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Technical Article

 Kart Steering, Physical Forces and Setup - Theory and Practice

by James Hughes

Page 1
 

Often when asking questions about chassis and steering settings, the usual answer is simply to say changing some setting or other causes an increase or decrease in grip. There is never an explanation of the physical principles involved in causing these changes. I hope in this article to explain the physical forces involved in driving a kart, along with how those forces are generated by the steering, and what the effect is on the track of changing the various parameters available as setup.

Although a kart may seem to be rather simple device, it is perhaps a more difficult subject to explain than an equivalent car. Both vehicles have many parts and principles in common but there two major differences, which account for a large divergence in design and in setting up. These differences are the karts lack of differential, and also its lack of any suspension components.

A good knowledge of the forces involved can help greatly when setting up a kart - giving the mechanic some knowledge as to what should happen when a parameter is changed. This should result in considerably less time spent on the track testing.

Steering Geometry

The steering geometry can be regarded at the movement and displacement of the front wheels as the steering wheel is turned. This movement is quite complex, and involves a number of different settings. There is one thing in common though, and that is the reason why we need a complicated geometry - We MUST lift the inside wheel while cornering.

The inside wheel lift is what enables a kart to go round a corner without using a differential.

Because of this lack of a differential, a karts natural direction of travel, forwards, is very difficult to change. This is down to the differing radii of turn experienced by the inner and outer rear wheels while turning a corner. The inside wheel is actually travelling a shorter distance than the outside, so therefor is needs to take fewer revolutions to go round the corner. However, the two rear wheels are attached by a solid axle, and must therefor move together, so in order to turn, one of the wheels need to skid over the track surface. In a car, the differential will allow the wheels to turn at different rates, without this skidding action.

This skidding action, or indeed the lack of it, is what make a stationary kart so difficult to turn round - you have to overcome the grip of one of the tyres, and with the sticky tyres used in many kart classes this can expend a lot of energy.

This is the reason for lifting the inside wheel and it effectively turns the kart into a tricycle during the cornering process! The steering geometry causes the inner rear wheel to lift off the ground while cornering, which means the wheel can rotate faster than it is passing over the ground. The rear inner wheel is no longer touching the track, and we therefor no longer need to overcome the grip from that tyre in order to turn.

In fact, depending on the power of the engine, we may be able to allow some scrub. For example, while a Prokart may need to entirely lift the inner wheel, because it does not have enough power to overcome the scrub, a more powerful kart may have power to spare in the corner, meaning that the power loss to scrub can be overcome. However, any scrub will start to cause understeer when entering a corner, so even though the engine may be powerful, it may still be necessary to completely lift the inner rear to maintain decent handling.

However, we haven't yet explained how the front geometry can affect the rear wheel lift, and in order to do this, lets define a few terms used when describing front end geometry.

Camber. This is the degree to which the front wheel lean in (or out) from each other. A camber setting of 0 means that both tyres sit flat on the track. Maximising the amount of rubber on the track is one of the aims of kart setup

Caster. This is the angle of the kingpin, which is the point around which the stub axles rotate. This is one of the most important settings for inducing wheel lift during cornering.

Figure 1

Toe In/Out. This is the angle at which the front wheels either point in towards each other, or away from each other. Zero degrees toe in/out means that the wheels are parallel. Toe in/out is set by changing the length of the tie rods.

Scrub Radius. This is distance from the centre of the tyre to the point where a line down the kingpin axis intersects the ground. Along with caster this affects wheel lift during cornering. Scrub radius is set using spacers on the stub axle.

Figure 2

King Pin Inclination. This is the inward lean of the kingpin, and it modifies the amount of camber change caused by the caster when steering. It is not usually possible (or necessary) to adjust the KPI although some camber adjust systems may let you do it.

Ackermann Steering. Ackermann steering uses the angle of the stub axle arms (and often an offset on the steering column) to make the inner wheel on a corner turn more than the outside wheel. With cars this is used to reduce tyre scrub on corner, but of more importance to karts is the greater wheel lift effect caused by increasing the inner wheels turn when compared to the outer.

Figure 3

To help explain how the front geometries affect the rear inside wheel, lets assume that the chassis is completely rigid - it is so stiff that it cannot bend in any direction. This assumption makes things a little easier to understand. Kart chassis are not actually this stiff - they flex in a number of areas. However, the differing effects caused by differing stiffness' in various parts of the chassis are beyond the scope of this article.

When we turn a corner, the steering geometry (but mainly the caster setting and scrub radius) causes the inside wheel to move down in relationship to the chassis, and the outside wheel to move up. As this happen, because our chassis is rigid, it pivots around a line from the inside front and outside rear, causing the inside rear to lift!

OK, so we have now explained how the front geometry is used to raise the inner rear wheel during cornering. There a quite a few other forces that come in to effect one a corner has been initiated, and that is what we will talk about next

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