# Caster (steering)

Steering geometry of a two-wheeler
The swivel castors of a shopping trolley swivel freely around a vertical steering axis and align themselves automatically in the direction of travel because the wheel contact point (around the caster) is offset from the steering axis. Due to the friction at the wheel contact point, the wheel has a tendency to follow the steering axis. With a driven wheel this would be reversed.

As a follow-up or lag distance is in road vehicles ( DIN ISO the distance between the intersection of the steering axis with the road surface 8855) (tracking point) and the wheel contact point in the side view of the wheel , respectively. The wheel contact point is usually in the longitudinal direction behind the track point (positive caster) so that the wheel lags behind the steering axle. It is therefore stable even without steering forces - such as when pushing a shopping trolley or when driving hands-free on a two-wheeler.

When the wheels are steered, the steering axis is usually inclined forward. This axis inclination is called the steering head angle for two-wheeled vehicles and the caster angle for automobiles . Caster distance and caster angle are two of several geometrical variables that determine steering and driving behavior . They can be selected independently of one another.

## purpose

When there is a lateral force, the overtravel creates a torque around the steering axis (spread axis in automobile construction ). The wheel tries to align this restoring torque in such a way that the lateral force is lower. If the wheel is not held in place (e.g. with a shopping cart), a state free of lateral forces is established.

If you let go of the steering wheel in two-lane vehicles, the rolling vehicle will automatically steer back to (almost) the straight-ahead position. A large overtravel distance causes good resetting, but causes high steering forces. The restoring torque when cornering can be felt on the steering wheel. Together with the lateral acceleration, it gives the driver feedback about cornering and the contact between the tires and the road. At the limit or when there is a low coefficient of friction between the tire and the road, the tire lag and thus the restoring torque are reduced . The driver receives feedback on tire-road contact via the steering wheel.

The caster distance is a function of the wheel lock because of the caster angle. For bicycles z. B. with a 90 degree wheel lock, the trailing distance is negative.

## Related sizes

• The "caster path" in a plane parallel to the road surface through the center of the wheel is referred to as the caster offset . It is positive when the spread axis is in front of the wheel center.
• If the spreading axis is not in the wheel plane, as is the case with cars, for example, then the distance between the track point and the wheel contact point in the view from behind is the scrub radius .

## Tire lag

Laterally deflected rubber particles of the tire profile of a tire that rolls at a slip angle (upwards) when cornering . The deflection of the rubber particles is symbolically indicated by springs.

Regardless of the geometric caster, the rubber tire has an additional effect, the so-called tire caster. The profile particles deform according to the skew angle elastic since they arriving in the Latsch first adhere to the road, and return to the starting position until the end of Latsch. A triangular tension is created. The center of gravity of this tension in the linear area of ​​the tire is shifted backwards about one sixth of the length of the contact area compared to the wheel contact point.

## Individual evidence

1. Wolfgang Matschinsky: Determination of mechanical parameters of wheel suspensions . Dissertation approved by the Faculty of Mechanical Engineering at the Technical University of Hanover to obtain the academic degree of Doctoral Engineer, 1992, p. 184.
2. Peter Pfeffer, Manfred Harrer (ed.): Steering manual : Steering systems, steering feel, driving dynamics of motor vehicles . Vieweg + Teubner, 2011, ISBN 978-3-8348-0751-9 , pp. 57 ( limited preview in Google Book search).
3. Jürgen Stoffregen: Motorcycle technology: Basics and concepts of engine, drive and chassis . 7th edition. Vieweg + Teubner, 2010, ISBN 978-3-8348-0698-7 , pp. 272 ( limited preview in Google Book search).
4. ^ Konrad Reif, Karl-Heinz Dietsche: Kraftfahrtechnisches Taschenbuch . 27th edition. Vieweg + Teubner, 2011, ISBN 978-3-8348-1440-1 , pp. 318 ( limited preview in Google Book search).