EX­PERT’S GUIDE TO... STEER­ING GE­OM­E­TRY

If you think rake be­longs in the gar­den shed and off­set is a lumpy cus­tard, this guide will set you straight

Motorcycle News (UK) - - Garage -

When dis­cussing steer­ing ge­om­e­try peo­ple of­ten only think in terms of rake and trail, but th­ese are re­lated by other pa­ram­e­ters as well. Specif­i­cally, tyre rolling ra­dius and the off­set of the wheel axle from the steer­ing axis. Wheel­base should also be re­garded as part of the equa­tion, too.

Tyre ra­dius and trail are the two most im­por­tant fac­tors which af­fect how we per­ceive the steer­ing feel. Trail is de­ter­mined by the rake, off­set and tyre size.

I am of­ten asked if there are op­ti­mum val­ues for th­ese pa­ram­e­ters. The short an­swer is no; val­ues have var­ied over time and each type of bike. Sports and rac­ing bikes need quick steer­ing bor­der­ing on in­sta­bil­ity, tourers are con­cerned with sta­bil­ity.

In­creas­ing power lev­els and tyre tech­nol­ogy have been the main fac­tors which have driven steer­ing ge­om­e­try val­ues. It used to be said that 27 de­grees was a magic value for rake an­gle, but nowa­days sports and rac­ing bikes use an­gles closer to 22/23 de­grees, and of­ten cus­tom bikes might be as high as 45 de­grees with tourers closer to 30. Trail val­ues gen­er­ally vary ei­ther side of 100mm. In gen­eral, less rake and trail will lead to a quicker han­dling bike, and higher val­ues give more sta­bil­ity. How­ever, steer­ing ge­om­e­try is not fixed, it is con­stantly chang­ing as we ride.

A few mm can make a big change

THE EX­PERT Tony Foale is a suc­cess­ful racer, chas­sis builder and au­thor of the sem­i­nal ‘Mo­tor­cy­cle Han­dling and Chas­sis De­sign’ book. The steer­ing ge­om­e­try of many of to­day’s bikes fol­low the the­o­ries he set out 30 years ago

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