Steering mechanisms

One of the most difficult things to design and build on a kart is the steering mechanism. The steering mechanism is used to turn the kart around the bends in the track. There are many different ways in which this can be done.

Steering controls
This is how the kart driver controls how much the wheels will turn around any corner. Most kart designs use handlebars to steer the kart like the ones used on bikes, but some use a steering wheel or long rods with handles that can be pushed forward and back to turn the wheels, as you can see in the photographs (right).

But how do we convert the turning of the wheels or handlebars, or the pushing and pulling of handles, into the right movement of the wheels? - We use a steering linkage.

Steering linkages
A steering linkage changes the movement of the driver's handle bar or steering wheel into the turning of the wheels to go round corners. Steering linkages on a kart are usually made from a system of rods and pivot points (moving connections) between the handlebars or steering wheel to the front wheels of the kart.

Some karts put the wheels in a diamond shape, with a single front wheel for steering. On a kart with a single front wheel, like a bike's, the steering linkage can be very simple, as there is only one wheel to turn. This can be done by having a set of handlebars connected to the front forks exactly as you would do with a bike. If the driver is sitting further back from the front wheel, two connecting rods can be used to steer the wheel either using simple handles, as shown in the photograph above, or handlebars as in the drawing below.

Having only three wheels can make the kart unstable when going round corners, increasing the risk of it falling over.

You can connect two front wheels to a single pivot point in the centre of the front axle, and use the same mechanisms. But this can cause the wheels to jam on sharp bends and it also makes the kart unstable when turning.

The kart is much more stable if it has two wheels at the front of the with their own pivot points, but the steering linkages get more complicated.

The names of many of the parts in a steering linkage using two front wheels are shown below.

The Pitman arm is used to connect the steering column to the tie rods. The arm changes the rotational (turning) movement of the handlebars and steering column to a sideways motion in the tie rods.

The tie rods are connected to the steering arms, which then change the sideways motion back into a rotational movement, which turns the wheels.

By changing the length of the Pitman arm you can change the amount the wheels turn.

The steering arms are connected to the wheels using a stub axle, or bicycle front forks in most cases and a pivot point called a king pin. It is called a king pin because in early vehicle designs, it was a large pin that held all the other parts together.

These linkages show how the kart driver can make the wheels turn from turning the handlebars or steering wheel, but by how much should we turn them to get round a corner?

Turning angle
You may think at first that both front wheels of the kart have to turn to the same angle to get round a bend, but that isn't correct.

If a kart goes around a circle or bend, the outside wheels have to travel further than the inside wheels – try running around a circle with a friend at different distances and see who has to run faster to keep up!

To make the wheels go round these different distances they need to turn by different amounts.

If we draw an angle between the rear axle of the kart, the centre of the circle and the centre of each front wheel, we see that this angle is different for each wheel. Using trigonometry, we can see that these angles are also the angles by which each wheel needs to turn into the curve. The inner wheel always turns more than the outer wheel; the difference between these two angles (Θ₁ minus Θ₂) is called the 'toe-in' of the wheels.

So, for example, if the inner wheel turned by Θ₁ = 45°, and we assume the kart dimensions to be x = 1m and y = 2m, then;

• r = y/tan (Θ₁) = 2/tan(45°) = 2/1 = 2m
• tan (Θ₂) = y/(x+r) = 2/3, so Θ₂ = 33.6°
• Toe-out = Θ₁ - Θ₂ = 11.4°

So how do we get the two front wheels to turn by different amounts?

Ackermann steering
Ackermann steering is named after Rudolph Ackermann, who designed a solution to the turning problem in London in 1817. The idea is to angle the steering arms of the steering linkage towards the centre of the kart so that the tie rods change the wheel angles by different amounts.

Calculating of the exact angle of each steering arm is complicated but angling the steering arms so that a line drawn from the centre of each arm meets at the centre of the rear axle gives a good result.

Conclusions
Using a design with four wheels at each corner makes the kart more stable but makes the steering more complicated.

The front wheels should turn at different angles to get round a bend.
This can be achieved by using Ackermann steering i.e. angling the steering arms towards the centre of the rear axle.