Tech Talk | All you need to know about Ackermann’s steering geometry

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ACKERMANN’S STEERING GEOMETRY

Carlo Forni

27 June 2017

Kart steering is far more complex than a simple system for turning the front wheels on a bend. Its operation uses the Ackermann system: a revolutionary solution invented and patented in 1817 to optimise the direction of the front tyres and, consequently, driving around a bend

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

The starting point for explaining the Ackermann steering geometry is obvious: while on a bend, the outer front wheel has a wider trajectory (that is, a wider curve) than the inner wheel (which has a narrower curve). It should also be considered that a turning kart must have a rotation centre around which to do so, and around which the front wheels can rotate.
That being so, it is evident that if the front wheels are perfectly parallel during the bend, no rotation centre would be created because the axles of the wheels would remain parallel and without a common rotation point (which would be obtained from the intersection of the two axles of the wheels). In this way, the front wheels would slide, generating friction between tread and asphalt, tyre wear and loss of performance.

Ackermann’s steering geometry serves to offset the different direction covered by the front wheels along the curved trajectory to avoid this happening.
How? By causing the front wheels to rotate in a non-linear direction when turning the steering wheel. In doing so, a theoretical point is created (“theoretical” because, with the slipping of a 4-wheeled kart, the point actually varies) in which the 2 axes of the front wheels and the axis of the rear axle intersect.
Because, among other things, because of the caster angle, the bend geometry of a kart’s steering will lower the inner front wheel and lift the outer front wheel, with the transfer of much of the load to the front. The grip on the front is accentuated and the wheel travel direction is even more decisive.

ACKERMANN’S STEERING GEOMETRY ALLOWS YOU TO HAVE A THEORETICAL COMMON ROTATION CENTRE FOR THE BACK AND FRONT WHEELS

Steering rods, stub axles and steering columns: these are the elements that determine Ackermann’s steering geometry

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

ACKERMANN’S STEERING GEOMETRY PARALLEL WHEELS NON-LINEAR SWERVE STUB AXLES AND STEERING COLUMNS

ACKERMANN’S STEERING GEOMETRY

PARALLEL WHEELS

If the wheels remained parallel during a bend, there would be two rotation centres that would cause the kart to swerve, making it slip and rub the front wheels on the asphalt.

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If the wheels remained parallel during a bend, there would be two rotation centres that would cause the kart to swerve, making it slip and rub the front wheels on the asphalt.

NON-LINEAR SWERVE

The Ackermann system allows the front wheels to swerve at different angles. This creates a single centre of rotation, which coincides with the common point between the two axles of the front wheels and the axis of the rear axle.

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STUB AXLES AND STEERING COLUMNS

The different attachments on the stub axle and the steering column plate allow convergence and Ackermann’s steering geometry to be modified quickly.

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The different attachments on the stub axle and the steering column plate allow convergence and Ackermann’s steering geometry to be modified quickly.

To ensure that the Ackermann steering geometry is more than zero and therefore the front wheels turn in a non-linear manner, the front stub axles are made with tie-rods directed towards the inside of the chassis, the ends of which have the holes the steering rods are hooked onto. In this way, a system is created in which the distance between the rotation axis of the stub axles is greater than that between the points of attachment of the steering columns with the steering rods. The steering geometry thus generated is, in fact, Ackermann’s steering geometry.

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

However, this is not enough. In fact, if it stopped there, the variation of the two angles of the front wheels with respect to the axis of the front stub axle would not be in optimal motion.

Therefore, in order to give greater progression to the variation of the two steering geometries of the front wheels (i.e. at the Ackermann angle) as the steering wheel rotates, the tie rods are connected to two different points on the plate of the steering system on the steering column instead of the same position.

This determines the non-linear variation of the front wheel steering angle, which will adapt in the best possible way to any steering geometry.

In order to have a single theoretical rotation centre there are equations that relate the front wheels (X), the kart’s pitch (Y) and the two angles, alpha and beta, of the two front wheels with the axis that joins the stub axles.

X/Y = cotg (alpha) – cotg (beta)

However, it needs to be said that, on the one hand Ackermann’s solution is simple and economical to achieve, but at the same time this system does not always ensure a perfect theoretical rotation centre: there is almost always a misalignment that does not allow the exact definition of a common point between the two front axle rotation axes and the axis of rotation of the rear axle.