Influence of Frictional Forces in Springs
The body springs of trucks usually consist of stacked leaf springs. The characteristics of these springs show hysteresis, resulting from the friction between the spring leaves. This leads to a temporary blockage of the body spring during the course of an oscillation, thereby turning the two-mass system into a single mass system. The body and wheel mass then oscillate in conjunction on the stiff and poorly damped tire spring.
The top half of the figure shows the velocity difference between body and wheel with and without friction. In the model which takes friction into account, the phases where the leaf springs block, as indicated by the velocity difference reaching zero, are clearly visible.
The bottom half of the figure shows the effect of the frictional force on the body acceleration of the examplary truck. Particularly in the range of the highest vibration sensitivity of humans, i.e., between 4-8 Hz, friction leads to a considerable degradation of the suspension behavior.