Vehicle Dynamics of a Jumping Rallycross Car

Abstract: This master thesis was performed in collaboration with Öhlins Racing AB. The company provides suspension systems to the automotive industry and to motorsport teams globally. Many of Öhlins’ customers compete in rallycross, a style of competition that generally involves highly modified production cars racing on closed, elevated and mixed surfaced circuits. Rallycross cars generally have an inherent problem and tend to over rotate mid-air after taking off from a jump. The vehicle often lands with a large pitch angle, damaging suspension components or losing valuable time when the driver waits for the vehicle to settle. The request from Öhlins was to analyse this phenomenon. The scope of the master thesis was to; investigate four different simulation software and choose the most appropriate software to simulate a rallycross car, perform a parameter study to analyse which parameters affect the jumping behaviour of the vehicle and study the force build up in the suspension during the landing phase. The four simulation software investigated were LMS Amesim, CarSim, Adams/Car and a 2D MATLAB model. The models were parameterised with vehicle data acquired from a rallycross car and validated against measured data obtained from tests with the same car. The MATLAB model was considered to be the best performing model given the criteria which were set up. A parameter study was conducted with the chosen simulation model where different vehicle parameters, driver behaviour and road profiles were analysed to investigate what impact they had on the jumping performance of the rallycross car. It could be concluded that the rear damper stiffness is critical for the jumping behaviour of the vehicle and that a stiffer rear damper generally gives the best performance. The longitudinal position of the centre of gravity also has a significant impact on vehicle jumping where a position in the middle between front and rear axle is preferred. The force build-up in the vehicle suspension was also analysed. Vehicle jumping and landing behaviour was compared to measurements from a human jumping in order to investigate if a human jumping utilised other force build-up strategies. It was found that the car force build-up during landing was similar to a human beings. Other force build-up strategies for the dampers were tested and it was found that dampers reacting to stroke position or hub acceleration can improve jumping performance of a rallycross car.