Caster and Camber – by Peter Winton

Arthur Krebs, the Frenchman who ran the car maker Panhard & Levassor, first patented the concept of an angle between the steering axis and the ground in 1896. Caster is the angle of a line between the steering pivots (their upper and lower ball joints) and the ground.

Trail is different to caster. A shopping trolley has mechanical trail – the axle is behind the steering axis, but it is undamped. Watch some trolleys as their wheels wobble from side to side when you push them. Trail makes the system stable (the trolley doesn’t make massive changes of direction depending on load) so adding some caster would damp out the wobble, and the steering would have better feel.

Trail and caster, in general, both aid steering. Trail adds feel, and caster adds damping; excessive caster makes the steering heavy. In racecars, caster can be used to optimize handling, as macho race drivers aren’t worried if the steering is a bit heavy, as long as the car is fast!

More caster improves camber gain in corners, so we use caster to adjust the handling of the car. On cars like the Corally SP12X and CEFX C12 with their swing-arm designs, there is a lot of camber gain in cornering, so small caster angles are used.

On the AE Dynamic Strut system (and its many imitators) moving the little white washers to various positions either side of the caster block adjusts the caster angle. Usually, we run one washer either side on AE systems. In general, more caster will give you more steering, but it isn’t an adjustment anyone makes regularly – the standard setting works for everyone, everywhere!

Oh, and those caster blocks – 10deg, 5deg and 0deg – change the caster as the car corners. They reduce the caster during the corner (as the suspension loads up) so giving the ideal steering characteristics through the turn. Changing the caster block doesn’t change the caster angle when the car is stationary.

Camber refers to a variety of curvatures and angles, and is used in road building (the camber of a road, producing a crown) and in aerodynamics to define the asymmetry between the top and bottom curves of an aerofoil. In automotive terms, it is defined as the angle of the wheel to the ground.

Camber can be positive or negative, and is used to ensure that the tyre is presented to the road as flat as possible. Camber angle is negative when the wheel leans in at the top, as the example of a wheel on the left side of the car below shows.

12th cars don’t use camber angle as a tuning aid, only to make sure that the front tyres wear flat. Tyres coned inwards (smaller diameter next to the chassis, larger diameter on the outside) need less camber. Reduce the camber angle on the AE front end by using the turnbuckles, and on Corally or CEFX swing-arm cars by reducing the camber angle when the car is static.

So, if increasing the caster angle increases the camber change, then what adjustment might you also make to keep the tyres flat on a 12th car? Caster and camber work together; make sure that when you change one, you check the other too.

Peter Winton