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Ackermann Steering - Why is it Different?

Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii.

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According to Cox Models...

"Most slot cars do not attain maximum speed because the front tires scrub sideways against the track surface during fast cornering on a twisting road course. The COX Ackermann front end minimizes this scrubbing by aligning the front wheels with the slot, regardless of how far the rear of the car drifts to the outside of a turn.

The rear of the guide is linked to steering arms extending back from the front wheels spindles and transmits a differential steering action to keep each wheel precisely aligned in any turn. The guide is vertically spring loaded to compensate for an uneven track surface or irregularities in contact strips along the sides of the slot. Kit contains a pair each of 1/24 scale Shelby type wheels and tires, plus all necessary front end parts molded in Coxalloy for minimum friction and long wear."

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Origin

This engineering solution is often attributed to Langensperger in 1816. However it was actually first introduced by Erasmus Darwin, grandfather of Charles Darwin, in 1758-1759. It was eventually patented by arrangement in London, in 1817, by Rudolph Ackermann, whose name stuck to it. The same idea was also developed in France in the late 1870s, by Bollée and Jeantaud.

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The Problem

Part of steering mechanism: tie rod, tie rod ends, steering arm. When a vehicle is steered, it follows a path which is part of the circumference of its turning circle, which will have a centre point somewhere along a line extending from the axis of the fixed axle. The steered wheels must be angled so that they are both at 90 degrees to a line drawn from the circle centre through the centre of the wheel. Since the wheel on the outside of the turn will trace a larger circle than the wheel on the inside, the wheels need to be set at different angles.

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The Solution

The Ackermann steering geometry arranges this automatically by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle. The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. This arrangement ensures that at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point.

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1:1 Usage

Modern full size cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed maneuvers.

Larger scale slot cars of the 1960s often contained this steering arrangement. Manufacturers such as Cox, Airfix and Marx used Ackermann style steering almost exclusively in their early products.

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Topside Illustration

Ackermann Steering Illustration