When discussing steering geometry people often only think in terms of rake and trail, but these are related by other parameters as well. Specifically, tire rolling radius and the offset of the wheel axle from the steering axis. Wheelbase should also be regarded as part of the equation, too.
Tire 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 tire 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 motorcycle. Sports and racing motorcycles need quick steering bordering on instability, touring motorcycles are concerned with stability.
Increasing power levels and tire 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 now-a-days sports and racing machines use angles closer to 22 or 23 degrees, and often custom motorcycles might be as high as 45 degrees with touring motorcycles closer to 30 degrees.
Trail values generally vary either side of 100m. In general, less rake and trail will lead to a quicker handling motorcycle, and higher values give more stability. However, steering geometry is not fixed, it is constantly changing as we ride.
TrailOne 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 tire 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 tire. This force acts behind the line of the steering axis and so it rotates the wheel back to the straight and narrow.
RakeRake is set by the angle of the headstock bearings, which means it will vary with the pitch attitude of the motorcycle. If the motorcycle pitches forward the rake will decrease and vice versa. A reduction in rake means a reduction in trail too. If the motorcycle ran those values all the time it would be less stable, but during the corner entry phase modern tire 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.
Adjustable YokesTrail can be changed independently of rake by changing the offset between the fork tubes and the steering stem. Most offset equates to less trail and the values only have to change by one or two millimeters to alter a sensitive rider’s feel. This is the preseve of racers, finessing their motorcycle from one track to the next.
The adjustment in these yokes is typically between 0 to 10mm, sometimes through eccentrically bored steering stem inserts, or offset inserts where the forks pass through the yokes.