Castor angle measurement – TC

Robert Lyell has kindly sent me an article on castor angle measurement; this is reproduced in the next few pages. However, I thought it would first be worth reminding readers of Eric Worpe’s writing on the subject of castor angle and self-centering, which was covered in Issue 20 of TTT 2 as part of the series of articles on TC steering. This follows:

The castor angle is made up from two components. The beam axle has an inherent castor angle of 3 degrees. (see Table 1) this is augmented by the slope of the front springs, which for the TA and TB was also 3 degrees.

However, when the rear trunnions were exchanged for shackles on the TC, it resulted in an increased spring slope of 5 degrees, giving a total of 8 degrees, as opposed to 6 degrees for the TA and TB. Subsequently, wedges of 2.5 degrees were offered to reduce the total castor angle of the TC to 5.5 degrees. (see Table 1 and the illustration at Figure 1)

Table 1

Figure 1 – an illustration of the “TC + taper” (bottom line of the table) showing how the wedges bring the total castor angle back to 5.5 degrees i.e. 5 degrees spring slope plus 3 degrees castor on axle less 2.5 degrees axle wedge.

Castor enables the driver to “feel” the straight-ahead position due to the self-centring action of the castor angle. Fig. 2 shows how the pivot centre line of the wheel intersects the tyre’s footprint ahead of the centre of contact.

Although the castor steering feature is similar to a castor wheel fitted to a trolley, where the wheel’s centre trails behind the pivot axis, an alternative explanation is more suited to the specific geometry of a car.

Figure 2 – showing the self-centring effect.

Turning the wheel about the pivot axis results in an edge of the tyre lifting up the wheel (see Figure 3); this can be illustrated by holding a tin can in the hand and holding one’s arm vertically with the can resting on a table. Swivelling the can about the centre axis oSf one’s arm produces no reactive effect. However, inclining one’s arm to the vertical and swivelling the can should cause one edge of the can to lift.

The weight of the car brings about a “reset” effect, forcing the wheel to return to its lowest (straight ahead) base level. Thus, the castor return action is mainly a function of the castor angle, weight of car and width of tyre.

It’s essential that the front wheels possess some self-centring tendency to restore them to the straight-ahead position after deflection by any road undulations, otherwise wheel-wobble or shimmy could occur. Too much castor produces hard steering, whereas too little causes wander.

Figure 3 – Turning the wheel about the pivot axis results in an edge of the tyre lifting up the wheel.

Robert Lyell’s article follows…………

Castor angle measurement – TC……….Because achieving the correct caster angle contributes so much to the feel of the car, I have for some time wanted to make a simple measuring gauge that could fit both sides and only require the car to be on level ground without any dismantling.

Staying at home was the catalyst and I have made one, but simple it is not. Anyway, I am pleased to share my design in case anyone else is inspired to make a copy or improve on it.

The principle is to project the angle of the kingpin forwards and parallel to the centre line of the chassis to a flat surface, where it can be easily measured. This is achieved by two arms of exactly the same length locating on the outside diameter of the short king pin protrusion at the bottom and the central ¼ BSF tapped hole at the top. Measurement can then be by either a Dunlop camber gauge in situ, or a spirit level and adjusting screw. With the screw locked in place the gauge can be removed to the bench and its protrusion measured with a Vernier.

Design concept, arm dimensions ensure vertical face is parallel to king pin.

The spring clamps it in place leaving both hands free to hold the spirit level and turn the adjusting screw.

Shown in place, wheel on full lock.

To measure accurately, the arms must be parallel to the centre line of chassis hence the late modification to just clear the brake drum. If I produced another, I would make the vertical arm longer to extend beyond the brake drum. The kingpin inclination requires the short ‘wing’ extensions at the bottom in order to take a vertical reading.

Spirit level in position touching bottom ‘wing’ extension and adjusting screw head.

Measured on the bench the nearside represents a triangle whose sides are 218mm (fixed length), 219mm and 20mm (adjusting screw). By using a piece of freely available software or a scientific calculator the acute angle is calculated at 5.2 degrees against a specification of 5.5 with taper plates on the axle (see Eric Worpe’s Table 1). The offside adjusting screw measured 21mm which calculates 5.5 degrees.