Simple explanations we love! We hope this helps everyone.
CASTER, CAMBER, ALIGNMENT AND OTHER VOODOO
This is the general term used to gloss over the next three points:
This is the forward (negative) or backwards (positive) tilt of the spindle steering axis. It is what causes your steering to ‘self-centre’. Correct caster is almost always positive. Look at a bicycle – the front forks have a quite obvious rearward tilt to the handlebars, and so are giving positive caster. The whole point of it is to give the car (or bike) a noticeable center point of the steering – a point where it’s obvious the car will be going in straight line.
Camber is the tilt of the top of a wheel inward or outwards (negative or positive). Proper camber (along with toe and caster) make sure that the tire tread surface is as flat as possible on the road surface. If your camber is out, you’ll get tire wear. Too much negative camber (wheels tilt inwards) causes tread and tire wear on the inside edge of the tire. Consequently, too much positive camber causes wear on the outside edge.
Negative camber is what counteracts the tendency of the inside wheel during a turn to lean out from the center of the vehicle. 0 or Negative camber is almost always desired. Positive camber almost always creates handling problems.
The technical reason for this is because when the tires on the inside of the turn have negative camber, they will tend to go toward 0 camber, using the contact patch more efficiently during the turn. If the tires had positive camber, during a turn, the inside wheels would tend to even more positive camber, compromising the efficiency of the contact patch because the tire would effectively only be riding on its outer edge.
As a car rolls in turns, or as the suspension compresses and extends over undulations in the road, the camber changes with respect to the ground (because the suspension mounts move relative to the road surface). To try to minimize this change, and keep the tire in the best alignment possible with the road, the suspension parts connected to the top and bottom of the wheel hub carrier are normally designed to move in a different arcs (relative to its lateral location). ie. the top control arms / McPherson strut etc. move in a different arc to the lower control arms and links. This causes a dynamic change in camber with suspension movement, meaning the more it compresses, the more it increases (typically) camber, referred to as camber gain. This counteracts the effects of the suspension mounting points moving in relation to the road.
TOE IN & OUT
‘Toe’ is the term given to the left-right alignment of the front wheels relative to each other. Toe-in is where the front edge of the wheels are closer together than the rear, and toe-out is the opposite. Toe-in counteracts the tendency for the wheels to toe-out under power, like hard acceleration or at motorway speeds (where toe-in disappears). Toe-out counteracts the tendency for the front wheels to toe-in when turning at motorway speeds. It’s all a bit bizarre and contradictory, but it does make a difference. A typical symptom of too much toe-in will be excessive wear and feathering on the outer edges of the tire tread section. Similarly, too much toe-out will cause the same feathering wear patterns on the inner edges of the tread pattern.
A nice description of toe-in and toe-out:
As a front-wheel-drive car pulls itself forwards, the wheels will tend to pivot around the king-pins, and thus towards the center of the car. To ensure they end up straight ahead, they should sit with a slight toe-out when at rest.
A rear-wheel-drive car pushes itself forward, and the front wheels are rotated by friction… thus they will tend to want to trail the king-pins, and therefore will want to splay apart. To ensure that they run parallel when rolling, they should be given some toe-in when at rest.
The perfect 4WD car will have neutral pressure on the front wheels, so have neither toe-in or toe-out… however very few companies make the perfect 4WD, so some will have a small amount to toe-in/out, depending on the dominant axle.