The spring characteristics indicate a linear behaviour in the operating range. By the modification of the vertical load FW as function of the compression zW (lowering of the wheel axle) the spring rate cW can be determined.
Since the force characteristic is not a constant function of compression, the spring rate is variable. Therefore a constant spring rate can be considered only as an approximation for a certain range of the curve, where the static wheel load FW,Stat and the compression zW,Stat according to the figure.
As a measure for driving safety and protection of the road, the ratio of total load to the static load is indicated.
This parameter is a measure for the additional load relative to the static load with a compression zdyn, e.g as a result of unevenness.
The wheel natural frequency is usually approximately feW = 8-14 Hz. This estimation proceeds from a static spring characteristic of the tire. In actual driving conditions, the radial spring rigidity of the tire is further influenced by a multiplicity of parameters. Due to the viscoelastic behavior of rubber, the spring stiffness increases e.g. when excited by road unevenness of increasing excitation frequency.