STATEMENT OF DR. DENNIS A. GUENTHER

I have been asked by counsel for Bridgestone/Firestone to carry out an investigation of the directional control of the Ford Explorer following a tire tread separation.  That investigation includes accident reconstruction, review of police accident reports, vehicle measurement, and dynamic testing.  I am presenting today the preliminary results of my dynamic testing; that testing is ongoing.  My resume is attached to this statement as Exhibit 1.

The hypothesis addressed in my testing is that the Explorer has a control problem leading to rollover crashes following tread separation.  I chose to explore the linear range of vehicle operation as a preliminary investigation because of the complexities and non-linearity of vehicles.

A description of the dynamic testing I have conducted is set forth below.

            Test Site

The tests were carried out at the Transportation Research Center, Inc. (TRC) test facility near East Liberty, Ohio.  The facility is used on a contract basis by automobile manufacturers, component suppliers, and state and national regulatory authorities to conduct automotive safety testing. 

            Study Objectives

The purpose of the testing program, which is ongoing, is to examine the margin of control in the Explorer as designed and, comparatively, in peer SUVs in the circumstance following rear tire tread separation.  As noted above, I have conducted my study to this point in the linear range.  In the linear range a principal parameter of control is the understeer/oversteer gradient (other parameters such as steering response time and gain, and steering frequency response are also being examined as they may relate to loss of control in the event of tire tread separation).

           Test Vehicles

            The vehicles evaluated are the following:

1996	Ford Explorer	4 dr	4 x 2
1996	Chevy Blazer	4 dr	4 x 2
2001	Jeep Cherokee	4 dr	4 x 2
2000	Ford Explorer	4 dr	4x 2

Each vehicle was tested with its original equipment (OE) tires.  The 1996 Explorer was tested with both OE Firestone tires and OE Goodyear tires recommended by Ford.

           Vehicle Instrumentation and Measurement

The data acquired for purposes of this analysis was the following:

Vehicle Input

Steering Wheel Angle
Vehicle Speed

Vehicle Response

Lateral Acceleration
Yaw Rate
Body Roll Angle

The instrumentation and sensors used to acquire this data is identified in Exhibit 2.

           Test Maneuvers

The tests conducted are universally recognized standard tests used by automobile manufacturers, including Ford, and other researchers in vehicle dynamics for establishing the values investigated.  The tests are as follows:

           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).  The test was run both with four good tires and with the left rear tire detreaded by cutting between the steel belts; test runs with the detreaded tire were done only at the slower 40 mph speed.  Test runs were done at both light load (curb plus driver and instrumentation) and heavy load (gross vehicle weight rating).

The test is used 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 (output divided by input).

           Constant Radius Circle

The vehicle is driven around a 200 foot constant radius circle with increasing speed.  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, with four good tires and with the left rear tire detreaded.  Test runs were done at light load (curb plus driver and instrumentation) and heavy load (gross vehicle weight rating).

The test is used to measure understeer and oversteer (degrees of road wheel steer per Gs of lateral acceleration).

Frequency Response

Sinusoidal sweep steering tests are frequently used to determine the linear response of vehicles.  The vehicle in these tests were 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 test measures lateral acceleration gain, yaw velocity gain, and phase angles at the frequencies tested (up to 3 to 4 hz).

           Results of Directional Control Tests

The results of the constant radius circle tests are set forth in data sheets and charts attached hereto as Exhibit 3.  Data reduction continues with respect to the step steer and frequency response tests.

In summary, the findings in the tests are as follows:

Constant Radius Circle – This standard method of measuring understeer/oversteer gradient establishes that the Explorer, with four good tires, has a relatively small amount of understeer compared to other SUVs – less than half the amount found in the Blazer and the Cherokee.  In fact, the Cherokee has about the same understeer with a detreaded tire as the Explorer with four good tires. 

The test results show that, unlike the other SUVs tested, the Explorer loses its small margin of understeer when it experiences a tread separation and becomes an oversteer vehicle.

This is true whether the Explorer is operated on Goodyear OE tires recommended by Ford or on Firestone OE tires.

The Explorer’s oversteer characteristic is worse in the loaded condition.  The only circumstance in which it does not become oversteer with a detreaded tire is when it is lightly loaded (curb plus driver and instrumentation) and the detreaded tire is on the inside rear position (left rear in a counter-clockwise turn); in test runs in that configuration the Explorer is almost neutral steer with respect to the understeer/oversteer gradient.

An oversteer vehicle is not safe at highway speeds in the hands of an average driver.  Sometimes a driver may achieve directional control, sometimes a driver may not, particularly where the driver has to deal with the unfamiliar and unpredictable oversteer handling.  The oversteer control problem is increased by virtue of the fact that the Explorer driver is used to a vehicle which is understeer and the vehicle has changed to oversteer without the driver’s awareness.

CONCLUSION

The Explorer is an oversteer vehicle in most circumstances after it experiences tread separation.  Oversteer can make a vehicle directionally unstable and subject to loss of control in the hands of most drivers.  This is a vehicle problem, not a tire problem.  The vehicle performs the same following tread separation on the Goodyear tire as it does the Firestone tire.  This must be regarded as a highway safety defect within the meaning of the National Highway Traffic and Motor Vehicle Safety Act.

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