Ackermann steering geometry

This article is about steering geometry. For other uses, see Ackermann (disambiguation).

The Ackermann steering geometry (also called Ackermann's steering trapezium)^[1] 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.

Ackermann geometry

It was invented by the German carriage builder Georg Lankensperger in Munich in 1816, then patented by his agent in England, Rudolph Ackermann (1764–1834) in 1818 for horse-drawn carriages. Erasmus Darwin may have a prior claim as the inventor dating from 1758.^[2] He devised his steering system because he was injured when a carriage tipped over.

Advantages

The first requirement of any steering geometry is to avoid the need for tyres to slip sideways when following the path around a curve.^[3] The geometrical solution to this is for all wheels to have their axles arranged as radii of circles with a common centre point. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle and where the axis of one front wheel meets that line, so must the other. A billy cart or four-wheel wagon, whose front axles are fixed to a solid beam with a central pivot, easily meets this condition but requires considerable steering effort due to the fore-and-aft movement of the wheels, and is heavily influenced by road surface variations.

Rather than the preceding "turntable" steering, Ackermann geometry gives each front wheel its own pivot, close to its hub. While more complex, this arrangement enhances controllability by reducing the fore-and-aft travel of the steered wheels. A linkage between these hubs pivots the two wheels together like a parallelogram, but with the tie rod (the moving link between the hubs) shorter than the distance between the pivots, so that when steering, the inner wheel turns farther than the outer wheel.^[3]

Design and choice of geometry

Simple approximation for designing Ackermann geometry

A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points^{[clarification needed]} inward so as to lie on a line drawn between the steering kingpins, which is the pivot point, and the centre of the rear axle.^[3] The steering pivot points^{[clarification needed]} are joined by a rigid bar called the tie rod. 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.

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 maneuvers. Some racing 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 maneuvers.^[4]

Extended Ackermann condition

Extended Ackermann condition of vehicle and trailer

The Ackermann condition of vehicle train is fulfilled when both the vehicle wheel and the trailer wheel axes are pointing to the theoretical turning center (momentan centrum).^[5]

See also

• Front axle assembly