4 THINGS YOU NEED TO KNOW
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.
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.
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!