September 25, 2009 by markarndt
Not all tire tread separations are equal and new testing documents previously unknown differences between a front tire failure and a rear tire failure. Almost universally, tread separation event testing is limited to rear tire failures. Most of the Ford Explorer/Firestone Tire crashes involved rear tires and the causes of these crashes are attributed to a variety of vehicle factors – the largest factor relates to adverse changes in vehicle controllability.
So why do vehicles that have front tire tread separations get into crashes?
The answer, in part, is explained because despite decreased sensitivity to steering the failure event is startling, produces violent vibration and loud noise and pulling. Pulling is turning of the vehicle without the driver turning the steering wheel. Of course, the vehicle steering characteristics also changed suddenly and nonsymmetrically, complicating the driving task. New testing of front tire tread separation demonstates for some vehicles a substantially increased pulling response comparable to equivalent rear tire failure. New testing also documents a torque response transmitted through the steering wheel that may jerk the steering wheel from the driver’s grip.
As a rule of thumb, when a rear tire experiences a tread separation the resulting change in the vehicle’s understeer gradient, a key measure of the vehicle turning characteristics, is roughly three degrees per G (3 deg/G) . Where, G is equal to the acceleration of gravity. And, when a rear tire experiences a tread separation event all vehicles ever tested respond in dynamic maneuvers with oversteer – in other words, they spin-out.
It is perplexing that the same changes at the tire that makes a vehicle spin-out when there is a rear tire failure also makes a vehicle less likely to spin-out when there is a front tire failure – in other words, when there is a front tire failure the vehicle will understeer more and become less sensitive to steering. The new testing results show that an external disturbance may play a greater role that previously understood.
