On Drive Force Distribution and Road Vehicle Handling - A Study of Understeer and Lateral Grip

Licentiate thesis, 2007

Computer controlled vehicle sub-systems aimed to support drivers in various driving situations are rapidly increasing in number and sophistication. These systems have evolved from anti-lock braking and traction control systems, to electronic stability control. Recent developments include active steering, suspension, and – of interest in this work – computer controlled drive force distribution. These systems are known to have the potential improve a vehicle’s handling, defined as the relationship between driver input and vehicle output, over a wide range of operating conditions. The focus of past and present research in this field has been on the interaction between combined lateral and longitudinal forces on a tire level. Nonetheless, the influence of the drive force and drive force distribution on vehicle level handling characteristics – of interest for this work – have been unsatisfactory described in the reviewed literature. Based on this fact, and the relevance of this needed knowledge for this study, these shortcomings are being addressed in this work. In the field of vehicle handling, two main aspects are; the understeer, i.e. how the turning radius changes with speed given a fixed steering input; and the lateral grip, which is defined by the maximum possible steady-state lateral acceleration. One objective of this study is to show how the drive force distribution can be optimized for maximum lateral grip and constant understeer. For this purpose the existing theory and methods are developed with the aim to show the influence of the drive force distribution on the understeer and lateral grip. Finally, by means of computer simulations, the developed theory was verified and the influence of effects which should be included in future development of the presented theory was identified. Overall, this present work has successfully expanded the knowledge in the field of combined acceleration and cornering, from tire level properties to vehicle level characteristics.