The behavior of Hooke's approach (a) expresses itself in a constant amplitude response without phase. In comparison, the tire models with damping elements (b)-(d), show frequency dependent responses, whereby it becomes evident that the Maxwell model is inapplicable as a tire model. The static spring rate disappears, while over the entire frequency range the standardised absolute value of the amplitude is smaller than unity.
The dynamic spring stiffening, which occurs with rubber tires as the excitation frequency increases, can be simulated using the Voigt Kelvin (b) and Gehmann (d) models, as their respective amplitude responses show. As this stiffening transitions into saturation, the Gehmann model describes the tire behavior in a more realistic manner.
In the previous chapter, it was shown that when tires are at standstill (slow rolling), the tire damping decreases with increasing excitation frequency. This effect is also reproduced by model (d), as the phase response of the Gehmann model shows. For high frequencies the phase response of model (d) goes down to zero.