reported understeer gradients for the MY 1996 4-door, 4X2 Ford Explorer results from Dr. Guenther and that of the MY 1997 4-door, 4X2 Explorer results from Ford, which is 1.06°/g. This difference is so close to the 1°/g variability criterion that ODI deems it to be insignificant. The other exception is the difference between the understeer gradients for the MY 2001 4-door, 4X4 Ford Escape reported by VRTC, 4.21°/g, and that reported by Ford, 2.50°/g. Despite these two instances, ODI believes that the understeer gradient results from Dr. Guenther, Ford, and VRTC are sufficiently consistent such that, for the purposes of analyzing the "findings" made by Firestone, it is legitimate to use the results reported by Dr. Guenther.

B. ODI's Review of the "Findings" and "Conclusions" Supporting the Firestone Request:

The first three of the four "findings" presented in the Firestone Statement are based entirely on the results of the tests that are discussed above. In examining these findings, ODI assumed that the reported test results upon which they are based are accurate and valid. The last of the findings presented in the Firestone Statement is based on Dr. Guenther’s review of technical literature, some of which is referenced in the Firestone Statement.

Additional conclusions are presented in the Firestone Update. The first is that the step steer test results show that the Explorers exhibit longer response times and higher yaw velocity gains with a “detreaded” rear tire when compared to the other SUVs tested. Since this finding is related to the issue of the control characteristics of an oversteer vehicle, it will be addressed below in the “Findings Related to the Controllability of an Oversteer Vehicle” section. The second is that the Explorers tested have “…two distinct peak ranges in the roll angle frequency response; one of them at a low frequency similar to the other SUVs tested and a second higher frequency response not shared by the other SUVs tested.”

Given the additional test results and revised conclusions included in the Firestone Final Report, ODI has assumed that Firestone now believes that these “findings” apply only to the “1995 model year and later 2WD Explorers tested.”

Each of these “findings” is examined below.

1. Finding that the Explorer Has Significantly Lower Understeer than the Other SUVs Tested:

As indicated above, understeer gradients with “4 normal tires” for several models of Ford Explorers and other SUVs are available from tests performed by VRTC and Ford, as well as from Dr. Guenther’s test program. The understeer gradients that were available from all of these sources for vehicles tested in the lightly loaded condition are included in Tables 3A and 3B. Table 3A includes understeer gradient values for those vehicles for which test results are available from more than one source, and Table 3B includes those for vehicles for which results were only available from one source.

The data in Tables 3A and 3B show that, in general, the MY 1995 and later 4X2 Ford Explorers have lower understeer gradients than the four peer SUVs tested by Dr. Guenther. However, they do not have significantly lower understeer gradients than many of the other SUVs tested by VRTC and Ford. Several recent and current model year SUVs have understeer gradients lower than or close to those of MY 1995 and later 4X2 Explorer models.

It should be noted that all of the vehicles included in Tables 3A and 3B whose understeer gradient is lower than or equal to 3.0°/g were introduced to the market or were redesigned for MY 1997 or later, and all but two of the vehicles, the MY 1994 Toyota 4Runner and the MY 1993 Ford Explorer XLT, included in the tables whose understeer gradient is lower than 4.0°/g were introduced to the market or were redesigned for MY 1996 or later. On the other hand, only three of the vehicle models included in the tables whose understeer gradient is greater than 4.0°/g were introduced or redesigned for MY 1996 or later. This seems to indicate a significant, industry-wide trend toward lower understeer gradients in more modern SUVs. This trend is also seen in the Ford Explorers, where the average understeer gradient for the MY 1995 and later 4-door, 4X2 Ford Explorers is about 2.3°/g and the understeer gradient for the MY 1993 4-door, 4X2 Ford Explorer, a model introduced in MY 1991, is 4.1°/g. This trend has positive consequences on vehicle response characteristics in that reduced levels of understeer gradient will tend to increase the level of vehicle control available to the driver. As long as the other aspects of the vehicle's response characteristics provide an adequate level of damping and stability (characteristics that are not directly related to or adequately quantified by the understeer gradient), drivers will be able to safely operate a vehicle with such lower levels of understeer gradient over the same range of driving situations as vehicles with higher levels of understeer gradient.

In summary, although the first finding made in the Firestone Statement is accurate when the comparison is limited to the small sample of vehicles examined by Dr. Guenther, data reflecting tests of a more representative sample of SUVs show that the understeer gradients of these Explorer models are higher than or similar to those of several other contemporaneous SUVs.

2. Finding that the Explorer Loses Much of its Understeer when Loaded to GVWR:

When an SUV is loaded to its GVWR, its center of gravity generally moves rearward; in fact, this is true of most light trucks and vans (see Measured Vehicle Inertial Parameters-NHTSA’s Data Through November 1998, Heydinger et al, SAE 1999-01-1336, page 5). Such rearward movement of a vehicle’s center of gravity would inherently tend to result in a reduction in the vehicle’s understeer gradient to some extent (see Fundamentals of Vehicle Dynamics, Thomas D. Gillespie, SAE R-114, page 226).

The understeer gradient results for the two load conditions of the three MY 1995 and later 4X2 Explorer configurations and the two other SUVs tested prior to the submission of Firestone’s request in the “4 normal tires” condition by Dr. Guenther and included in the Firestone Statement are shown in Table 4A along with the absolute and relative (percentage) changes from the lightly loaded condition to the GVWR condition. Also included in Table 4A are the understeer gradient results of VRTC’s analysis of Dr. Guenther’s data for the “4 normal tires” tests that were discussed earlier.

First, examining Dr. Guenther’s results alone, only one of the six comparisons for the Explorer configurations has a difference greater than the typical test-series-to-test-series variability of +/-0.25°/g discussed earlier. That is the MY 1996 Ford Explorer with Firestone tires tested in the clockwise direction. If the typical industry practice of averaging the results for clockwise and counter-clockwise tests were applied to this data, the difference of 0.40°/g between the lightly loaded and GVWR conditions would also fall within typical test-series-to-test-series variability. The largest absolute and relative differences between the lightly loaded and GVWR conditions is seen for the MY 1996 Chevrolet Blazer, where its understeer gradient changes by 1.22°/g for both the clockwise and counter-clockwise tests, but its understeer gradient decreases in the clockwise direction and increases in the counter-clockwise direction.

Using the VRTC’s reduction of Dr. Guenther’s data, in all but one of the lightly loaded-to-GVWR comparisons, the differences are smaller than those reported by Dr. Guenther. In no case is the difference significant based on the 95% confidence limits that were calculated. Table 5 presents an analysis of understeer gradient data from tests performed by Ford and by Dr. Guenther. These data also do not indicate a consistent or significant reduction of understeer with increasing load for various MY 1995 and later 4-door, 4X2 Explorer models. In fact, for the seven 1996 to 2000 model year Explorers included in Table 5, the average change in understeer gradient from lightly loaded to GVWR condition is a decrease of 0.146°/g, with the 95% confidence interval being from –0.45 to +0.16°/g. If the analysis is expanded to include all MY 1995 and later 4X2 Explorers for which understeer gradient results are available, the average change in understeer gradient from lightly loaded to GVWR condition is a decrease of only 0.077°/g, with the 95% confidence interval being from –0.33 to +0.17°/g.

The understeer gradient results for the two load conditions of five of the Explorer configurations and of the two peer SUVs tested in the “4 normal tires” condition by Dr. Guenther and included in the Firestone Update, Firestone Supplement, and Firestone Final Report are shown in Table 4B along with the absolute and relative (percentage) changes from the lightly loaded condition to the GVWR condition. The results for the “right rear” MY 1996 Explorer are not included in the table, since these are the same for the MY 1996 Explorer tested with OEM Firestone tires included in Table 4A.

Only two of the ten comparisons for the Explorer configurations have a difference greater than the typical test-series-to-test-series variability of +/-0.25°/g discussed earlier. These are the two MY 1993 Ford Explorers tested in the clockwise direction, and the differences are only very slightly greater than 0.50, specifically 0.51 and 0.53. Also, if the typical industry practice of averaging the results for clockwise and counter-clockwise tests were applied to this data, the differences between the lightly loaded and GVWR conditions would fall within typical test-series-to-test-series variability.

Table 6 presents the results of an analysis of the differences in understeer gradient between lightly loaded and GVWR conditions of 24 non-Explorer SUVs. For these vehicles, the average change in the understeer gradient from lightly loaded to GVWR condition is –0.347 °/g, with the 95% confidence interval being from –0.59 to +0.11°/g.

This data does not support Firestone's assertion that the MY 1995 and later 4X2 Explorer models lose much of their understeer when loaded to GVWR. For that matter, the data indicates that the changes in understeer gradient for the Explorer models tested by Dr. Guenther are not as great as those for the other three SUVs tested by Dr. Guenther, as well as those of many other SUVs tested by Ford.

3. Finding that, Unlike the Other SUVs Tested, Explorers Become Oversteer Following a Rear Tire Tread Separation:

The understeer gradient results in the “detreaded” tire condition for the two load conditions and two turn directions for the nine Explorer configurations and the four peer SUVs tested by Dr. Guenther are included in Table 7. Seven of the nine Explorer configurations exhibited an understeer gradient less than zero, i.e., oversteer, in at least one load/turn direction condition, and all of these were MY 1996 or later Explorers, which are equipped with a short-long-arm (SLA) front suspension. The understeer gradient for these vehicles in the “detreaded” tire condition ranged from –0.04°/g to –2.91°/g, i.e., oversteer. The understeer gradient exhibited by the MY 1997 4-door, 4X4 Explorer in the “left rear tire detreaded,” GVWR, clockwise turn direction test condition, 0.04°/g, is so close to zero that the vehicle could be characterized as a neutral steer vehicle. One of the peer SUVs, the MY 1992 Nissan Pathfinder, also exhibited an understeer gradient of less than zero in both turn directions in the GVWR load condition. Neither of the two MY 1993 Ford Explorers, which are equipped with a “twin I-beam” front suspension, were found to have understeer gradients of less than zero in any of the “detreaded” tire test conditions. The lowest understeer gradient measured for either of these Explorers in the "detreaded" tire test condition, 0.20°/g, was exhibited by the MY 1993 Explorer XLT. This value is close to or greater than the lowest understeer gradients exhibited by two of the peer SUVs, the MY 1996 Chevrolet Blazer and the MY 1996 Jeep Grand Cherokee, when they were tested in the “detreaded” tire condition.

Ford provided results of understeer gradient tests for 25 SUVs, 13 with 4X4 drivetrains and 12 with 4X2 drivetrains, two front-wheel drive compact passenger cars, and two front-wheel drive minivans that included data for tests conducted in the GVWR condition both with all intact tires and with a tread separation of the tire that was on the outside rear wheel during the tests (i.e., the left rear tire for a clockwise, right, turn or the right rear tire for a counter-clockwise, left, turn). Based on this data, most of the SUVs (20 of the 25 in the data), as well as one of the passenger cars and both minivans, exhibit linear range oversteer characteristics following a rear tire tread separation. Table 8 shows the maximum, minimum, mean, and median values of the understeer gradient with intact tires (“tread on”) and with a tread separation on the outside rear tire (“tread off”) and the change in the understeer gradient from the “tread on” to the “tread off” condition for all 29 vehicles for which results were provided by Ford and for the 13 vehicles for which results were provided by Firestone. For the maximum and minimum values shown in Table 8, the model year of the particular vehicle is shown in the table. All of the vehicles that represent these maximum and minimum values are 4-door SUVs with wheelbases between 104 and 116 inches and curb weights between 3,500 and 4,700 lbs. The 15 Explorer models represented in the data include six of the seven vehicles included in Table 5 (one of the MY 1997 “Ford” Explorers included in Table 5 was not tested with a “detreaded” rear tire), four additional Explorer vehicles tested by Ford (a MY 1993 4-door 4X2, a MY 1997 2-door 4X2, and a MY 1996 and a MY 2000 4-door 4X4), and five additional Explorer vehicles tested by Dr. Guenther (a MY 1993 and a MY 1996 (tested with a right rear “detreaded” tire) 4-door 4x2, a MY 1999 2-door 4X2, and a MY 1993 and a MY 1997 4-door 4X4). The average “tread off” understeer gradient for these Explorers is –1.38°/g. Of the sixteen non-Explorer SUVs that became oversteer in these circumstances, ten are more oversteer than the average for the Explorers and four had more oversteer than the Explorer that exhibited the most oversteer.

These data indicate that Explorers are not unique among SUVs in exhibiting linear range oversteer characteristics following a rear tire tread separation. The data also show that linear range oversteer following a rear tire tread separation is not unique to SUVs. (Although the data in the Firestone Final Report indicates that the MY 1992 Nissan Pathfinder exhibits linear range oversteer in the GVWR condition with a rear tire tread separation, Dr. Guenther asserts that the results of the “constant radius circle” tests show that “unlike the other SUVs tested, the 2WD 1995 model year and later Explorers lose their margin of understeer when they experience a tread separation” (emphasis added). As such, this finding is not supported even by the results of the tests conducted by Dr. Guenther.)

4. Findings Related to the Controllability of an Oversteer Vehicle:

The Firestone Statement asserts that “[a]n oversteer vehicle is not safe at highway speeds in the hands of the average driver.” It also states that an oversteer vehicle is difficult to drive because the driver “has to deal with…a slow response time and the following large gain.” In particular, Firestone contends that an oversteer vehicle is extremely difficult for most drivers to control, particularly at high, “interstate highway” speeds where it can become directionally unstable. Firestone revised this contention in its August 22, 2001 Update, stating that “[a]t or above critical speed the Explorers are not controllable by any driver.”

ODI is not aware of any published results of tests of passenger cars or light trucks that exhibit linear range oversteer characteristics that result from operating factors such as loading or component degradation/failure (excluding tire failures). However, based on information provided by vehicle dynamics experts, it is likely that station wagons and light trucks with heavy loads in the rear of their cargo areas and light duty vehicles towing relatively large trailers (particularly, if the trailers are improperly loaded) can exhibit linear range oversteer, yet such vehicles are driven by average drivers without crashing. Drivers may complain about the difficulty of driving such vehicles for extended periods and can become fatigued driving them, since constant, minor steering corrections are necessary to drive such a vehicle at speeds approaching, at, or above its critical speed, but this is not relevant to the Firestone request, since it is unlikely that drivers will continue to drive vehicles that have experienced a rear tire tread separation for any longer than necessary to get to the side of the road and change the tire.

The first specific statements made by Firestone about the safety consequences of the changes in transient response times and steady-state gains appear in the Firestone Update, and are repeated in the Firestone Final Report. The Firestone Update states that the step steer test results show that the Explorers exhibit longer response times and higher yaw velocity gains with a “detreaded” rear tire when compared to the other SUVs tested by Dr. Guenther. Given the understeer gradients of these vehicles, this is not at all surprising since, as discussed in the Appendix, vehicles with lower levels of understeer will inherently exhibit longer transient response times and higher steady-state gain characteristics at highway speeds than similar type/class vehicles with higher understeer gradients. However, such response characteristics are not unique to Explorers and would be exhibited by any light duty vehicle whose understeer gradient is significantly reduced due to vehicle loading or component degradation/failure.

Although not mentioned by Firestone, ODI notes that, in addition to the reduction in the vehicle's understeer gradient, a tread separation has an additional effect on a vehicle’s control and stability characteristics. That is the result of the “detreaded” tire’s significant loss of traction, as well as a dramatic loss of cornering stiffness. This results in not only a significant reduction in the vehicle’s understeer gradient, but also a substantial reduction in the lateral acceleration capability of the vehicle. Test data available from both VRTC and Ford show that most SUVs are normally capable of controllably achieving lateral acceleration levels of about 0.7 g, and their linear range would typically extend to about 0.4 g on dry, paved road surfaces. However, information provided by Ford indicates that following a rear tire tread separation, some SUVs exhibit limits of lateral acceleration of only 0.35 g, and linear ranges of lateral acceleration response as low as 0.2 g. This effect likely contributed to the loss of directional stability of the Explorer shown in the videotape presented at the December 4, 2001 meeting. Based on the 40 mph linear range yaw response gain measured by Dr. Guenther for that vehicle, the 21° steering wheel input used in the 40 mph step steer test of the Explorer would have resulted in a steady turn with a lateral acceleration of 0.33 g. Based on the similarly measured gains for the other two SUVs shown in the video, the Blazer's and the Cherokee's responses to the 24° steering wheel inputs would be steady turns with lateral accelerations of about 0.13 g and about 0.2 g, respectively. These lateral accelerations were well within the linear range maneuvering capabilities of each of these vehicles with “good tires,” and the lateral accelerations for the Blazer and Cherokee remained in the linear range even with "detreaded" rear tires. However, the lateral acceleration achieved by the Explorer reached the non-linear range, causing the vehicle's response gains to increase very rapidly, leading to eventual loss of directional stability. (During this demonstration the driver was instructed not to make any effort to change the steering wheel angle in an attempt to correct any undesirable vehicle response. ODI believes that such a scenario is not representative of real world driver behavior.)

The above discussion is not meant to minimize the fact that the driver of a vehicle that has become oversteer as a result of experiencing a rear tire tread separation is confronted with substantially different vehicle response and maneuverability characteristics than those with which the driver is familiar. Particularly if the vehicle is being driven above its critical speed, this could lead to loss of control, if the driver does not make appropriate corrections to the steering input in reaction to the vehicle's response.

With regard to a driver’s ability to control such a vehicle, a crucial factor is the control tasks with which the driver is confronted. When the driver of a vehicle that has become oversteer after experiencing a rear tire tread separation is confronted with other than simple, driving tasks (i.e., straight line driving, and possibly ”slow” lane changes and negotiating typical curves of modern high-speed highways), the vehicle can be very difficult to control, especially when the vehicle is being driven above its critical speed. In such situations, there may be a loss of directional stability, e.g., a “spin-out” or a loss of path control, resulting in an impact with another vehicle or roadside object. Whether or not a given driver is able to adequately control such a vehicle depends upon the vehicle’s speed, the precise traffic/driving situation, the vehicle’s response characteristics, which are affected by the level of oversteer of the vehicle, and the driver's experience, knowledge, and alertness.

Ford has made available to ODI videotapes of tests that it conducted involving various SUVs, including various models of Ford Explorers with a rear tire tread separation, being driven at high speeds, in some cases significantly higher than their critical speeds. These tests indicate that moderate lane changes and relatively hard braking maneuvers can be performed by alert, experienced drivers (not necessarily professional test drivers) without loss of control or stability.

Firestone claims that a driver may lose control when “he has to deal with the unfamiliar and unpredictable oversteer handling through a steering input/vehicle response characterized by a slow response and a following large gain.” This is correct. However, such loss of control situations also can occur with vehicles that are not oversteer. For example, Dr. Guenther's test results for the two MY 1993 Explorers suggest that MY 1991 through 1994 Explorers that are equipped with a “twin-I beam” front suspension do not become oversteer following a rear tire tread separation. However, a substantial number of these vehicles that were equipped with Firestone ATX or Wilderness AT tires were involved in loss of control crashes following rear tire tread separations.

Based on the above, ODI believes that the value of a vehicle's linear range understeer gradient, and in particular, whether it is positive or negative (i.e., understeer or oversteer, respectively) does not provide a sufficient measure for evaluating the risk of a loss of control in the event of a rear tire tread separation.

5. Discussion of Other Issues Raised in the Firestone Presentation:

Firestone provided a list of 16 crashes of Explorers equipped with non-Firestone tires that experienced rear tire tread separations. Seven of these crashes involved pre-MY 1995 Explorers, vehicles that are not the focus of the Firestone request, and 10 of the 17 fatalities and 9 of the 35 injuries included in that list involved these vehicles. Another of the crashes included in the list involved an Explorer with an unknown model year. As stated earlier, ODI agrees that a tire tread separation can lead to a crash. That is why it is important to minimize the number of such tire failures. However, these incidents do not indicate that an Explorer is more likely to experience a crash under this scenario than other SUVs.

ODI believes that Firestone's analysis of the FARS data is of limited, if any, use in understanding the issues related to Firestone’s request. In past analyses of crash avoidance issues, NHTSA has found that analyses based on FARS data are usually of limited value. The likelihood of a fatality in a crash is very dependent upon the vehicle's crashworthiness characteristics and safety belt use. Often, analyses of fatal crashes yield results that conflict with results of analyses based on more general state crash data files that include information on crashes of all levels of severity.

The Firestone Presentation included a discussion of various causes of tire tread separations, other than those related to design and manufacturing faults in the tires. NHTSA is in the process of several initiatives that will reduce the incidence of such tread separations, in response to mandates established by the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act. These include an upgrade of the Federal motor vehicle safety standards applicable to tires, a requirement for tire pressure monitoring systems on vehicles, and improved labeling of tires. NHTSA is also taking steps to encourage the proper maintenance and inspection of tires and proper vehicle loading. Nevertheless, ODI agrees that because various factors related to in-service tire use and abuse can lead to tread separations, tread separations will continue to occur. However, this has no direct relevance to the issue of whether the Explorer contains a safety-related defect.

VI. Field Experience of Ford Explorers and Other SUVs that Experienced a Tread Separation:

An examination of the Firestone claims database provided to ODI during its investigation of Firestone ATX and Wilderness tires, EA00-023, supports the conclusion that Explorers do not respond in a significantly different manner than other SUVs after experiencing a tire tread separation. Table 9 (Table 7 from the “Engineering Analysis Report and Initial Decision” for EA00-023) shows that the number of crashes per 100 “tread separation” claims for Explorer vehicles is similar to that of other compact SUVs and all other SUVs. The ODI consumer complaint database for the tires that were the subject of that investigation, summarized in Table 10 (Table 8 from the “Engineering Analysis Report and Initial Decision” for EA00-023), also does not indicate a significant difference in the likelihood of a crash following a tread separation between Explorer vehicles and other compact SUVs.



VII. Conclusions:

The foregoing analysis should not be construed as suggesting that tread separations will not lead to losses of control and crashes, particularly in SUVs. The many crashes following tread separations of tires on these vehicles that are documented in the Firestone claims database and that have been reported to ODI by consumers and others demonstrate that such a tire failure 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 fact that a vehicle exhibits linear range oversteer characteristics following a rear tire tread separation does not, in itself, indicate that the vehicle contains a safety-related defect. Moreover, the data available to ODI does not indicate that Explorers in general, or even MY 1995 and later 4X2 Explorers in particular, are more likely to exhibit linear range oversteer characteristics following a rear tire tread separation than many of their peers.

For the foregoing reasons, Firestone’s request for a defect investigation is denied. Appendix - Discussion of Some Basic Concepts of Dynamic Systems and Vehicle Dynamics:

This Appendix is intended to allow those who are unfamiliar with the basic concepts of dynamic systems in general, and vehicle dynamics in particular, to better understand the issues raised by Firestone's request for a defect investigation and ODI's analysis of that request. It provides a discussion of some basic concepts with respect to the responses of dynamic systems, including “linear range” versus “non-linear range” dynamics, “steady-state” versus “transient” response, “steady-state” response gains and “transient” response times, and “open loop” versus “closed loop” control of dynamic systems. It also includes a discussion of some basic concepts of automotive vehicle dynamics, including understeer gradient, the steady-state and transient stability and control characteristics of understeer and oversteer vehicles, and the concept of “critical speed” for an oversteer vehicle.

The concepts of “linear range” versus “non-linear range” dynamics, “steady-state” versus “transient” response, and “open loop” versus “closed loop” control apply to the analyses of any dynamic system. However, in order to simplify the following discussion, the examples used to describe general system dynamics concepts are derived from automotive vehicle dynamics.

A. Basic Concepts of Dynamic Systems and their Counterparts in Vehicle Dynamics:

1. “Linear range” versus “Non-Linear range” Dynamics:

Linear range refers to an operating range of a dynamic system over which, in response to incremental changes in an input to the system, the changes in the responses (motions) of the system are proportional, with a constant ratio, to each successive input increment. That is, it is a range of system response over which the ratio of the amount of change in the response of the system to the amount of change in an input to the system is equal to a constant over the entire range.

In the case of an automobile, the linear range usually refers to that range of vehicle operation over which incremental increases in the steering wheel angle will result in incremental increases in the vehicle’s lateral acceleration, such that the ratio of each successive response increment to its corresponding input increment is constant over the entire range of the increments. As an example, at a given speed, assume that a vehicle responds with a lateral acceleration of 0.2g to a 20° steering wheel input, and at that same speed, its response to a 40° steering wheel input is a lateral acceleration of 0.4g. In both cases, the vehicle’s response to a 20° steering increment is a 0.2g lateral acceleration increment. Since the ratio of the response, lateral acceleration, to the input, steering wheel angle, is 0.01g/° for the two successive increments of steering wheel angle, the vehicle’s response up to at least 0.4g would be “linear.” If, in response to a 50° steering wheel input, the vehicle’s response is 0.45g, the vehicle’s response would no longer be linear, since the ratio of the incremental response to the incremental input, 0.005g/°, has changed from that of the previous increment, 0.01g/°. The level of lateral acceleration at which this incremental response-to-input ratio is no longer constant is the upper bound of a vehicle’s “linear range.” All levels of lateral acceleration above that level would be considered to be in the “non-linear range.”

In reality, the response of a vehicle is never perfectly linear. As a practical matter, a vehicle's response is measured over a range of inputs and then a linear fit to the measured data is generated. Also, while various testing techniques have been standardized, the results always vary from one test to another, due to a variety of factors, some of which are discussed in the main document.

2. “Steady-State” versus “Transient” Response:

Steady-state response refers to an operating condition of a dynamic system characterized by constant values of its dynamic response variables (the measures of the system’s various responses, usually in terms of measures of motion and/or position).

In the case of an automobile, steady-state response refers to an operating condition of a vehicle in which its motion is characterized by constant values of the vehicle’s various dynamic response variables, forward velocity (or longitudinal acceleration, i.e., braking or accelerating), lateral acceleration, radius of turn, yaw velocity (yaw rate), sideslip (yaw) angle, and roll angle. The sideslip angle of a vehicle is the difference between the direction in which the vehicle is “pointed” (the orientation of the vehicle’s longitudinal centerline), and the direction in which the vehicle's center of gravity is moving.

The steady-state response of a vehicle to zero steering wheel angle, constant throttle position, and no brake pedal application on a flat road surface (with no external disturbances, such as a wind) would be straight line driving at a constant speed. This “steady-state” condition would be characterized by constant velocity (zero longitudinal acceleration), zero lateral acceleration, an infinite radius of turn, zero yaw rate, zero sideslip angle, and zero roll angle. If a vehicle is being operated in this condition and then the steering wheel is turned a predetermined number of degrees and held at that position, the vehicle will within a period of several seconds achieves a “new” steady-state condition, characterized by constant, or nearly constant, values of the motion variables above. This new steady-state condition would be characterized by constant velocity, a non-zero, finite level of lateral acceleration, a finite radius of turn, a non-zero, finite level of yaw rate, a non-zero, finite sideslip angle, and a non-zero, finite roll angle. When a vehicle is operating at a steady-state condition, it is said to be "trimmed."

The period of time between the two steady-state conditions discussed above constitutes the vehicle's transient response and is characterized by values of forward velocity, lateral acceleration, radius of turn, yaw rate, sideslip angle, and roll angle that vary with time.

3. “Steady-State” Response Gains and “Transient” Response Times:

One basic category of measures of the steady-state response of dynamic systems is steady-state response gains. This type of measure can be used to characterize the response of any of a system’s dynamic response variables. It is a measure of the level of steady-state response (output) of any response variable to an input to the system and is expressed as a ratio of the level of that response to the level of the input to the system.

In the case of an automobile, the most common input variable used to characterize a vehicle’s steady-state response gains is steering wheel angle. As such, these gains are expressed as a response level per unit of steering wheel angle (SWA) expressed in degrees (°). An example is the yaw velocity response gain, which measured in degrees of yaw angle per second (°/sec) per degree of steering wheel angle (SWA°, or °), or more simply, °/sec/°. This is one of the response gains that figures prominently in the discussion in the Firestone Update.

Another basic category of measures of the transient response of dynamic systems is response time. This type of measure can also be used to characterize the response of any of a system’s dynamic response variables. It is a measure of how quickly a dynamic system responds to an input and achieves a new steady-state condition. Not all of a system’s dynamic response variables respond at the same rate, and a system’s various response variables may reach new steady-state values at different times.

For automobiles, response times are measured during transient response testing, such as the “step steer” tests conducted by Dr. Guenther, and are very basic measures of an automobile’s transient response characteristics. They are measures of how quickly the vehicle is able to respond to very rapid steering input. If the input were a true “step input” (taking zero time) as described above, the response time would be measured from time, t, equal zero. Since this is not possible, the usual convention in automotive vehicle dynamics is to turn the steering wheel as rapidly as possible and to measure the response time from the time that the steering wheel angle reaches 50% of its final value (this reference time is called t0). The response time of an automobile is characterized by the time between that t0 and the time at which the vehicle response first reaches a given percentage of its steady-state response. The percentage most often used is 90%; however, the 50% response time is sometimes used. The results of Dr. Guenther’s “step steer” tests included both the 90% and 50% response times.

4. Distinction between Stability and Control

In the broadest sense, the term “stability” is used to describe the response of a trimmed system to disturbances external to the system. The term “control” is used to describe the response of a system to changes within the system that attempt to disturb it from the trimmed condition. The following descriptions are taken from Chapter 5 of Race Car Vehicle Dynamics by Milliken and Milliken, SAE R-146, 1995:

Basically, the term "static stability" refers to the tendency of a system to return to a previously established equilibrium when disturbed. One cannot talk about stability without first having established the equilibrium or initial trim. The initial trim can be defined by the control position, a steady-state attitude or acceleration.

In the case of the automobile the basic directional control is provided through the steering wheel:

Static directional control is the control moment on the vehicle as a result of steering the front wheels (emphasis in original).

The terms stability and control are often confused or used interchangeably, but it is essential to make the distinction between these concepts when discussing vehicle dynamics.

5. “Open Loop” versus “Closed Loop” Control:

The concepts of “open loop” and “closed loop” control relate to whether the inputs to a system are adjusted in response to the outputs (responses) to achieve a specific desired outcome. If such an adjustment to the input to a system that is dependent upon the output of a system exists, the system is being operated with “closed loop” control. The output “signal” that is used to adjust the input is known as “feedback,” and “connection” from the output back to the input is the “control loop.” “Open loop” control refers to the situation in which the control loop is not “closed” and there is no “feedback,” i.e., the control loop is “open.”

In the case of an automobile, “open loop” refers to a situation where a predetermined input or series of inputs are made to the vehicle, such as moving the steering wheel to a new fixed position or moving the steering wheel in a particular manner, such as a sine wave pattern, and the vehicle is allowed to respond without the intervention of a driver who “closes” the control “loop” by altering the input to achieve a predetermined vehicle output, such as following a particular path like a lane change maneuver. A vehicle being operated by a driver on the highway such that the vehicle will negotiate the path desired by the driver is being operated in a “closed loop” situation, i.e., the driver’s inputs to the system (the vehicle) are being adjusted in response to the output (the vehicle’s motion) to achieve a desired outcome (follow a particular path).

A special case of open loop control is “fixed control.” As its name implies, “fixed control” is a situation where a single “fixed” input is made to a system in order to determine the system’s response. In the case of an automobile, the “step steer” tests discussed earlier are an example of a fixed control situation.

“Loss of control” can occur for a variety of reasons. For example, in the open loop case, loss of control occurs when a change in the steering wheel angle no longer results in a change to the control moment—the heading and path of the vehicle can no longer be adjusted by changes in the steering wheel position. In the closed loop case, loss of control may be due, for example, to a change in the vehicle's response characteristics that is beyond that to which the driver can adapt.

B. Additional Vehicle Dynamics Concepts:

1. Understeer Gradient:

The simplest means to describe the concept of understeer gradient is to examine one of the tests used to measure it, the “constant radius circle” test. In this test, a vehicle is driven in a circular path of constant radius starting at a low speed and slowly accelerating, usually to a speed at which the vehicle is no longer controllable. As the vehicle’s speed increases while it negotiates the constant radius circle, the vehicle’s lateral acceleration increases with the square of the speed. During the test, the vehicle’s steering wheel angle and lateral acceleration are measured (the latter can either be measured directly or it can be calculated from the vehicle’s speed and the known radius of the circle). When the lateral acceleration is plotted versus the steering wheel angle, the slope (gradient) of the line of that plot is known as the “steering wheel angle gradient.” Dividing the value of this slope by the vehicle’s overall steering ratio, which is the number of degrees of steering wheel angle necessary to change the vehicle’s front wheel angle by one degree, yields the “reference steer angle gradient.” In the special case of the “constant radius circle” test, this “reference steer angle gradient” is the vehicle’s understeer gradient.

The general terms, "understeer" and "oversteer," are often used to describe a vehicle’s control and stability characteristics. A vehicle is said to be “understeer” if its understeer gradient is positive and is said to be “oversteer” if its understeer gradient is negative. The distinction between these terms is that "understeer gradient" is the metric used to quantify this vehicle performance characteristic and "understeer" and "oversteer" denote a category or type of vehicle performance characteristic. A positive "understeer gradient" characterizes an "understeer" vehicle, and a negative "understeer gradient" characterizes an "oversteer" vehicle.

2. Steady-State and Transient Response Characteristics of Understeer and Oversteer Vehicles:

Even within the linear range, a vehicle’s understeer gradient alone does not provide adequate information to provide a complete description of the control and stability characteristics of a vehicle as it relates to the ability of the average driver to safely control the vehicle. However, a reduction of a vehicle’s understeer gradient sufficient for the vehicle to approach a neutral steer condition, or in a more extreme case, for the vehicle to become oversteer, as a result of the degradation or failure of a vehicle component (such as a tire tread separation) could result in substantial changes in a vehicle’s control and stability characteristics; e.g., increased transient response times in the vehicle’s transient responses, and higher sensitivity of steady-state response gains to vehicle speed (which could result in higher than desirable gains at typical highway speeds). Under some circumstances, the vehicle’s response to otherwise reasonable and appropriate steering and/or braking inputs could result in loss of vehicle control and/or stability.

The vehicle responses for which these changes in response times and response gains are most likely to affect the ability of the average driver to safely control the vehicle are its primary control responses; i.e., lateral acceleration and yaw velocity. Of secondary influence, and much less importance, are a vehicle’s roll, pitch and ride responses, which are only coupled responses that result from the fact that automobiles have a “sprung” suspension system in order to achieve a comfortable ride over irregular road surfaces.

Factors other than understeer gradient affect a driver's ability to safely control a vehicle. For example, the overall steering ratio is a basic vehicle characteristic measure that has a significant effect on the vehicle control characteristics sensed by a driver, since the vehicle’s steady-state response gains are proportional to that ratio. Simply changing the overall steering ratio of a vehicle will change those gains without affecting the understeer gradient.

The following discusses the effects on driver/vehicle performance of transient response times and the speed sensitivity of steady-state response gains.

Transient response times that are substantially longer than usual are considered undesirable, since it is more difficult for the average driver to adequately predict a vehicle’s response when the response times are relatively long. The ability of a driver to anticipate and adjust for the delay in a vehicle’s response that is characterized by its transient response time is referred to as “lead equalization.” Although there are no specific guidelines for what levels of transient response times would be best for normal, safe vehicle operation, response times much greater than 0.5 seconds are usually considered less desirable.

Relatively high sensitivity of a vehicle’s steady-state response gains to changes in vehicle speed is considered undesirable, since this would require drivers to use significantly different steering inputs at different speeds in order to achieve the same vehicle response. However, for the sensitivity of a typical light duty vehicle’s steady-state response gains to changes in vehicle speed to be great enough to result in undesirably high gain characteristics at typical highway speeds, a vehicle’s understeer gradient would need to approach 1°/g. Such a vehicle with an understeer gradient of 1°/g would exhibit an increase in its yaw rate gain (the vehicle steady-state response gain that is emphasized in the Firestone Update) of about one-third when a vehicle’s speed increases from 40 mph to 80 mph. For comparison, yaw rate gain of a vehicle with an understeer gradient of 2°/g would increase by about 10% when the speed increases from 40 mph to 80 mph, and the gain for a vehicle with an understeer gradient of 3°/g would decrease only slightly for the same change in speed.

As discussed earlier, for typical vehicles whose understeer gradient is reduced as a result of the degradation or failure of a vehicle component (such as a tire tread separation), the reduced values of understeer gradient would result in higher sensitivity of steady-state response gains to vehicle speed and increased transient response times of the vehicle’s transient responses. For such vehicles whose understeer gradient is reduced to zero, i.e., neutral steer vehicles, the vehicle’s steady-state response gains would increase along with increases in the vehicle’s speed, and the transient response times would approach one second or more.

In a situation where a vehicle understeer gradient is reduced to the point that it becomes negative, i.e., it becomes an oversteer vehicle, the vehicle’s steady-state response gains would increase at a rate greater than increases in the vehicle’s speed and the transient response times would usually exceed one second.

For such an oversteer vehicle being driven at or above its “critical speed,” the vehicle’s steady-state response gains would become so sensitive to vehicle speed that the gain would become theoretically infinite, and the vehicle’s transient response times would also become theoretically infinite. The concept of “critical speed” is discussed in detail below.

3. "Critical Speed" for an Oversteer Vehicle:

One approach to explaining the implications on vehicle control and stability of an oversteer vehicle is to examine the moments acting on a vehicle in a “steady-state” turn, i.e., at constant radius and constant speed. These can be represented using what is known as “derivative” notation. In such notation, the forces and moments acting on a vehicle are represented by multiplying each appropriate “derivative” coefficient by its corresponding basic state variable. There are three basic state variables that can together describe the operating conditions of a vehicle that affect the forces and moments acting on the vehicle. They are the road wheel steer angle, d; the yaw velocity, r; and the sideslip angle, b. Their corresponding “derivatives” are the control moment derivative, Nd, the yaw damping derivative, Nr, and the static directional stability derivative, Nb.

Using this notation, the moments acting on a vehicle are:

These moments and their direction of action for vehicles negotiating a steady-state turn are shown in Figure A-1 for the cases of understeer, neutral steer and oversteer vehicles.

In the case of an “understeer” vehicle (i.e., a vehicle with a positive understeer gradient), all three of these moments act on the vehicle with the yaw damping moment and the static directional stability moment both acting to resist the control moment and the vehicle’s turning motion, i.e., trying to return the vehicle to a straight path. The control moment acts in the opposite direction to the other moments and maintains the vehicle’s curved path.

As a vehicle’s understeer gradient becomes smaller, the static directional stability derivative decreases, and when the vehicle’s understeer gradient becomes zero (i.e., a neutral steer vehicle), the derivative becomes zero. Therefore, the static directional stability moment also becomes zero, and the only moments acting on the “neutral steer” vehicle are the yaw damping moment and the control moment, which in the case of a steady-state turn as shown in Figure A-1, are equal and act in opposite directions.

In the case of an “oversteer” vehicle (i.e., a vehicle with a negative understeer gradient), the static directional stability derivative is negative, and its corresponding moment will act to reduce the radius of whatever turn the vehicle is negotiating (i.e., tighten the turn). As such, it will oppose the stabilizing influence of the yaw damping moment. As shown in Figure A-1, in the special case of an oversteer vehicle being driven at the “critical speed,” only the yaw damping moment and the static directional stability moment act on the vehicle in a steady-state turn. In this case, these moments are equal and acting in opposite directions. While the yaw damping moment still acts to resist the vehicle’s turning motion, the static directional stability moment acts to resist the yaw damping moment and maintain the curved path.

The yaw damping moment decreases as vehicle speed increases while the static directional stability moment does not change substantially with speed in the linear range (in fact, the simple two degree-of-freedom model upon which Figure A-1 is based predicts that the static directional stability moment does not change with speed in the linear range). Therefore, there exists (only for an “oversteer” vehicle) a “critical speed” at which the yaw damping moment is exactly balanced by the static directional moment. At the “critical speed,” if the steering wheel is turned, there are no stabilizing moments available to balance the control moment. As such, any fixed steer angle input would be unbalanced, and the vehicle’s response would be an ever-increasing lateral acceleration and yaw velocity, and an ever decreasing radius of turn until the vehicle experiences a “spin-out.” At “critical speed,” an oversteer vehicle can only maintain a steady-state response if the control moment is zero; i.e., if the steering wheel angle is zero. In others words, an “oversteer” vehicle traveling at the “critical speed” cannot maintain a steady-state response with a non-zero steady steering input.

For “oversteer” vehicles traveling in a curved path at speeds above the “critical speed,” the static directional moment will always exceed the yaw damping moment. Therefore, to maintain such a steady-state turn, a control moment opposite to the direction in which the vehicle is turning is needed to balance the moments acting on the vehicle; i.e., the driver must steer the vehicle “out of the turn.” Without a driver’s intervention with such “reverse” steering, the vehicle could no longer maintain a steady state response, and the “fixed-control” response of the vehicle would become unstable.

However, unstable does not mean uncontrollable. The above discussion is based on the analysis of the steady-state vehicle; i.e., one for which the steering input is fixed (does not vary with time). During real driving, the steering input is not fixed; i.e., the driver can and usually does turn the steering wheel to respond to an evolving driving situation. For example, if an “oversteer” vehicle starts to diverge from the desired course, the driver can turn the steering wheel so as to bring the vehicle back to the desired path. This is an example of “closed-loop” control, as discussed earlier.

4. Linear Range versus Non-Linear Range Vehicle Characteristics:

A vehicle’s directional control and stability characteristics in the linear range are not the sole determinant of how the vehicle will respond following a rear tire tread separation. Another key factor is how the vehicle behaves at its maneuvering limit. A vehicle's response at its maneuvering limit is called its “limit response.” A directionally unstable limit response leads to a spin out, while a directionally stable limit response would result in a “plow” or “push.” In either case, the vehicle would not be controllable through steering inputs at its maneuvering limit, regardless of whether it exhibited understeer, neutral steer, or oversteer characteristics in the linear range.

In the linear range, higher measured understeer gradients correlate with higher levels of vehicle stability. However, it is not appropriate to attempt to apply the understeer-oversteer concept to the nonlinear range because in that range one cannot assess stability by measuring the vehicle's response to control inputs — which is the way in which understeer gradients are determined.

The information available to ODI indicates any vehicle that has experienced a rear tire tread separation would exhibit an unstable limit response. Thus, even if a vehicle exhibits linear range understeer characteristics in the linear range and is therefore stable and controllable in that range, if a large enough steering input is made such that the vehicle's response reaches the non-linear range; the vehicle would exhibit a directionally unstable limit response and would spin out.


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[1] Throughout this document, Ford Explorer will be used to refer not only to the Ford Explorer vehicles, but to Mercury Mountaineer and Mazda Navajo vehicles that are the same vehicles that are "badged" for the other two "makes."