Designing for Vehicle Stability During Rear Tire Tread Separation Events

Abstract

Rear tire separation or delamination events affect the rear wheel adhesion and thus the vehicle stability. Maintenance of the lateral force of the tires on the rear axle of a vehicle is paramount to vehicle control. Tire tread separation events are, unfortunately, a foreseeable event for which allowance in the design of the vehicle must be made. Testing of vehicles illustrates how loss of control of a vehicle can occur if, during the process of delamination, the excitation of the rear wheel due to the delaminating tread causes tramp on the rear axle of the vehicle. This tramp occurs if the excitation is at the natural frequency of the rear axle. For solid rear axle vehicles, this natural frequency coincides with the rotational frequency of the wheel when traveling at highway speed. The tramp motion can be so severe to cause the tires to alternately leave the ground. Testing of the vehicle with excitation within the natural frequency of the axle/tire combination during an SAE J266 circle test, showed an oversteer gradient of 12 to 19 degrees per g, resulting in a critical speed of approximately 28 mph (45 kph), a highly unstable situation while traveling at highway speed. This paper reports from testing results how the vehicle may, be designed to eliminate the dangerous tramp mode by proper installation of the shock absorbers and proper shock damping characteristics. The test results show that the vehicle can maintain a continuous understeer characteristic during the rear wheel excitation at the natural frequency.