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

Computer controlled vehicle sub-systems aimed to support drivers in various drivingsituations are rapidly increasing in number and sophistication. These systemshave evolved from anti-lock braking and traction control systems, to electronic stabilitycontrol. Recent developments include active steering, suspension, and – ofinterest in this work – computer controlled drive force distribution. These systemsare known to have the potential improve a vehicle’s handling, defined as the relationshipbetween driver input and vehicle output, over a wide range of operatingconditions.The focus of past and present research in this field has been on the interactionbetween combined lateral and longitudinal forces on a tire level. Nonetheless, theinfluence of the drive force and drive force distribution on vehicle level handlingcharacteristics – of interest for this work – have been unsatisfactory described in thereviewed literature. Based on this fact, and the relevance of this needed knowledgefor 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 theturning radius changes with speed given a fixed steering input; and the lateral grip,which is defined by the maximum possible steady-state lateral acceleration. Oneobjective of this study is to show how the drive force distribution can be optimizedfor maximum lateral grip and constant understeer. For this purpose the existingtheory and methods are developed with the aim to show the influence of the driveforce distribution on the understeer and lateral grip.Finally, by means of computer simulations, the developed theory was verifiedand the influence of effects which should be included in future development of thepresented theory was identified.Overall, this present work has successfully expanded the knowledge in the fieldof combined acceleration and cornering, from tire level properties to vehicle levelcharacteristics