The caster is one of the key parameters of the front axle: this is the angle formed by the axis of the bolt that connects the stub axles to the C of the chassis relative to a hypothetical axis perpendicular to the ground. For a kart, which has no differential on the rear axle, the caster is a key factor which determines different aspects of the set-up. It is thanks to the caster, for example, that on bends, the front wheels assume different heights when the steering wheel is turned, allowing the correct lifting of the inner rear wheel.
Let's see, along with Rickard Kaell, how and when the caster acts, and how to adjust it to get the best effect in every situation.
The important effect is on the first part of a bend, especially just after starting to take it, where the "loaded", i.e. increased caster, allows immediacy and precision of insertion as soon as the steering wheel is turned.
This is due to the fact that, with a "loaded" caster, there is a greater difference in height between the front wheels: therefore, there is greater ground pressure and at the same time a greater and more immediate rise of the inner rear wheel. This frees the rear axle and helps bends to be taken.
As mentioned, the caster has its maximum effect when going into bends, up to half-way; then the effect of this angle is less felt. In this situation, it works differently with respect to the camber, which, on the other hand, has a great influence on the behaviour of the kart also from the middle of the bend and coming out of it.
Loading the front axle, increasing the caster angle entails harder steering, also because the front wheels have a higher cantilever height and, therefore, it is more difficult to turn the steering wheel. At the same time, the front tyres have more grip on the ground. The kart in general will be less stable, because the inner rear wheel will rise first and higher, and in this condition the rear wheel tends to slide more easily and more suddenly.
A very heavy caster greatly reduces the sliding of the front wheels and they heat up. Conversely, the back axle is less loaded and tends to slip more easily, firstly warming up the rear tyres, which heat up more tha the other tyres. Of course, an unloaded caster will give the opposite effect, with more rear grip and greater sliding (and heating) of the front tyres.