On the torque steer problem for front-wheel-drive electric cars

Abstract

Beyond the well-known benefits of electric and hybrid powertrains in terms of environmental impact, the peculiar torque curve of electric motors for automotive applications offers extensive opportunities for improved vehicle dynamics control, such as yaw moment control through the application of different tractive forces across the axle: the so-called Torque Vectoring [1]. This is made possible for instance by fitting an independent motor on each wheel of a drive axle. On the other side, whenever Torque Vectoring is achieved on the front axle it can give birth to the so called torque steer effect: an undesirable influence of tractive or braking torque on the steering [2]. This is perceived by the driver as a “tugging or pulling sensation in the steering wheel, or a veering of the vehicle from the intended path” [3]. The steering feedback and self-aligning properties, often considered a vital portion of the feedback required for safe and intuitive driving control [4, 5], are thus jeopardized, especially under heavy acceleration. This is very similar to what can be experienced on front-wheel-drive cars featuring a high torque-to-weight ratio, often requiring the adoption of a LSD or active differential device in order to optimize traction capabilities [6]. The paper presents an approach to the torque steer problem on high-performance electric and hybrid vehicles, where the effects of suspension and steering geometry as well as tyre contact patch load variation are taken into account and various design solutions are proposed as an improvement. The VI-grade® software tools for vehicle dynamics analysis are adopted, also in co-simulation with MATLAB-Simulink® whenever an active control strategy is used. The impact on steering feedback quality is assessed through a testing campaign carried out on a state-of-the-art driving simulator.