This is a major parameter of front-end geometry and overall weight distribution on a kart. The caster is the tilt angle (front/back) of the kingpins (the bolts that the stub-axles pivot around). It regulates tire behavior through bends by raising the outside front wheel and lowering the inside front wheel. In turn, this causes more or less of a weight transfer to the front end and more or less of a load reduction on the rear inside wheel, sometimes great enough for it to lift off the ground.
Verify how the kart reacts on track depending on weather caster is positive or negative
Test 1: fully discharged (negative) caster, meaning: top eccentric shifted forward
Test 2: fully charged (positive) caster, meaning: top eccentric shifted backward
At the Adria Karting Raceway in Adria (RO), Italy
Well-kept even surface with good, but not excessive grip after a full race weekend. The test was performed using high-performance tires (specifically, Vega Whites)
Unloading the caster means reducing corner entry speed and having the rear tires gripped to the ground.
Theoretically, this results in heavy under-steering when you let off the throttle and a braking effect through corners due to the extra grip on the rear end (since both tires touch the ground and karts don’t have the differential). On the other hand, unloading the caster should also improve corner-exit performance, with the simultaneous low-grip / low brake-effect on the front end and the extra bite at the rear allowing for more immediate and free acceleration.
The kart is easy to handle: the steering feels light, the rear end is glued to the ground and there are no sudden slips. The set-up feels perfect through wide bends, but when you really start pushing, the kart tends to under-steer and loses drive at corner entry. So, to get the correct corner speed and trajectory, we need to brake earlier and for longer before entering a bend. The first direction change is when you feel the caster give out, but at mid-bend the kart become stable again and hold on well, also thanks to the rear track width setting. Corner exit is nice and easy: you can accelerate at any time without losing the rear end. Yet, if you let off the brake too early, you go out of line and cut the tail of the corner off.
As you can see, with the “front end unloaded” set-up (red line) we need to go through the right hairpin  slower than we would with a “maximum caster” set-up (blue line), even if we brake earlier going in. On the following hairpin , we again have to brake sooner, but our slow speed allows us to enter the corner well, even with a low caster. Actually, the kart feels easy to drive and, at the same time, handles the track very well: it has better corner-through speed than with a loaded set-up and accelerates more easily at corner-exit. As we approach the right bend , we need to let off the throttle earlier and going slower. From mid-bend , we get the opposite situation: the unloaded kart glides better and accelerates more easily when coming out of the bend.
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