Experimental identification of shock absorber knocking noise using various input waveforms

Shock absorber transient noise, often referred as clonk or knock noise, has been a challenging vehicle noise and vibration concern. As the comfort standards have been rising and quieter power trains (quieter engines, hybrid power trains and electric drives) have been introduced, secondary noise sources could become a significant concern. This study investigates the shock absorber knocking noise on 11 fully adjustable, twin-tube gas-filled automotive shock absorbers, using industry standard damper dynamometer, and a new experimental setup involving the adaptation of one of the shakers of a four post rig to perform tests in isolation. Four of the shock absorbers were returned from the field for knocking noise. The noise was successfully reproduced using triangular and sinusoidal wave inputs with frequency of 10 to 20 Hz on the four post rig setup, while no noise was detected on the same shock absorbers with the damper dynamometer setup. Unlike previously published results, the “noisy” shock absorbers could be successfully identified with a high degree of certainty based exclusively on the performance curve that includes the effects of higher frequencies. The inter-cycle inconsistency in performance curve was the main differentiating aspect. The statistical measures proposed have robustly identified the “noisy” shock absorbers.