The Explanation of the Caster Angle

The term tracking incorporates camber, caster and toe. Toe and camber (by the adjustment of ride height) are the only two components of tracking that can be adjusted on the MGF.

What is Caster?

Caster refers to the angle that the steering axis makes with the perpendicular. I shall explain this abstract statement.

Each individual wheel is bolted onto a wheel hub. The hub also carries the brake disc. Naturally, for the wheel to turn, the road wheel is not mounted rigidly to the car. Instead it is mounted on bearings to allow free rotation (or, additionally, to a drive shaft from the gearbox/ differential to supply drive). The hub carrier is not mounted directly to the body work either- instead it is attached to wishbones by ball joints that allow the wheel to move up and down so that the wheel can ride over bumps (see figure 2; A: steady state, B: wheel deflected by bump). Figure 1. Caster angle on the MGF's front wheel
From figure 2 you can see that the upper and lower wish bones do not meet at the same point, but are separated by some distance. The hub-carrier therefore can be likened to a rod that links the upper and lower wishbone, locating the freely rotating hub, and wheel itself. On the steered wheels, the wheel rotates about this axis: the steering axis.

But the upper and lower mounting points are not directly above one another when the car is viewed side on. The top wishbone pivot point tends to be further back than the lower pivot point. The steering axis therefore leans back from the vertical (the perpendicular). This is demonstrated in figure 1.

 Figure 2: Double wishbone suspension

(Thanks to Dieter Könnecke for the animated image)

Caster can be seen on bicycles- have you noticed how the front forks project forward of the handle bars? That is because of the caster. And it is no coincidence that the casters on your sofa have that name...

What use is caster?

Caster provides the self centring of the steering. More caster, the more self centring. By fine tuning of the caster and toe angles, very particular steering feel characteristics can be tuned in.

On the MGF it is not adjustable. Therefore this fine tuning is done at the vehicles' development stage by the manufacturer.

The problem with Caster

On the steered wheels, caster can influence the camber angle when steering lock is applied. This is usually not a problem, but when the camber is marginal (due to a low ride height) excess load can be placed on the edges of the tyres leading to excess wear. This is the primary reason for the front tyres suffering more from the dreaded tracking problems than the rears.