Caster always reminds me of bicycles - yes, the humble bike has to 'handle' as well, and in fact bikes of the human powered variety have all of the same variables and make the same compromises as do motor bikes and, yes, cars. Stiffer suspension? Sharper handling? Lower centre of gravity? Steering speed and feel? Carbon fibre construction? Need I go on? In many ways, including historically, bicycles lead the way and a top of the range racing bicycle will have more in common with Formula One motor racing than your average tin-top motor vehicle. And what of this connection betwixt caster and bikes? Well positive caster is a self-centering force, and if you want to ride your bike hands-off, you need some of that force!

What is caster? If you look at your typical steering setup, you will see that the upper balljoint sits behind the centre of your front wheel and the lower balljoint sits ahead of centre. So if a line is drawn vertically through the wheel's centre and another line is drawn between the upper and lower balljoints, the angle between those lines is the caster angle, and in this example it's a positive value. The line between the balljoints is the steering pivot axis, by the way.

Positive caster is a self centering force. Think about this for a while. The wheels are in balance, steering straight ahead and you turn the wheel to the right. If there was no natural self-centering, your right turn continues on, ad infinitum, until you apply a correction. If you let go of the steering wheel, you remain turning. You probably haven't thought about this before, but in fact your car has a tendency to return to straightahead, ie it has some degree of positive caster.

A high degree of caster will give a strong self centering effect and great stability at high speed, but at the cost of heavy steering. However a small caster angle will give lighetr steering but some straightline wander and instability. Its suddenly not as simple as you thought. There are decisions to be made here, and they are made by automotive engineers... unless you do some modifying of your own, for better or worse!

Typically, the shorter the wheelbase the more 'twitchy' the car, so a large caster angle is employed to give stability. Alas, you also get heavy steering, but that's why power steering was invented, wasn't it? Of course, if you're racing said car you may not want the additional drain of the power steering pump. Another decision. Or it may be an extra-cost option on a cheap small car, like the Hyundai Excel (aka Accent).

You may find a large car will have small, even negative caster values. In fact, tyres generate a 'caster effect' of sorts, possessing a natural elasticity that will pull the car back into line; so the larger car, with mass and momentum on its side, may well have enough caster action from the tyres alone to satisfy the driver.