The Ackermann steering geometry (also called Ackermann's steering trapezium) is a geometric arrangement of linkages in the

steering Steering is the control of the direction of motion or the components that enable its control. Steering is achieved through various arrangements, among them ailerons for airplanes, rudders for boats, cylic tilting of rotors for helicopters, ...

of a

car A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people rather than cargo. There are around one billio ...

or other

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

designed to solve the problem of wheels on the inside and outside of a turn needing to trace out

circles A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. The distance between any point of the circle and the centre is called the radius. The length of a line segment connecting t ...

of different

radii In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is the line segment or ...

. It was invented by the German carriage builder

Georg Lankensperger Georg Lankensperger (also: Lankensberger), (31 March 1779 – 11 July 1847) was a German wheelwright who invented the steering mechanism that is today known as Ackermann steering geometry. He patented the invention in Germany, but his agent Ru ...

in Munich in 1816, then patented by his agent in England,

Rudolph Ackermann Rudolph Ackermann (20 April 1764 in Stollberg, Electorate of Saxony – 30 March 1834 in Finchley, London) was an Anglo-German bookseller, inventor, lithographer, publisher and businessman. Biography He attended the Latin school in Stollberg, ...

(1764–1834) in 1818 for horse-drawn carriages.

Erasmus Darwin Erasmus Robert Darwin (12 December 173118 April 1802) was an English physician. One of the key thinkers of the Midlands Enlightenment, he was also a natural philosophy, natural philosopher, physiology, physiologist, Society for Effecting the ...

may have a prior claim as the inventor dating from 1758. He devised his steering system because he was injured when a carriage tipped over.

Advantages

The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve. 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. Intersecting the axes of the front wheels on this line as well requires that the inside front wheel be turned, when steering, through a greater angle than the outside wheel. Rather than the preceding "turntable" steering, where both front wheels turned around a common pivot, each wheel gained its own pivot, close to its own hub. While more complex, this arrangement enhances controllability by avoiding large inputs from road surface variations being applied to the end of a long lever arm, as well as greatly reducing the fore-and-aft travel of the steered wheels. A linkage between these hubs pivots the two wheels together, and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated. This was achieved by making the linkage not a simple parallelogram, but by making the length of the track rod (the moving link between the hubs) shorter than that of the axle, so that the steering arms of the hubs appeared to "

toe Toes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plantigrade''; ...

out". As the steering moved, the wheels turned according to Ackermann, with the inner wheel turning further. If the track rod is placed ahead of the axle, it should instead be longer in comparison, thus preserving this same "toe out".

Design and choice of geometry

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, which is the pivot point, and the centre of the rear axle. The steering pivot points are joined by a rigid bar called the

tie rod A tie rod or tie bar (also known as a hanger rod if vertical) is a slender structural unit used as a tie and (in most applications) capable of carrying tensile loads only. It is any rod or bar-shaped structural member designed to prevent the separ ...

, which can also be part of the steering mechanism, in the form of a

rack and pinion rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert between rotational motion and linear motion: rotating the pinion causes the rack to be d ...

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. 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 In vehicle dynamics, slip angle or sideslip angle is the angle between the direction in which a wheel is pointing and the direction in which it is actually traveling (i.e., the angle between the forward velocity vector v_x and the vector su ...

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.

Extended Ackermann condition

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). In German:

References

External links

2002 technical paper on Ackermann steering linkage designAckerman? Or not? Does it matter?True Ackermann & Dynamic generated Ackermann

* ttp://www.racingonthecheap.com/recommended-reading/carroll-smiths-handling-guide/ Problems experienced that may be due to excessive Ackermann or insufficient Ackermann
Ackermann Steering and Racing Circle (oval) Tracks, includes the toe out effect when Ackermann steering geometry is included and its use in racingErasmus Darwin House, Lichfield UK. Museum. Darwin Page
* ttps://blackboots.co.uk/tyre-and-alignment-technical-information/ Wheel Alignment Explained {{Powertrain Automotive steering technologies

Advantages

Design and choice of geometry

Extended Ackermann condition

See also

References

External links