Solid Axle Tramp Response Near The Natural Frequency and its Effect on Vehicle Longitudinal Stability

Rear axle tramp can be excited by rough roads or cyclic vertical inputs from tire failure. If the excitation frequency is at or above the natural frequency of the axle/tire spring-mass system, the response can be sufficient to cause loss of control due to axle tramp, which will cause decreased lateral friction of the rear tires resulting in oversteer. Recognition of this problem led to this study of the mechanics of the rear axle motion during controlled cyclic inputs. Tramp response to cyclic impacts or imbalance is analyzed theoretically and compared to measured responses on a vehicle. The effects of axle moment of inertia, spring stiffness and placement, and shock stiffness and placement are discussed. Testing of a vehicle with a controlled vertical impact to the rear axle was conducted in an SAE J266 circle maneuver to determine the effects of tramp magnitudes and frequency to the understeer characteristic of the vehicle and reported in a previous paper [1]. Tramp responses from instrumented tests are compared to those obtained from a theoretical physics model of the axle tramp. From these tests and the theoretical model, a characteristic damping of the axle, as a percentage of the critical damping, is proposed. This will allow vehicle designers to select shock absorber stiffness and placement to best control vertical displacement and lateral load handling capability of the vehicle in the event of tramp mode excitation at the natural frequency and beyond.