Commercial vehicles are mostly equipped with pneumatic spring elements which lead to a perfect height levelling and spring rate adjustment under different loading conditions. However, pneumatic springs are not common in light commercial vehicles where passive spring elements, e.g. single- and multi-leaf springs, are still be used. Since those vehicles are covering a wide range of different loads the spring elements frequently exhibit a progressive spring characteristic, i.e. the spring rate is adjusted under deflection as soon as the load is increased. The need for light weight design also relates to light commercial vehicle so that glass fibre reinforced plastic (GFRP) has become a suitable substitute for high strength steel. Furthermore GFRP allows for innovative as well as functionally and technologically improved constructional solutions of progressive spring elements, e.g. the single-leaf spring approach by Schürmann et al. [MBS11].
However, the above mentioned solution is sometimes rather solitaire and no systematic approach for its genesis exits. Hence, this contribution shows an approach for a more systematic development of progressive light weight spring element concepts in vehicle construction. Different approaches of implementing a progressive spring rate characteristic are presented in the introduction. A simple analytical model of a bending beam considering a variety of boundary conditions has been set up to discuss the effect of bearing stiffness on the spring rate. This simple model has been systematically expanded by adding further beam elements which allows the calculation of the load depending composite spring rate.
The model serves as a basis for a morphological box as a systematic approach for the development of the desired progressive spring elements. It allows to identify and to select a balanced concept for a progressive light weight spring element which also considers the application of the appropriate spring material at any specific part of the construction.
[MBS11] Muhr und Bender KG. Leaf Spring for motor vehicles. Inventor: Helmut Schürmann. European Patent application WO 2011/110611 A3. 15.09.2011