The normally linear characteristic of the coil spring can be changed into a progressive characteristic by the variation of the overall diameter, the wire thickness and the gradient
This effect is based on the fact that an increasing spring load leads to some of the windings coming into contact each other, whereby the effective length l of the wound torsion bar shortens.
The combination of all three measures results in the so-called mini block spring. Apart from its progressive behavior, the low overall height of the mini-block spring is also an advantage, since the coils partially move into one another under a load.
Since coil springs can essentially withstand higher forces along their longitudinal axis in comparison with their transverse axis, they are used similar to torsion bars in combination with the struts of the wheel suspension, which support those components of forces which cannot be withstood by the springs, as in the figure.
The spring is connected on the one hand with the body and on the other hand with the strut, on whose axle-side the wheel load acts as an external force.
Depending on the kinematics of the wheel suspension and the arrangement of the springs, the ratio i represents the relationship between the displacement of the body spring Δf and the displacement of the tire contact patch ΔzW in independent suspensions:
The spring ratio i is usually smaller than 1 and not constant, but changes depending on the instantaneous position of the transmission components of the wheel suspension, hence dependent on the instantaneous compressed condition.
The state of equilibrium exists between the wheel load FW and the spring load FS for a leverage i is shown in the figure.
This way, the so-called wheel specific spring stiffness of an independent suspension for a specific state of compression can be determined.
A progressive characteristic of the suspension can also be obtained possibly by an appropriate configuration of the wheel suspension kinematics.