As a result of the linear spring characteristic, two main disadvantages result:
Spring travel
When a load is applied on the spring, the available spring travel DzReserve can often reduce to insufficient values (recommended values for passenger car: DzReserve 50mm, truck: DzReserve 70mm)
Natural frequency const.
Since the spring stiffness does not change with load, the body natural frequency in the unloaded condition is larger than the natural frequency in the loaded condition because of mB,loaded > mB,unloaded. The acceleration of the body mass is described in chapter 1.4.
These disadvantages of multi layer leaf springs can be reduced through a progressive design of the spring characteristic. This can be achieved by the application of varying pre-loads on individual leaves and/or by the parallel addition of auxiliary springs, which come into operation only when the load exceeds a certain limit as shown in the figure.
The natural frequency remains constant, if the gradient of the spring characteristic changes proportional to the load.