Effect of Shock Absorber Friction on Vehicle Vertical Dynamics

In order to efficiently predict and investigate a vehicle’s vertical dynamics, it is necessary to consider the suspension component properties holistically. Although the effects of suspension stiffness and damping characteristics on vertical dynamics are widely understood, the impact of suspension friction in various driving scenarios has rarely been studied in both simulation and road tests for several decades. The present study addresses this issue by performing driving tests using a special device that allows a modification of the shock absorber or damper friction, and thus the suspension friction to be modified independently of other suspension parameters. Initially, its correct functioning is verified on a shock absorber test rig. A calibration and application routine is established in order to assign definite additional friction forces at high reproducibility levels. The device is equipped in a medium-class passenger vehicle, which is driven on various irregular road sections as well as over single obstacles. For all tested road sections, a linear decrease of ride comfort in terms of specific relevant vertical objective values is found by increasing the friction force. This emphasizes a definite link between suspension friction and vertical vehicle body vibration, resulting in a negative impact on vertical ride comfort. However, the longitudinal vehicle body vibration is not significantly affected. The relevance of friction in terms of transmitting the energy associated with road unevenness to the chassis in the frequency range of the chassis’ natural frequency is found to be remarkably high on smooth roads, and still considerably high on bumpy roads. The chassis and wheel resonance frequencies are significantly friction-dependent due to the damper’s slip or stick states. The results obtained from smooth road tests demonstrate the practical relevance of accurately considering friction for the given suspension type in terms of vertical ride comfort prediction.