Ackermann steering geometry 1

Ackermann steering geometry

Ackermann geometry

Ackermann steering geometry is a geometric arrangement oflinkages in the steering of a car or other vehicle designed to solve theproblem of wheels on the inside and outside of a turn needing to traceout circles of different radius. It was invented by the German carriagebuilder Georg Lankensperger in Munich in 1817, then patented by hisagent in England, Rudolph Ackermann (1764–1834) in 1818 forhorse-drawn carriages. Erasmus Darwin may have a prior claim as theinventor dating from 1758.[1]

AdvantagesThe intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path arounda curve. The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with acommon 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 is turned, whensteering, through a greater angle than the outside wheel.Rather than the preceding "turntable" steering, where both front wheels turned around a common pivot, each wheelgained its own pivot, close to its own hub. While more complex, this arrangement enhances controllability byavoiding large inputs from road surface variations being applied to the end of a long lever arm, as well as greatlyreducing the fore-and-aft travel of the steered wheels. A linkage between these hubs moved the two wheels together,and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated. This wasachieved by making the linkage not a simple parallelogram, but by making the length of the track rod (the movinglink between the hubs) shorter than that of the axle, so that the steering arms of the hubs appeared to "toe out". Asthe steering moved, the wheels turned according to Ackermann, with the inner wheel turning further. If the track rodis placed ahead of the axle, it should instead be longer in comparison, thus preserving this same "toe out".

Design and choice of geometry

Simple approximation for designingAckermann geometry

A simple approximation to perfect Ackermann steering geometry may begenerated by moving the steering pivot points inward so as to lie on a line drawnbetween the steering kingpins and the centre of the rear axle. The steering pivotpoints are joined by a rigid bar called the tie rod which can also be part of thesteering mechanism, in the form of a rack and pinion for instance. With perfectAckermann, at any angle of steering, the centre point of all of the circles tracedby all wheels will lie at a common point. Note that this may be difficult toarrange in practice with simple linkages, and designers are advised to draw oranalyze their steering systems over the full range of steering angles.

Modern cars do not use pure Ackermann steering, partly because it ignoresimportant dynamic and compliant effects, but the principle is sound forlow-speed manoeuvres. Some race cars use reverse Ackermann geometry to

Ackermann steering geometry 2

compensate for the large difference in slip angle between the inner and outer front tyres while cornering at highspeed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromisesperformance in low-speed maneuvers.[2]

References[1] Erasmus Darwin's Improved Design for Steering Carriages (http:/ / www. jstor. org/ pss/ 532121) by Desmond King-Hele , 2002, The Royal

Society, London. Accessed April 2008.[2][2] Milliken, William F, and Milliken, Douglas L: "Race Car Vehicle Dynamics", Page 715. SAE 1995 ISBN 1-56091-526-9

External links• RcTek article about Ackermann steering geometry (http:/ / www. rctek. com/ technical/ handling/

ackerman_steering_principle. html)• 2002 technical paper on Ackermann steering linkage design (http:/ / www. sciencedirect. com/ science/ article/

pii/ S0094114X0200071X)• javascript model of Ackermann Steering (http:/ / alokmenghrajani. github. io/ steering/ )

Article Sources and Contributors 3

Article Sources and ContributorsAckermann steering geometry  Source: http://en.wikipedia.org/w/index.php?oldid=575558083  Contributors: Adrian Robson, AndrewDressel, Andy Dingley, AtholM, Betacommand,Bromskloss, CharlesC, Chienlit, Chris the speller, CommonsDelinker, CsDix, Dinomite, EuTuga, Falcald, Fox Wilson, GRAHAMUK, Greglocock, Isnow, JP82747919, Jake Wartenberg, Just zisGuy, you know?, KVDP, Kingpin13, Kuhn3, Lavanya83, Legion fi, Lumos3, Mac Dreamstate, Materialscientist, Morven, Nlu, Pjrobertson, Sanguinity, Sfoskett, Sonett72, Stabbington, Stickee,Tanvir Ahmmed, Trekphiler, Viharm, Weregerbil, Widr, Yamamoto Ichiro, 81 anonymous edits

Image Sources, Licenses and ContributorsFile:Ackermann turning.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Ackermann_turning.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Ackermann.svg: User:Bromskloss derivative work: Andy Dingley (talk)File:Ackermann simple design.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Ackermann_simple_design.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:Ackermann.svg: User:Bromskloss derivative work: Andy Dingley (talk)

LicenseCreative Commons Attribution-Share Alike 3.0//creativecommons.org/licenses/by-sa/3.0/