Castor

Castor

Caster angle is the tilt of the king pin or ball joint centre line from the vertical towards either the front (negative caster) or rear (positive caster) of the vehicle (Fig. 27.15).



The weight of automobile having positive castor tends to turn a wheel inward to allow the body to lower. Negative castor causes an outward turning effect. Wheel castor enables the driver to feel the straight ahead position so that he may steer in a straight path.   



 During cornering, a torque must be exerted on the steering wheel to overcome the self centring or castor action, which tends to keep the wheels pointing straight ahead. The castor angle produces a trailing effect and hence gives the directional stability. Incorrect castor can produce difficulties like hand steering, pulling to one side when brakes are applied and shimmy. To understand the action of this steering feature, the operation of a simple furniture castor (Fig. 27.15A) fitted to a trolley may be considered. Force acting on the trolley causes it to move in the direction of the force. The figure illustrates the effect of this force on the castor. The force, F, acting at the pivot and the resistance acting at the wheel produce a couple, which rotates the castor to a position where the wheel is following the line of thrust.





On a motor vehicle the pivot centre line normally strikes the road in front of the centre of contact of the wheel, so that the wheel follows the path taken by the pivot centre line, which is always in front of the wheel. Castor can be obtained by mounting a vertical king pin infront of the wheel (Fig. 27.15B) or by inclining the king pin forward at the bottom (Fig. 27.15C).



 The latter arrangement is simpler, and hence used by most manufactures. The caster angle of between 2 to 5 degrees is generally provided and the tolerance is normally +0.5 degree for all steering angles.

 Fig. 27.15. Wheel castor.

In Fig. 27. 15D the pivot centre line makes contact with the road in front of the centre line of the wheel, providing positive castor. This layout is normally used for a rear wheel drive arrangement but is unsuitable for front and four-wheel drive vehicles. This is because the conditions shown in Fig. 27.15D are then reversed.

Vehicles using the front wheels drive generally have negative castor (Fig. 27.15C), however, the manufacturer's data should be referred while checking the steering geometry. Castor adjustments can be made on most cars by repositioning the inner pivots of the upper control arm to move the upper ball joint forward and backward.