Having looked at the wheel and tyre combo from front-on, let's now look from above. If you have wheels that point absolutely straight-ahead, ie are in parallel, you have zero toe angle. If you don't turn the steering wheel, you'll just keep going straight ahead... However, it is more usual that you will find toe-in on the front wheels and zero-toe on the rears. Why is it so?

Firstly, what is toe-in? Well, looking again from above, if your wheels are pointing inwards, ie that a line drawn forward from the centre of each wheel will at some point ahead intersect, you have toe-in. If these same wheels are pointing outwards, such that the imaginary centre-lines forever increase the distance between them, you have spotted some toe-out.

Great. Obviously either situation will scrub the tyres to some degree, so it's yet another plot to line someone's pockets, in this case the tyre industry's. But there's more. Toe settings must be equal on each side or the car will wander, which is not good. You want to be in control of the car, not the other way around. In fact, toe settings can change as you drive, such that various forces will cause a toe change which results in wandering or unwanted directional changes. If the toe change was caused by body roll then you have experienced rollsteer. If it's caused by vertical suspension movement then we call it bumpsteer. The suspension designer will attempt to counteract these effects by various means, but it's always a compromise.

What effects are felt with toe changes? Well, toe-out tends to encourage oversteer whereas toe-in builds in stability and some initial understeer. Suddenly we can see why toe-in is common on the front wheels. And why toe settings overall are important in handling terms. But there's more! Under strong acceleration you will get a weight transfer front to rear. So the front suspension will lift and the rear will squat. Think of a Porsche 930 Turbo blasting off from the lights - do you get the picture? Now imagine what's happening front and rear to the respective suspension settings - bingo! - toe changes! Once again the suspension designer attempts to build in settings and methods to control these changes.