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Track Test Tech | The caster: how to work on it

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The caster: how to work on it

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.

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What

Verify how the kart reacts on track depending on weather caster is positive or negative

How

Test 1: fully discharged (negative) caster, meaning: top eccentric shifted forward
Test 2: fully charged (positive) caster, meaning: top eccentric shifted backward

Where

At the Adria Karting Raceway in Adria (RO), Italy

Conditions

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)

Weather
26°C / 79°F
IN THEORY
LOADED CASTER

The caster angle affects weight transfer through bends, with obvious consequences on grip.
A fully loaded caster (meaning, with the eccentric shifted backward to accentuate the pre-set angle formed by the Cs in the chassis) causes the maximum possible load transfer to the front tires and so better corner-entry performance. At the same time, it unloads the rear tires, especially the inside tire, causing it to lift off the ground. The result is more bite up front and less at the rear, which helps corner-entry, allowing to keep good speed, but reduces traction (and acceleration). Of course, a fully unloaded caster has the exact opposite effect.

TIRES AND GRIP

A greater caster angle causes front tires to work harder, so the surface tread becomes grainy. Vice versa, rear tires glide more easily, so the tread becomes very slick and smooth. Obviously, setting the caster as low as possible produces the opposite effect.
This said, on a track with low grip conditions and tight corners, caster will help corner entry, but the effect will have to be counter-balanced by an increase in rear-end grip.
Today, caster settings usually range from neutral to high, in order to keep good speed going into corners and be able to let off the brakes as late as possible coming out of them.

REAR END

As weight is transferred to the front end, the rear inside tire gets “unloaded”, losing basically all grip and often coming off the ground. On the rear, all work through bends will be done by the outside tire. The kart will get enough grip to keep in line, but won’t be able to accelerate until some weight shifts back onto the inside tire. This explains why a high caster setting results in delayed acceleration coming out of corners. In part, though, delayed corner entry is also a consequence of tire behavio: the elevated load on the front end causes the front tires to have excessive grip and so to function as brakes as soon as the kart steers even slightly in acceleration.

CASTER AND BENDS

Getting down to details: as you steer to enter a corner, if caster is set high, the front outside wheel “unloads” (lifts) more and the front inside tire goes down lower, generating a braking effect (keep in mind that with Ackermann angles, steering has greater effect on the front inside than the front outside tire). So, the higher the caster, the greater the weight transfer to the front-end of the kart and thus the greater the vertical force that pushes down on the front inside tire, loading it to the ground. The result of all this is an increase in front-end grip.

FULLY UNLOADED CASTER
53’’250
Set-up
Front width 5 notches
Rear width 140 cm
Camber Neutral
Caster Unloaded
Rear height Low
Front track Neutral
What we expected

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.

Driving impressions

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.

Analysis

As you can see, with the “front end unloaded” set-up (red line) we need to go through the right hairpin [1] slower than we would with a “maximum caster” set-up (blue line), even if we brake earlier going in. On the following hairpin [2], 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 [3], we need to let off the throttle earlier and going slower. From mid-bend [4], we get the opposite situation: the unloaded kart glides better and accelerates more easily when coming out of the bend.

Tires

The insufficient grip on the front tires results in a smooth tread and vice versa, the heavy load on the rear tires and relative bite leads to a very botchy tread surface.

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