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
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.