Variable caster steering in automotive dynamics
Creators: name: Vo, Quoc Dai
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400204
subject: Automotive safety engineering
classificationCode: 400703
subject: Autonomous vehicle systems

description: The tyre lateral force is a function of side slip and camber. While the lateral force due to side slip is the main part of the total cornering force, it saturates at a certain level of side slip angle even when the available tyre-road friction has not been fully utilised. In such a situation, the use of an appropriate extra camber could increase the safety, stability, and manoeuvrability of the car. This research serves as a further step to put the theory of variable caster steering into eect. The key idea of the theory is actively varying the caster to produce the required extra camber in a good fashion. In this investigation, the theory was examined in a specic case: developing a variable caster scheme to counteract roll camber - a phenomenon that generally limits the tyre lateral force and hence the maximum lateral acceleration of the car. By applying the variable caster scheme, the lateral force is increased. Therefore, the lateral grip capacity of the car is expanded. The benet of the variable caster was also further exploited to improve the steering returnability during low speed cornering.<br><br>The theory of variable caster steering began with the development of road steering wheel kinematics. In order to do that, a number of coordinate systems was introduced to suciently describe the steering motion of a road steering wheel. The homogeneous transformation was then utilised to map coordinates between the systems. By doing so, the kinematics was developed. It was then used in two ways. The rst was to determine the camber, which is the orientation of the wheel, as a function of steering axis orientation, vehicle motion, suspension geometry, and steering angle, for a cornering car. The other was for developing a novel method to determine kingpin moment which aects the returnability of the steering wheels during low speed cornering.<br><br>Then a rollable vehicle model, which is capable of capturing important characteristics of a turning car such as load transfer and roll motion, was constructed. The Magic Formula was used for tyre force modelling to take the camber contribution and the non-linear characteristics of the tyre into account. MATLAB/Simulink was used to simulate the dynamic response of the vehicle to steering input. The steering wheel kinematics and the dynamics model of the car were later validated using both multi-body and experimental data. More specically, the validation of the wheel kinematics was done by a road steering wheel model built in ADAMS software; the dynamic vehicle model was validated using data from eld tests and from a full car model constructed in the CarSim environment. A kinematics analysis of the camber function determined earlier was carried out. <br><br>On the basis of the analysis, a scheme of varying the caster with the primary aim of countering the roll camber was proposed. The dynamic responses of the vehicle to dierent steering inputs were examined to evaluate its dynamic performance with and without the variable caster strategy. The simulation results show that the roll camber phenomenon, for the caster-controlled car, is reduced signicantly. The associated lateral acceleration and yaw rate increase without compromising other handling characteristics. The variable caster strategy, therefore, provides a more manoeuvrable car with expanded turning capacity compared to the passive car. To take advantage of the variable caster, a caster conguration that can improve the returnability of the steering wheels in low speed cornering manoeuvres was also suggested. Using the novel method for determining the kingpin moment, we showed that the caster conguration provides a better steering returnability during low speed cornering.