Simulation and optimisation of off-road vehicle performances based on user expectations

This master thesis was performed in collaboration with Öhlins Racing AB, a company that develops advanced suspension technologies for automotive and motorcycle applications, mostly within the motorsport sector. Shock absorbers, which highly influence the vehicle behaviour and thus the global performances of the vehicle, are all the more important for off-road applications. In the past years, Öhlins Racing mostly focused on on-track applications for its automotive sector, but it recently showed its will to be a powerhouse in the off- road sector. The request from Öhlins Racing was to investigate and optimise the performances of off-road vehicle dampers. This research project started by looking into the modelling of off-road tracks and the modelling of dampers. The next step was to investigate which vehicle performances were relevant for off-roading and which metrics were necessary to evaluate them. Five tests were conducted in order to analyse the following performances: chassis stability, bump absorption, jump, acceleration/braking and steering. For each one of them, the optimised damper parameters were found and their effects on the vehicle’s performances were studied. A comparison has then been conducted in order to analyse the acceleration/braking and steering performances of a vehicle optimised for the chassis stability test and for the jump test. The influence of the soft soil was evaluated by conducting a test that involved the vehicle running on rigid soil and on soft soil (dry sand). The influence of the damper’s internal friction was also evaluated by conducting tests with and without friction. The chassis stability test, which consists of driving on an uneven “dune” profile, showed that a soft setup is better for pitch dynamics but much worse for roll dynamics. On the other hand, a setup deemed too stiff induces high damping forces. When facing a bump or a jump, a vehicle equipped with soft damper settings risks, in addition of having a longer settling time, to hit the bump stops. A setting that is too stiff results in high damping forces and high damper velocities and vertical acceleration of the chassis. Low compression damping is used to keep the damping forces low enough. For the jump, a high rebound damping was found necessary to optimise the landing and to avoid a bounce back. Concerning the acceleration and braking behaviour, it was found that the dampers almost exclusively work in the low-speed region and that a soft setup results in many oscillations while a stiff setup slows down the response. The same effect was established for the steering response: a comparison showed that the chassis stability optimisation is too soft and results in too many oscillations for the acceleration/braking and steering responses, while the jump setting seems to have a behaviour similar to the optimised settings for these tests. The soft soil test showed that the soft soil is acting as if an extra damping was added to the vehicle, with damping forces and velocities being greatly reduced. Finally, it was shown that the internal friction of the damper does not influence its performances, as the friction forces are kept low by Öhlins Racing’s technologies. The study and modelling of the soft soil was quite complicated due to complexity of the field and the lack of concrete studies about its application to vehicle simulations. The soft soil model developed in this thesis is limited and can thus be improved by future works.