Firestone Statement Concerning Dr. Dennis A. Guenther's
Engineering Analysis of Ford Explorer Margin of Control in the Event
of Belt/Tread Separation
SUMMARY
This analysis is focused on the loss of control
experienced by the Explorer in normal highway driving following a rear
tire belt/tread separation (hereinafter "tread separation").
Loss of control in this circumstance often results
in the Explorer leaving the highway and rolling over or spinning into
an angle relative to its path of travel on the roadway sufficient to
cause rollover, with or without tripping, or other serious accidents.
Because loss of control is a precursor to rollovers and other serious
accidents, the hypothesis is suggested by common Explorer accident scenarios
that the Explorer has a control problem leading to rollover and other
crashes in the event of tread separation.
Dr. Guenther has tested that hypothesis and found that:
- the Explorer models he has 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;
- 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;
- 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;
- an oversteer vehicle is extremely difficult for most drivers to
control, particularly at interstate highway speeds where it can become
directionally unstable;
His conclusion based on these findings is 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.
Relevant Engineering / Accident Reconstruction
Concepts
Understeer/Oversteer
The terms "understeer" and "oversteer",
while not particularly descriptive in themselves, are engineering terms
that are used to characterize what is one of the most significant control
relationships in driving an automobile in the linear range‡
- the amount of steering wheel input necessary to produce an amount
of G's of lateral acceleration, the side force that accomplishes turning
of an automobile. It is measured and expressed in degrees of steering
wheel input per G of lateral acceleration.
†Left
rear tread separation is the most common finding in Explorer accidents
involving tread separation and is the condition examined to date.
‡Linear range
in this context refers to normal everyday driving by average drivers.
The amount of understeer or oversteer in a vehicle
is measured by driving the vehicle in a constant radius circle at an
increasing speed and recording the degrees of steer input per G of lateral
acceleration. In an understeer vehicle a test driver, in terms of what
he perceives and does in that circumstance, must steer toward the center
of the circle, with increasing steer input as he increases speed, in
order to stay on the path of the constant radius circle; that is the
same thing the average driver experiences as he drives around a curve
- he must steer to the inside of the curve in order to generate the
side force necessary to turn the vehicle and stay on the curving path.
An oversteer vehicle behaves just the opposite. The
test driver would have to steer away from the center of the circle in
order to stay on the constant radius circle - he would have to "take
steer out" in order to keep the car on the path of the circle as
he increases speed. See Exhibit 2 for a technical definition of "understeer"
and "oversteer".
Automobile manufacturers do not intentionally design
an oversteer characteristic into cars intended for ordinary drivers
because "a vehicle that oversteers, by design or circumstance,
is highly undesirable." Exhibit 3. See also Bergman, "The
Basic Nature of Vehicle Understeer-Oversteer" at page 11, col.
1 (1965). Ford, like any other automobile manufacturer, tries to build
understeer into its cars. See, e.g., Exhibit 4. They do this because
understeer is essential to safely control an automobile.
Car designers can increase or decrease the amount of
understeer in a vehicle by many different means - by adjusting spring
rates, shock absorber stiffness, frame stiffness, roll damping, tire
properties, tire pressure, weight distribution, and other component
functions. They adjust these and other elements which result in the
amount and character of control available. Automobile designers, of
course, may adjust these elements for reasons other than achieving or
influencing controllability; they may, for example, make such adjustments
to seek ride comfort, to achieve a "flat" European cornering
feel, to improve rollover resistance, or for other reasons.
Each of those trade-offs for such reasons, however,
potentially affects the amount of understeer and the amount of control
safety margin.
Cars differ from each other in how much control margin,
or understeer, they have. How much understeer is necessary to provide
a safe margin of control? The answer from an engineering perspective
is: Enough to provide vehicle control in all foreseeable driving circumstances
for the drivers intended for that vehicle.
The foreseeable circumstances of driving include many
things - the coefficient of friction of the roadway surface, wind gusts,
ice and snow, vehicle load, component wear and failure, the effect of
heat and use on the fit and flexibility of suspension system components,
and many others. One foreseeable circumstance, of course, is tires wearing
out and eventually failing, including tread separation, the most common
mode of wearout failure for steel belted radial tires. All of these
circumstances can cause an increase in the need for understeer or directly
decrease the amount of understeer available in the vehicle. For example,
tread separation will change tire properties related to understeer,
decreasing cornering stiffness and traction provided by belt and tread.
One of the car designer's engineering obligations is
to quantify the amount of understeer and other vehicle control characteristics
required to compensate and account for such varying and foreseeable
events, the inevitable changes in driving circumstances. By that quantification
he determines the amount of understeer, the margin of control, that
must be designed into the car.
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* Dr. Guenther is a Professor of Mechanical Engineering
at The Ohio State University. He is frequently engaged by NHTSA and automobile
manufacturers as a consultant on automotive safety matters. He has testified
on numerous occasions at the request of automobile manufacturers as an
expert on automobile control issues, rollover, and accident reconstruction.
He has published over 100 articles on automotive safety matters. His resume
is attached as Exhibit 1.
