Degraded vehicle shock absorbers represent a risk for road safety. The effect of degraded vehicle shock absorbers on road safety is still the subject of research. This work aims to estimate the effect of degraded twin-tube shock absorbers of passenger cars on road safety. A model was built that represents the vertical dynamic properties of vehicle axles. Part of the suspension model is a damper model that simulates the properties of a shock absorber with different oil levels. The damper model calculates the filling levels of the individual working chambers with the deflection of the damper and distinguishes whether oil, gas or foam flows through the damper valves. The suspension model was parameterised with data from a parameter space, which was estimated by test rig measurements and database analyses and is intended to represent the vehicle field in Germany. With the suspension model, a total of 1.4 million simulations were carried out in the parameter space, which calculate the transmissible lateral forces of the various suspension configurations as a function of vehicle speed, road surface, damper conditions and suspension parameters. The parameter variation included stochastic excitations of the road surface classes A to D with speeds from 30 to 100 kph. The effects of one and two degraded shock absorbers with oil levels of 0 %, 40 % and 60 % were investigated. Degraded shock absorbers have the greatest effect on the transmissible lateral forces at high speeds and on poor road surfaces. If a vehicle is travelling at a speed of 100 kph on a class D road, two degraded shock absorbers without oil reduce the transmissible lateral forces of an axle by 20 % on average. For individual parameter configurations, this value increases to 50 %. If one shock absorber of an axle is degraded in such a way that it no longer carries oil, the transmissible lateral forces of a vehicle axle are reduced by 10 % on average and by a maximum of 20 %. The suspension parameters that most influence the effect of a degraded shock absorber are the vertical stiffness of the tyre, the unsprung mass and the anti-roll bar stiffness of the axle. The damper model used shows good results for highly dynamic excitations except for the configuration with 60 % oil fill level. Measurement results indicate that the damper oil foams strongly at a filling level of 60 % during highly dynamic excitations and thus generates lower damper forces than simulated by the model. The axle model itself has not yet been completely validated. Dynamic measurements on a complete vehicle test bench are planned, which should validate the model for dynamic excitations up to 20 Hz. The work attempts for the first time to describe the effects of degraded vehicle shock absorbers on the vehicle lateral dynamics in a large parameter space. Thus, statements about parameter dependencies and the effect on the entire vehicle field are possible. The work shows that degraded vehicle shock absorber represent a safety risk. However, the effect of degraded shock absorbers on the driving physics are strongly dependent on vehicle parameters and road excitation.
Dipl.-Ing. Tobias Schramm, Research Assistant, TU Dresden