An Explanation of 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 of rear ward lean of the kingpins (the bolts about which the stub axles pivot around).

Each individual wheel is bolted onto a wheel hub, on each 'stub axle'. The stub axle also carries the brake disc and the bearings to allow free rotation of the road wheels (and additionally at the rear, to a drive shaft from the gearbox/ differential, to supply drive). The stub axle is not mounted directly to the body work - instead it is attached to wishbones by the kingpins that allow the wheel to move up and down so that the wheel can ride over bumps (see figure 1).

From figure 1 you can see that the upper and lower wish bones do not meet at the same point, but are separated by some vertical distance. The stub axle 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.

Figure 1: Double wishbone suspension

(Thanks to Dieter Könnecke for the animated image)

But the upper and lower mounting points are not directly above one another when the car is viewed side on. The top wishbone pivot (or king pin)  tends to be further back than the lower pivot (king pin). The axis therefore leans back from the vertical (the perpendicular) - which is described as positive caster. This is shown in figure 2. Figure 2. Caster angle on the MGF's front wheel

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 (Kingpin inclination also has an impact on this). 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/TF 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. The greater the caster angle, the greater the increase of negative camber on the outside, laiden wheel, and the more positive camber on the inside, unladen wheel. 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.

Caster also results in changes of the front wheel trim height with changing steering angles: the more camber, then at a given steering angle, the lower the inside wheel and the higher the outside wheel ride height.