Motorcycle News (UK) - - Garage -

1 Trail

One of the ef­fects of trail is to give a mea­sure of straight line sta­bil­ity, that means if the steer­ing is de­flected by some in­flu­ence, such as un­even­ness in the road, the steer­ing will re­turn to the straight-ahead po­si­tion with­out rider in­put. When the tyre is de­flected, it forms an an­gle to the di­rec­tion of mo­tion, this is called a slip an­gle, which cre­ates a lat­eral in­ward force on the tyre. This force acts be­hind the line of the steer­ing axis and so it ro­tates the wheel back to the straight and nar­row.

2 Rake

Rake is set by the an­gle of the head­stock bear­ings, which means it will vary with the pitch at­ti­tude of the bike. If the bike pitches for­ward the rake will de­crease and vice versa. A re­duc­tion in rake means a re­duc­tion in trail too. If the bike ran those val­ues all the time it would be less sta­ble, but dur­ing the cor­ner en­try phase mod­ern tyre grip lev­els and so­phis­ti­cated sus­pen­sion mean that re­duced in­sta­bil­ity trans­lates into rapid turn-in and the abil­ity to change line through the turn; just watch a top racer like Marc Mar­quez use that ‘in­sta­bil­ity’. It’s rel­a­tively sim­ple to re­duce rake by pulling the forks up in the yokes or rais­ing the rear ride height. Some Du­catis have the head stock bear­ings mounted in change­able cups which con­trol the rake an­gle.

3 Ad­justable yokes

Trail can be changed in­de­pen­dently of rake by chang­ing the off­set be­tween the fork tubes and the steer­ing stem. More off­set equates to less trail and the val­ues only have to change by one or two mil­lime­tres to al­ter a sen­si­tive rider’s feel. This is the pre­serve of rac­ers, fi­ness­ing their bike from one track to the next. The ad­just­ment in th­ese yokes is typ­i­cally be­tween 0-10mm, some­times through ec­cen­tri­cally bored steer­ing stem in­serts, or off­set in­serts where the forks pass through the yokes.

4 Con­clu­sion

Chang­ing yoke off­set for a trail change will change the wheel­base and weight dis­tri­bu­tion. Change the rake and we change the trail, wheel­base and weight dis­tri­bu­tion. Re­mem­ber ev­ery­thing is linked!

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