This is a complex subject, but a basic understanding of it is useful to appreciate its importance, particularly in relation to car behaviour and tyre wear.
There are two fundamental aspects to be appreciated – the positioning of the front wheels relative to the axis on which they rotate when the steering wheel is turned, and the positioning of the wheel in relation to the road, and the direction of travel, all of which have significant influence on steering feel, directional stability, tyre wear, and road grip, and several others.
Camber is the angle of any wheel, front or rear, relative to vertical, the top of the wheel being tilted out with positive camber, or tilted in for negative camber. Both wheels on an axle need identical camber, or steering pull will occur, which can often arise with unrectified accident damage. Wheel camber usually alters with directional change, and with vertical loading, as the wheel pivots, rises, and drops on its suspension. The old Vokswagen Beetle and Triumph Herald both had swing-axle rear suspension that developed extreme positive camber on braking, with considerable weight transfer from rear to front, that reduced back wheel grip and often resulted in a rear wheel skid. Early Hillman Imps, with swing axle front suspension, exhibited the opposite extreme under power, with loss of front wheel grip, due to extreme positive camber at the front wheels. Today’s multi-link rear suspensions avoid such dangerous changes of camber, although some cars are still set up with some positive camber to induce understeer, rather than oversteer.
As the camber changes with suspension movement, so the Toe may change, this being the direction the tyres point forwards relative to the centreline of the vehicle. Both wheels pointing inwards, or pigeon-toed, is toe-in, while wheels pointing outwards, or duck-footed, is toe-out. Toe is the most critical factor in setting up steering, and the setting is easily disturbed with even minor kerb collisions, resulting in changes in steering feel and tyre wear. Toe settings can also be used to alter a vehicle’s handling traits. Increasing toe-in will reduce oversteer, steady the car, and enhance high-speed stability and self-centre effect. It is Toe that garages may alter when checking steering geometry, and provision for its adjustment is made by threading a link to the ends of the steering arms, and providing a suitable locking nut. If toe is not corrected by equal adjustments on both steering arms, the proper steering wheel central position may be disturbed.
Caster is the angle of the steering pivot viewed from the side; when the wheel is in front of the axis (best visualised on a bicycle) the caster is positive, as on nearly all road cars, although negative caster was used to lighten steering before power steering arrived. Three to five degrees of positive caster is a typical range for most cars, and it is most unusual for the factory specified caster angle to be changed. So the steering axis, most usually now a suspension strut, is not vertical, but forward inclined at the bottom, and the same goes for its position from side to side; the strut’s Steering Axis Inclination will usually incline outwards at the bottom, for stability and reduced steering effort.
The somewhat mystical Scrub Radius describes the relationship between the tyre contact patch and the point where a projection of the strut’s steering axis touches the ground. If this distance is large, it can lead to instability in braking. With MacPherson strut front suspension, and since the introduction of anti-lock braking systems, negative scrub radius has become a standard design feature. Scrub radius can be significantly affected by fitting alternative road wheels with different offset and such changes should only be made after taking professional advice.