A Thermomechanically Coupled Model for Automotive Shock Absorbers: Theory, Experiments and Vehicle Simulations on Test Tracks

In the development of cars numerical simulation plays a more and more important role. A method commonly used in this context is based on the formalism of multibody dynamics. In this approach a vehicle is described as a system of rigid bodies connected by joints, bushings, springs and dampers. For the purpose of estimating the time-dependent dynamical loads when riding, for example, over potholes or other obstacles we need enhanced models representing the mechanical behavior of the shock absorbers. A model of this type should describe the nonlinear rate dependence of the force in combination with friction effects and thermomechanical coupling phenomena. Due to the dissipation of energy the temperature of the shock absorber increases and influences its damping behavior. When riding over long rugged test tracks these effects are strongly pronounced. Thus we develop a thermomechanical model representing all these phenomena with good approximation and being compatible with the natural laws of thermodynamics. To validate the theory, we investigate the thermomechanical behavior of two automotive shock absorbers in detail. We measure the velocity dependence of the force under different temperature levels as well as the dissipative change in the temperature during cyclic excitations. Finally we carry out a vehicle test on a rugged test track and record the temperature of the front and rear dampers. As we show, the model describes all phenomena with sufficient approximation, especially the evolution of temperature.