Oversteer (vehicle)

Oversteering Maserati 250 F

Oversteer describes the behavior of a vehicle on bends in which the slip angle on the rear wheels is greater than on the front wheels. The rear pushes outwards, the steering angle is less than the curve radius would correspond to. This behavior is typical for vehicles with a rear engine , but also occurs in vehicles with a front engine and rear-wheel drive.

The opposite behavior is called understeer . Understeering vehicles push their front wheels to the outer edge of the curve in a curve. This behavior is typical for front-wheel drive vehicles.

Vehicles that break out slightly at the rear are the VW Beetle and various series of Porsche sports cars (such as the 356 and 911 , especially the GT2 and GT3 models ), i.e. vehicles with a rear engine and a correspondingly high load on the rear axle .

The tendency of the vehicle to “make more curves” than is set on the steering wheel is considered less safe than understeer. The correction requires practice and an appropriate driving technique (see drifting ).

Balancing springs on the rear axle and stabilizers on the front axle counteract oversteer, and in modern vehicles electronically controlled systems such as the electronic stability program  (ESP) by automatically braking the front wheel on the outside of the bend.

definition

Stationary circular travel on a constant radius.

According to DIN ISO 8855, there is oversteer if the difference between the gradients of the steering angle and the Ackermann angle with regard to the lateral acceleration is less than zero during stationary circular travel . ${\displaystyle \delta }$ ${\displaystyle \delta _{A}}$  ${\displaystyle a_{y}}$

${\displaystyle {\frac {1}{i_{S}}}{\frac {d\delta _{H}}{da_{y}}}-{\frac {d\delta _{A}}{da_{y}}}<0}$

This definition has replaced an older one according to Olley, in which the slip angle difference of the front and rear axles was used. The basis of the evaluation is a diagram in which the steering angle and the Ackermann angle are shown as a function of the lateral acceleration. The Ackermann angle is constant on a constant curve radius. If the driver has to turn the steering wheel back with increasing lateral acceleration, there is oversteer. This definition according to Bergmann is based on the terms of the single-track model and describes the self-steering behavior over the entire lateral acceleration range. Since vehicles with a negative self-steering gradient become unstable at the critical speed , all cars are designed to understeer up to at least medium lateral accelerations. In contrast, in racing, often only the behavior at the adhesion limit ( slip limit ) is considered. A vehicle in which the rear axle first reaches the slip limit is referred to as oversteering. The vehicle becomes unstable and can only be stabilized again through active counter-steering or ESP.

literature

• Konrad Reif (ed.): Bosch basics of vehicle and engine technology. 1st edition, Vieweg + Teubner, Wiesbaden 2011, ISBN 978-3-8348-1598-9 .
• Manfred Mitschke: Dynamics of Motor Vehicles. 2nd completely revised edition, Volume C Driving behavior, Springer Verlag, Berlin 1990, ISBN 978-3-642-86471-1 .

Web links

Commons : Override  - collection of images, videos and audio files

• Definition of override / underride (accessed November 30, 2015)

Individual evidence

1. ^ Karl-Ludwig Haken: Fundamentals of automotive engineering . 4th edition. Hanser, 2015, ISBN 978-3-446-44216-0 , pp. 252 . ( limited preview in Google Book search)
2. ↑ Erich Schindler: Driving dynamics: Fundamentals of steering behavior and their application for vehicle control systems . expert verlag, 2007, ISBN 978-3-8169-2658-0 , p. 36 . ( limited preview in Google Book search)