The Toe

Toe is a fundamental parameter of kart performance that affects corner entry handling, straight-line speed and tyre wear

WHAT IS IT?

The toe is the angle formed between front wheels and the kart’s longitudinal axis.

This angle is responsible for how fast the kart goes on straights, how it handles corner entry, and how tyres wear.

Taking the distance between the front tyres and that between the rear tyres, toe angle can be calculated based on this formula:

TgA = (Lf – Lr)/2/ D

A = arctg ((Lf – Lr)/2/D)

TgA is the tangent of the toe angle; Lf and Lr are front and rear distance between tyres; D is tire wheel diameter.

Maths aside, measuring toe by using alignment discs on the front hubs instead yields more precise and effective results.

Toe-out (positive toe angle) is when front tyres are farther apart compared to rear tyres, i.e. they are angled outwards. Vice versa, toe-in (negative toe) is when they are closer together than rear tyres, i.e. they are angled inwards. Zero toe is when Lf = Lr, i.e. front tyres are perfectly parallel to the kart, just like rear tyres.

THE STRUCTURAL ELEMENTS RESPONSIBLE FOR TOE ARE THE STEERING RODS, WHICH CONNECT THE STUB AXLES TO THE STEERING COLUMN

HOW DOES IT WORK?

Zero toe is not always ideal. First, even if you have zero toe (perfectly parallel front tyres) when the kart is still, the interplay between spindles and hubs and the normal deformation of the chassis in motion will offset the angle. In fact, the rolling resistance of tyres (drag) generates forces that are parallel to the kart’s line of motion, but that act in the opposite direction.

The torque produced by these forces acts on the spindle’s rotation axis, causing the toe angle to “open” out. So, you lose the zero setting you gave the kart in the pit box.

To make sure toe remains at zero also while driving (dynamic toe), you must set a negative value (toe-in) when the kart is the static phase. And the higher the grip and track speed, the greater the toe-in (negative angle) you must set on the kart.

Now, why is a dynamic zero toe desirable? Because it guarantees good results in terms of straight-line speed, keeping the front wheels be perfectly parallel to the direction of travel. So why ever set a positive toe (toe-out)? Because with a positive setting, the kart loses some speed, but, on the other hand, it handles corner entry better. In fact, it widens the outward angle of the inside front tyre relative to corner trajectory, increasing the braking and rotation effect it has on the kart.

IN PRACTICE

On a track with long straights and fast wide bends, you need to start with a slight toe-in (negative) setting. If there are high grip conditions, you need to “close in” even more (go further on the negative scale).

Vice versa, on a track with low-grip and that is highly technical (i.e. with a lot of bends that call for slower driving), you need to optimize corner entry performance. And to do so, you need to set the kart at toe-out. Why? In poor grip and low speed conditions, the load transfer from the inside tyres to the outside tyres is smaller and angling the front tyres outward (toe-out) helps the inside front tyre “work better” through bends.

Basically, it allows it to have a greater effect on the kart’s overall grip and directionality through corners.

Second, with the tyres angled outwards, you get high tyre temperatures on the straight, because a greater portion of the footprint rubs against the track surface, and this yields extra bite when going into corners right after the straight. Keep in mind that, of course, toe-out also leads to greater tyre wear up front.

Third, in low grip conditions, positive toe settings also help compensate for the fact that tyres have lower drag on straights and wide bends. In addition, on technical layouts with low grip the toe-out effect tends to be reduced to a minimum; so set it “wider”, because you’ll lose some anyways.

Finally, keep in mind that when it rains, the low grip/low speed effects we just described are amplified, turning tyre temperature into a key performance factor – on a wet track, the recommended toe-out setting is as far as 8 mm.

QUICK TIPS

On a dry track, zero toe or even a slightly negative toe (toe-in) is recommended

On a wet track, you need to open the toe (toe-out) – if necessary (i.e. very wet and slippery) moving up in the positive range

On low-grip tracks, toe-out yields better corner handling

On high-grip tracks, toe-in will optimize straight-line speed

On “technical” tracks (layouts with many bends), toe-out is generally better

On “fast” tracks (with long straights and wide bends), toe-in is better

SET YOUR TOE RIGHT: HERE IS HOW

There are different ways to set the toe: from the proper and precise alignment-disc method to the quick and “dirty” do-it-yourself strategy.

The most effective and common method to set the toe is with alignment discs.

Remove the front wheels and set the discs on the stub axle pins, making sure you have the same number of spacers on each side and that the discs are equally spaced relative to the kart’s central longitudinal axis. Set the steering wheel straight and block it.

Now measure how far apart the discs are, from one median point to the other, first on the front, then on the back: the ratio between the two measurements gives you the toe. If the discs are perfectly parallel (same distance front and back), you have zero toe.

To adjust the toe, act on the steering rods: measure them (from the uniball bolts on the steering column to the spindle bolts), loosen up the bolts (with a 13 mm key) and increase or decrease rod extension (identical on each side). Secure the bolts, leaving enough leeway for the uniballs to move.