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Written by Vic R   

As with all racing vehicles, correct and accurately set front-end geometry is vital to get the most out of any kart. Handling, grip, tyre wear and even engine performance can be adversely affected by poor settings caused by neglect and damaged or worn components.

A kart chassis is quite different to a car chassis, and because of this, so are the steering geometry settings required to make the kart work properly. Karts share all of the same steering geometries as a car, but by comparison, a kart's settings are somewhat exaggerated. A kart's combination of solid rear axle (no differential) and very short wheelbase, in relation to a very wide track, presents particular problems for front-end geometry.

If a kart were to have similar geometry settings to road or racing cars, it would suffer from severe understeer at the moment of corner turn-in. This is because the combined grip of the rear tyres would simply push the front wheels straight ahead. To overcome this problem, karts have steering geometry designed to lower the inside front wheel and raise the outside front wheel in relation to the chassis at corner turn-in (know as jacking effect).

This change of front wheel heights causes a weight transfer from the inside rear wheel to the outside rear wheel and the inside front wheel. This mechanically lifts the inside rear wheel off, or nearly off the track surface at the moment of turn-in. Once the kart is turned into the corner, this mechanical weight transfer becomes less important and is largely superseded by weight transfer due to cornering forces. Because of this weight transfer to the inside front tyre, most of the initial turn in front grip comes from this tyre. However, most of the mid corner and exit grip comes from the outside front tyre.

FWHeight

Almost every steering geometry angle and setting is designed around this need to unload weight from the inside rear tyre at turn-in. A kart that does not do this enough, or does it too much, will not handle well. If it does not lift the inside rear wheel enough, the front may tend to slide at turn-in, then suddenly gain front grip and flick the rear end into a slide. This can be one of the most difficult handling problems to drive with and is often mistaken for a rear grip problem since the rear slide can be the dominant sensation. If it lifts the inside rear too much, it can make the kart twitchy and difficult to drive smoothly. Getting this initial turn-in weight transfer just right is one of the most important factors in tuning the chassis and is mostly a function of correct front-end set-up.

Although it is very important to drive karts smoothly, it is also very possible to be too smooth when turning into a corner. If you turn the steering wheel too gently at turn-in, the kart may behave in a similar manner as if it won't transfer weight from the inside rear wheel. But this article is not about driving skills!

So let's have a look at what these various settings are and how they work:

  • Toe is the degree to which the front wheels point toward or away from each other. Front wheels pointing toward each other is toe-in and toe-out is the opposite of toe-in. Toe-in makes a kart more directionally stable, but can contribute to poor turn-in to corners. Toe-out can cause the kart to be directionally unstable, but can assist the kart turn-in to corners well. With toe-out, the inside front wheel moves down in relation to the chassis more than it will with zero toe or toe-in.
TOE_out
  • Toe either in or out, creates friction at the road contact patch of the tyre. This friction generates heat in the tyres (contributing to overheating in some conditions) and can lead to excessive wear. This heat energy comes from engine power that is not being used to accelerate the kart. Toe is adjusted by lengthening or shortening the tie-rods.
  • Camber is the degree to which the front wheels lean toward or away from each other, if the tops of the tyres are closer together than the bottom, then camber is negative and positive camber is the opposite of negative camber. To maximise grip when cornering, it is highly desirable to have as much of the two outside tyre's rubber on the track as possible. Camber is the setting mostly responsible for maintaining maximum rubber on the road in corners.
  • Camber is adjusted by rotating the camber adjusters at the stub axle mounting. If the king-pin bearings are housed in the stub axle, caster and king-pin inclination (see below) will also change when you adjust camber settings (karts with the king-pin bearings housed in the chassis have no adjustment for caster or king-pin inclination). Not all karts have adjustable camber, caster or king-pin inclination, but it is not usually difficult to fit adjusters if you need them. Generally SL tyres should have zero camber and Open CIK set at 0-2mm positive (wider at tope of tyre)
  • Caster angle is the rearward lean of the king-pins (the bolts that the stub-axles pivot around). Caster angle is responsible for most of the self-centring action of the steering and is an important factor in lowering the inside front wheel and raising the outside front wheel of the kart at corner turn-in. The greater the caster angle, the greater the height changes of the front wheels.

Caster also causes change of camber when the steering is turned, resulting in more negative camber on the outside front wheel and more positive camber on the inside front wheel. Caster is adjusted using the 'camber' adjusters.

 

Scrub_Radius

King-pin inclination (KPI) is the inward lean of the king-pins (up, towards the centreline of the kart). KPI causes some of the self centring action of the steering. It also modifies the amount of camber change caused by the caster angle when the steering is turned, lessening negative camber gain on the outside front wheel and increasing positive camber gain on the inside front wheel. It would be unusual to deliberately alter KPI, but it can be adjusted using the 'camber' adjusters.

  • Scrub radius (also called king-pin offset) is the distance from the centre of the tyre at ground level to the point where a line drawn through the king-pin axis intersects the ground. Scrub radius works with the caster angle to alter front wheel heights in relation to the chassis. The greater the scrub radius, the greater the front wheel height change. Increasing scrub radius will also widen the front track. This track increase effectively softens the front end of the chassis, possibly increasing grip. Scrub radius is adjusted using the track spacers on the stub axles.
  • Ackermann effect is caused by the relationship between the position of the king-pins and the position of the outer tie-rod ends (the inward angle of the steering arms) and by the use of two separate inner tie-rod end mounting positions. Ackermann effect causes the inside front wheel to turn substantially more than the outside front wheel. Karts employ far more Ackermann effect than almost any other type of vehicle and it is used for somewhat different purposes than on a car.

A car uses Ackermann geometry to minimise tyre scrub when the vehicle is turning a corner. A kart (mostly) uses Ackermann effect (in conjunction with caster angle and scrub radius), to maximise the inside front wheel's downward movement as much as possible (by making it turn more), in order to raise the inside rear wheel at turn-in (jacking effect). Some karts have adjustable Ackermann, involving the use of different length tie-rods, and mounting them in different holes on the steering arms and/or steering column. Increasing Ackerman, increases jacking effect which is beneficial on tracks with tight turns.

Look around the pits and you will probably see quite a few karts using noticeably negative camber settings. Some of these karts will be this way through neglect, but many will have been deliberately set-up this way. Some kart racers see many full sized racing cars using pronounced negative camber settings and conclude that if it works for them, it should work for me too. Unfortunately the only problem with this theory is that most racing car tyres use radial construction and have very soft sidewalls, while kart tyres are made with cross-ply (or bias-belted) construction and have much stiffer sidewalls. A cross-ply racing tyre doesn't work well at large camber settings for this reason.

Many racers will spend a lot of money having their engines blueprinted and getting the latest new pipe for their kart, in the belief that the only way to go any faster is to get more out of the motor. Yet many of these racers are simply wasting some of the engine power they already have, and every little bit counts!

Engine power can be unnecessarily wasted in three ways, brake pad drag, friction in the wheel bearings, and incorrect wheel alignment. These problems cause an increase in the rolling resistance of the kart, which means more power is required to allow the kart to maximise it's speed and acceleration. Since the engine doesn't magically gain power because the kart has more rolling resistance, it goes just that bit slower. If the increased rolling resistance is due to bad alignment, the kart will probably handle poorly as well.

Try thinking about it this way, if you're losing only ½% per lap to the kart in front due to poor alignment (or any other reason) then in ten laps, on a 700 metre track you will lose 35 metres. It doesn't even take a kart length to lose a race!

So what is good alignment? A well aligned kart will have the toe and camber settings at, or close to zero toe and camber. This will ensure that the tyres are being used as they were designed, not slowing the kart on the straights and also maintaining a wide patch of rubber on the track in corners. It should also have enough scrub radius and caster to adequately transfer weight from the inside rear tyre at turn in (a kart should effectively be almost a three wheeled vehicle in corners).

Most karts, most of the time, will probably handle and accelerate better with toe set to absolute zero. Sometimes slight toe-out will help turn-in to corners, but rarely more than two millimetres (except in wet conditions, when larger toe-out settings can be helpful). Setting camber to zero will nearly always be the best starting point, and can be fine tuned using tyre wear as a guide, or tyre temperatures across the tread. A very general rule of thumb is; the less available grip, the more scrub radius, caster and tyre pressure should be used. So how do I achieve accurate alignment? There are several wheel alignment methods and tools available for this purpose.

Methods not requiring special equipment:

  • The cheapest way (free!) and most commonly used to set toe, is the old scribe a line around the tyre method. This involves (oddly enough) scribing a line around the circumference of the tyre, setting the steering straight ahead (or as straight as you can guess), then measuring between the scribed lines at the front and the back of the tyres with a tape measure. Any difference in these measurements (in mm's) is the toe setting (more or less). The downsides of this method are that it is not good for measuring camber and can be somewhat time consuming and awkward. It does not set the front wheels equally with the kart's centreline or rear axle and the effect of Ackermann geometry can throw the alignment off.
  • Also free, and somewhat more accurate, is stringlining the kart. This involves accurately arranging parallel stringlines along each side of the kart and measuring in from the stringlines to the appropriate points on the wheel rims. The downsides of this method are that it requires careful preparation, a flat floor and lots of patience. A distinct advantage of this method is the ability to align the kart with the driver seated in it (dynamic alignment).

Dynamic alignment is definitely an advantage because the geometry settings of most karts will significantly alter when the driver's weight is placed in it. Some karts won't change toe at all with driver weight, but most will suffer some amount of toe change, up to three millimetres is not uncommon. This is equivalent to about 8 or 10mm of inaccuracy on full sized car tyres. Just about every kart ever made will change camber with driver weight, more so than toe change. Most karts will toe-in and gain negative camber with driver weight. To achieve dynamic zero toe and camber, most karts will need some amount of toe-out and positive camber without the driver seated. The biggest downside of stringlining is that it is almost impossible do at the track.

Special tools available:

  • Laser Alignment System uses laser beams for easy and quick alignment.  They can be fitted and removed without having to remove front wheels,
  • At the stratospheric top end of the market, is the 'Pro-K Laser Toe Gauge'. This device is also produced by an American company, 'Advanced Racing Technologies', and is designed to measure toe only. It utilises a laser beam passed underneath the chassis to a second unit that reflects the beam back to the first unit. This is an extraordinary piece of gear, accurate to a claimed 2000th of a degree. This, I'm sure is accurate enough for anybody, as you would expect from a company that makes larger versions of this tool for measuring Indycars and a product costing in excess of $1,200.00 (Australian $). The ability to measure toe to such a degree of accuracy, does not mean that it is necessarily easy to set the toe this accurately. This is because the toe setting will slightly toe in when the tie rod end lock nuts are tightened. This change must be allowed for, and is basically something of a guess regardless of the means used to measure it (and will be different from kart to kart). The "Pro-K Laser" is perfect for measuring dynamic toe settings. A.R.T. also produce the 'Smart Camber II", an electronic tool for measuring camber / caster settings, and the "Rear Axle Alignment Fixture" for accurately measuring wheelbase (used with a tape measure). The downsides of the 'Pro-K' are cost, and it is not designed to measure camber.
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