Small Amplitude Torsional Steering Column Dynamics on Smooth Roads: In-Vehicle Effects and Internal Sources

Internally excited torsional steering wheel vibrations at frequencies near 8-22 Hz on smooth roads can produce driver disturbances, commonly described as “SHAKE”. These vibrations are primarily excited by the rotating front suspension corners and are periodic in the rotational frequencies of the tire-wheel assemblies. The combination of vehicular dynamic amplification originating in dominant suspension and steering system vibratory modes, and a sufficiently large 1^st harmonic non-uniformity excitation of the rotating corner components, can result in periodic vibrations exceeding thresholds of disturbance. Controlling the periodic non-uniformity excitation through individual component requirements (e.g., wheel imbalance, tire force variation, wheel runout, concentric piloting of wheel on hub) is difficult since the desired upper limits of individual component requirements for vibration-free performance are typically beyond industry capability. Alternative approaches include de-sensitizing the vehicle, or the vehicle subsystems, at the critical frequency bands, which yield a lower overall vibration level for statistically relevant amplitudes of periodic excitation. One vehicle subsystem, the steering column assembly in a rack-mounted assisted steering system, may have a torsional dynamic response that can produce unwanted dynamic amplification. This condition significantly contributes to the overall sensitivity of the vehicle. Comparisons of ostensibly identical steering columns demonstrated substantial differences in dynamic amplification in both on-road and laboratory subsystem tests. The paper describes the on-road effects of these conditions at typical highway speeds, a laboratory test procedure for evaluation of the column contributions, and the source for the dynamic amplification within the column.