# Ackermann steering geometry

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Ackermann steering geometry

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. It was invented by the Anglo-German inventor Rudolph Ackermann (1764–1834) in 1810 for horse drawn carriages. Erasmus Darwin may have a prior claim as the inventor dating from 1758. [ [http://www.jstor.org/pss/532121 Erasmus Darwin's Improved Design for Steering Carriages] by Desmond King-Hele , 2002, The Royal Society, London. Accessed April 2008.]

A simple approximation to perfect Ackermann steering geometry may be generated 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. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyze their steering systems over the full range of steering angles.

Modern cars do not use "pure" Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed manoeuvres. Some race cars use "reverse" Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tires while cornering at high speed. The use of such geometry helps reduce tire temperatures during high-speed cornering but compromises performance in low speed manoeuvres. [Milliken, William F, and Milliken, Douglas L: "Race Car Vehicle Dynamics", Page 715. SAE 1995 ISBN 1-56091-526-9]

References

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