Quick Fork Geometry Lesson

What are rake and trail? And does angling the triple clamps relative to the frame really change the rake angle? Here’s a quick review of front-end geometry.

Rake is determined by the angle of the steering head tube as it relates to the surface on which the motorcycle is sitting. There are three ways to change the rake: 1) shorten or lengthen the forks, 2) shorten or lengthen the height of the rear suspension or, 3) cut, re-angle and re-weld the steering head tube. Any combination of these can change the rake angle, as well as ride height. Changing tire/wheel sizes can also influence rake and trail, but only to a relatively small degree.

Trail is defined as the point at which the front tire contacts the ground behind an imaginary line drawn straight through the center of the steering stem to where it contacts the ground. "Trail" is so named because the tire’s contact point rails the steering axis, just as in a caster on a grocery cart.

There are at least three ways to change trail. 1) Change the offset of the triple clamps by varying the distance between the centerline of the fork tubes and the stem centerline. 2) Change the angle of the fork tubes relative to the pre-set frame rake angle. 3) Change the rake as described in the above rake paragraph.

As a general rule, less fork tube to steering stem offset yields more trail and hence, up to a point, more stability. More fork tube to steering offset yields less trail and less stability. These changes also change the wheelbase.

Angling the fork tubes relative to the frame only changes trail, not rake. This is because the axle position, and hence the tire¹s contact point, is changed relative to the steering axis. As an example, look at the top triple clamp on a leading axle fork. Because the axle is moved forward relative to the steering stem, compensation has to be made for more trail by reducing the offset between the centerlines of the fork tubes and the steering stem, producing a triple clamp that is very Œflat¹ as viewed from above. Angling the fork tubes with different offsets on the top and bottom triple clamps only creates the visual image of changing rake.

One last way of changing trail is to change the true frame rake angle. Steeper steering head angles reduce trail (and stability) if all other factors remain the same. Notice how modern bikes with their steeper head angles employ less fork-tube-to-steering-stem offset in their triple clamps in order to generate enough trail. Compare this to older bikes that had shallower head angles and more triple clamp offset.

Draw yourself a diagram with extremes of rake and/or trail and the above explanation will make a whole lot of sense.