The angle that determines how inclined and how high front tires are when going through bends: this is a good working definition of “caster”. Technically, it is the angle that forms between the axis vertical to the ground and the axis parallel to the imaginary line that connects tires and chassis. As we said when discussing the camber, the C bends
of kart chassis are manufactured so that camber and caster are optimal for standard driving. However, if you race competitively, adjusting the caster allows to optimize the kart’s performance for specific pavement conditions, track layout, and engine type (like KF or KZ). This is why manufacturers equip their products with caster management systems.
In theory, what holds for the camber also holds for the caster. You can adjust either angle by shifting the eccentrics, or similar components, that control the tilt angle of the bolt that connects the spindle pins (and therefore the tires) to the C bends of the chassis. Just like with the camber, fitting the C bends with two eccentrics each, one above and one below, offers a wider adjustment range for each angle and, more importantly, allows to adjust each one independently of the other.
Yet, to make life easier, all the main manufacturers have developed alternative camber and caster adjustment systems that we compare in detail at the end of this article. So far everything seems the same for camber and caster, so let’s see specifically how the caster works. This angle determines a change in the height of the front tires when steering. This factor is key on karts, since they don’t have a differential system that compensates for the fact that when you steer through a bend the rear inside tire covers less distance (rotates less) than the outside rear tire. The only way to solve this “problem” is to allow the inside rear tire to lift up, or at least to release some of the vertical weight that pushes it into the ground. This way, the tire that leans into
the bend is the only one that really grips to the ground. Of course, the tighter the bend, the greater the difference in distance covered between the two rear tires. Not only, a tighter bend also means harder steering, which increases the difference in height between the front tires and this, in turn, causes the rear inside tire to lift off the ground more. In fact, the uneven height of front tires causes a weight shift that sums up to the effect of centrifugal forces, which tend to push down on the outside tires. Basically, also due to the kart’s deceleration, when you go through a bend, the weight shifts to the front tires and the rear outside tire, causing the inside rear tire to lift. So, to come full circle, the wider the caster angle, the greater the weight shift on these three tires, especially the two up front.
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