Physical Modeling and Simulation Analysis of an Advanced Automotive Racing Shock Absorber using the 1D Simulation Tool AMESim

Abstract: Shock absorbers are crucial components of a vehicle’s chassis responsible for the trade-off between stability, handling, and passenger comfort. The aim of the thesis is to investigate the physical behavior of an advanced automotive racing shock absorber, known as TTR, developed by Öhlins Racing AB. This goal is achieved by developing a detailed lumped parameter numerical model of the entire TTR suspension in the advanced 1D simulation tool, AMESim. The shock absorber is mainly composed of the main cylinder with through-rod piston design and the gas reservoir located at the low pressure hydraulic line, which connects the compression and rebound sides. The mentioned sides are identical in terms of the components which are a High Speed Adjuster, a Low Speed Adjuster, and a check valve mounted in parallel. The adjusters are special hydraulic valves, which can be modified in terms of flow metering characteristics by means of external accessible screws. Adjustment is done in a series of discrete numbers called ‘clicks’. A fixed orifice and a spring-loaded poppet valve are responsible for controlling the piston low and high speed regions respectively. The developed AMESim numerical model is capable of capturing the physics behind the real shock absorber damping characteristics, under both static and dynamic conditions. The model is developed mainly using the standard AMESim mechanical, hydraulic and hydraulic component design libraries and allows discovering the impact of each single hydraulic component on the TTR overall behavior. In particular, the 1D model is presented in two levels of progressive physical complexity in order to improve the dynamic damping characteristics. Several physical phenomena are considered, such as the hydraulics volumes pressure dynamics, the contribution of external spring and pressure forces to the dynamic balance of the moving elements, the static and viscous frictions, and the elastic deformations induced by solid boundaries pressure. In this thesis, progressive model validation with different types of measurements is as well presented, covering the individual hydraulic components models as well as the entire shock absorber model. The measurements have been performed on the flow benches and dynamometers available at the Öhlins Racing measurements laboratory. These comparisons, deeply discussed in the thesis, allow discovering the impact of specific physical effects on the low and high speed hydraulic valves static performance and on the shock absorber dynamic behavior. Numerical results show good agreement, especially at low and medium frequencies and symmetric ‘click’ adjustments on compression and rebound sides. Further model development is necessary in the other areas, for example by considering more complex models of the valve dynamics and fluid flow patterns, i.e. flow forces, together with more advanced models of the sealing elements viscous friction, and thermal effects. Finally, the AMESim environments offered a good level of flexibility in designing the TTR hydro-mechanical system, by allowing the user to choose between different levels of model complexity.