# On the Influence of Suspension Geometry on Steering Feedback
## 1 Introduction  
As stated by the authors in [1, 2], the steering system and geometry have a primary impact on the tactile feel perceived by the driver through his hands acting on the steering wheel. This perception --often called \"steering feel\"-- is considered to be vital because steering is the driver's main line of communication with the car; distortion in this guidance channel makes every other perception more difficult to comprehend [3]. Friction in the steering line, for instance, must be accurately calibrated in order to prevent stick and slip effects that could heavily affect returnability and the on-centre feel, while providing enough damping at the same time [2]. According to [4], steering feel or steering torque feedback is widely regarded as an important aspect of the handling quality of a vehicle, as it is known to help the driver in reducing path-following errors. Some authors even suggest that, apart from eyesight, the driving action is mainly based on feedback communication through the steering system [5]. Therefore, any interaction between the steering system on one side, and the effects ofpower delivery, braking torque, load transfer and road surface irregularities on the other, should be very carefully handled in order not to alter such a vital feedback. The disturbance related to unequal braking forces on the two sides is called \"brake pull\", while the disturbance induced by uneven tractive forces on the front axle is known as \"torque steer\" [6; 7]. They are both undesirable because they upset the correlation between lateral acceleration (hence cornering speed), steering angle and steering effort. Standard front-wheel drive (FWD) cars are normally equipped with a so-called open or free differential delivering torque in equal amounts to the wheels on both sides. Therefore, torque steer effects can be a consequence of rarely found non-symmetrical designs between the left and right half-shaft geometry, such as different shaft length, rotational inertia and inclination angle [8]. Transient phenomena like powertrain movements over elastic mountings and large roll angles also play a role, while minor effects can be due to suspension geometry tolerances, tyre conicity and wear and asymmetric mass distribution as a consequence of vehicle loading [9]. Finally, whenever occasional wheelspin occurs under heavy acceleration, inertia effects can also induce unpleasant self-steering transients. Where present, a limited-slip differential (LSD) can help to mitigate traction problems in cars featuring a high torque-to-weight ratio or on low-friction surfaces. The LSD can also be tuned to improve handling and stability by generating a yaw moment influencing the handling characteristics [10; 11; 12; 13]. However, relevant torque steer problems were encountered when limited-slip differentials first appeared in front-wheel drive rally cars in the early 1970s, given the large asymmetry in terms of torque and tractive force generated under certain conditions [14; 15]. Peculiar geometries have been designed with the aim of minimising these effects. For example, the McPherson suspension with separate steering axis found in sporty Honda cars (see Figure 1) is aimed at reducing the wheel center arm (KO, see chapter 2). The very same concept can be found on the so-called RevoKnuckle(r) Ford geometry [16]. It should also be stated that it is not possible to eliminate the asymmetry of driving or braking torque applications completely. As a matter of fact, the resultant forces exchanged between the tyre and the ground can move laterally across the contact patch, especially on uneven road surfaces and with wide tyres, thus inducing torque steer or brake pull [6]. High-performance or special application vehicles can feature active differentials or torque vectoring devices aimed at extending the performance and stability envelopes by further enhancing yaw motion control [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28].