Many highway traffic accidents occur as a result of sudden tire failures. One particular kind of tire failure is called tread separation, and it can be especially dangerous in some circumstances. This kind of failure involves the outer steel belt layer of the tire peeling away violently, often without causing the tire to go flat. A tread separation that occurs at one of the rear tires of a solid-rear-axle vehicle - such as a pick-up truck or SUV - is a special type of case that has resulted in many highway traffic fatalities.
When a tire tread separation is ongoing, the imbalance of the partially detreaded tire causes that tire to bounce, or "hop," while the tire remains inflated. This hopping of the one tire can cause the entire axle to "tramp," or move in such a way that both tires across the axle are bouncing in an alternating fashion. Rear axle tramp can result in a vehicle that is almost impossible to control at highway speeds, because both rear tires are bouncing out of firm contact with the roadway. Even after several years of research conclusively demonstrating this tread separation axle tramp phenomenon, there are automotive experts who continue to deny such a thing ever occurs. Others in the field of traffic accident investigation remain unaware of the phenomenon. Highway patrol investigators sometimes even cite the driver in these tragic crashes for "failure to maintain lane" or "unsafe turning movement."
Research performed over a decade ago, around the time of the Ford/Firestone recall campaigns, was focused primarily on what happens after the entire circumference of tread has peeled away from the tire. But this research has almost no relevance to accidents involving only a partial detread, where a flap of tread remains attached to the failed tire throughout the accident sequence. Furthermore, very little of this research offered even a glimmer of insight into how solid-rear-axle vehicles respond during the violent detachment of the tire tread, and why such vehicles are so prone to catastrophic loss of control.
One such fatal accident in July, 2002, involved a 1996 model year Ford Explorer, driving on Interstate 10 near Las Cruces, New Mexico, where the legal speed limit was 75 mph. This accident involved a partial tread separation of the left rear tire. Approximately half of the tire's outer tread circumference was ejected from the failing tire; approximately one-quarter of the tire's tread remained fully attached; and approximately one-quarter of the tread remained attached as a loose flap throughout the duration of the accident sequence. The detreaded tire never lost its air, and was thus free to bounce violently due to the tread imbalance. As the driver lost control of the vehicle, it ultimately yawed counterclockwise and rolled over 4 1/2 times through the median. Photos taken by highway patrol in the immediate aftermath of the accident showed definitive evidence of axle tramp: wheel hopping of the undamaged right rear tire on the outboard side of the final yaw, and intermittent tire marks deposited by the imbalanced, partially detreaded left rear tire.
At the time of this accident in July 2002, most of the published research on tire tread separation accidents could offer no explanation for why the axle was tramping. This was despite the fact that the automotive industry had been running highway-speed tread separation tests for several years.
By 2006, researchers from the Engineering Institute and others were publishing test results that showed the solid-rear-axle tramp problem during highway-speed tire tread separations. Professional societies such as SAE, ASME and the ESV Conferences have accepted Engineering Institute publications detailing both the problem and design solutions in recent years.
This year, Paul T. Semones and Dr. David Renfroe of the Engineering Institute published new research titled "Magnitude of Axle Tramp Response to Partially Detreaded Tire Imbalance in Highway-Speed Driving" that, for the first time, quantified the magnitude of highway-speed axle tramp due to a simple tread imbalance caused by a partial tread separation. This newly compiled data confirmed a key vehicle design factor in helping to maintain control during tread separations:
"...increasing shock damping force at the higher operational velocities of the shock absorber reduces the magnitude of axle tramp and assists in keeping the rear axle tires in contact with the ground. Additionally, increasing the distance between the shock absorbers by moving them closer to the wheels provides the same advantage."
In addition to presenting this research at the 14th International Conference on Advanced Vehicle Technologies this summer in Chicago, Mr. Semones was also invited to present a tread separation reconstruction seminar at the 11th Annual ARC-CSI Crash Conference in Las Vegas. During the conference, Mr. Semones presented the evidence from real-world crashes involving tread separation tire failures, where the occurrence of axle tramp from the tire imbalance was indisputable. The audience consisted of accident reconstructionists and police officers from all over North America who learned how and why these crashes occur, and why decade-old research is only part of the explanation.
Through presentations and publications, the Engineering Institute is educating the accident investigation community about the tread-separation-induced axle tramp phenomenon and its hazards. You can learn more about the issues involved in highway-speed tire tread separation accidents by contacting Paul T. Semones at the Engineering Institute.
EI Consultants, LLC offers a variety of engineering research, consulting, and public services related to the automotive industry, tire failure and design, fire investigations and materials failure. The engineers have expertise in accident reconstruction, automotive mechanical failures; and deposition and trial testimony.
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