Technical Research: Trying to Reuse
Old Forks
In the past, many custom bike builders cut and welded
the frames to change the rake angle to look the way they thought
was cool. Unfortunately a lot of those bikes were unrideable due to the
altered rake. The drawings below demonstrate why stock forks and
triple-clamps don't work when only one part the geometry is changed.
The forks and triple-clamps of any motorcycle are
designed by the manufacturer who wants a certain amount of rake and trail.
The drawing at left shows a typical wheel and fork. The angled centerline
runs though the head bearings (where the forks rotate) and down to the
ground. The vertical centerline shows the location of the wheel's axle
directly above the ground. The distance between both lines is the amount of
trail for that fork's design.The drawing on the
right shows what happens when the forks are raked without correcting the
trail. The centerline running through the head bearings now extends further
past the axle's centerline, which increases the trail. Too much trail can
make the motorcycle difficult to steer, or keep it from steering at all.
This chopper bicycle is a good example of poor
engineering. The forks are stretched way out (which I like) but the centerline of the head bearings are far, far beyond the axle. It
looks fun but I'll bet it steers like crap.
