Stiffness and strength aspects in the design of automotive coil springs for McPherson front suspensions: a case study

The current paper deals with the elastic and stress analysis of a side-load spring, which is employed in cars belonging to the medium segment of the European automotive market. In the design of this kind of spring both the elastic characteristics and the fatigue strength have to be considered as significant aspects. Side-load springs, indeed, have a thrust axis which results in skew with respect to the damper axis; for this reason they are widely employed in front McPherson automotive suspensions in order to minimize the side force acting on the strut damper, which is responsible for the undesired damper-sticking phenomenon. In addition to this particular elastic property, as a consequence of the research effort in reducing the mass of components typical of the automotive industry, these springs have to face very high working stresses. The structural reliability of the spring must therefore be ensured by high residual stresses, which are induced by presetting and shot peening treatment at the end of the manufacturing process. The evolution of the thrust axis during suspension travel was analysed by a finite element (FE) model of the spring-seats assembly, built starting from a three-dimensional (3D) scan of the spring. In addition an experimental campaign was planned in order to measure the residual stresses at several points along the spring by X-ray diffraction. The actual stresses were then obtained as the superposition of those produced by the external loads, computed by means of the FE model, and the measured residual stresses. The beneficial effect of residual stresses on the structural reliability is discussed with a theoretical fatigue analysis on the basis of different multiaxial criteria. A mechanism of residual stress generation, based on the superposition of stresses coming from presetting and shot peening, is also suggested with the aid of FE simulations.