tech14

At a recent BSAOC Dual Sport ride a friend showed me his latest
modifications to his Victor. He had made new billet triple clamps that
changed the angle of the forks by three degrees. He said that by
making the forks more vertical he had reduced the rake from 30 degrees
to 27 degrees. At that point I realized that he and many other people
don't really understand the geometry of a motorcycle. His modification
was absolutely first class, including new billet triple clamps that
moved the top of the legs forward and pulled the bottom in toward the
motor. This indeed changed the fork angle by three degrees but not the
rake. What he had done was to increase the trail, the exact opposite
of what he thought he was doing. He made other changes like
lengthening the swinging arm by one inch, retaining the same wheel
base as before. So what he really did was to increase the trail,
adding high speed stability and shifting weight to the front wheel.
The swing arm change added high speed stability, increasing the speed
at which the rear end would try to swap in rough ground, and further
shifting weight to the front wheel. What he got was more high speed
stability (slower steering), not faster turning as he had hoped.
Perhaps the weight distribution change will enhance the bikes turning
by loading the front wheel more, making it "stick" better and not wash
out (??). This relationship of fork angle, rake and trail are highly
misunderstood by the general motorcycling public so my friend need not
feel bad. No mater what the fork legs do to put the tire patch on the
ground, it is the resultant trail and the steering axis angle that
count, not the angle of the fork legs. At least he did things to
increase the stability rather than make it worse. Lets talk terms here.

Wheel base is the distance between the center of the front and rear
axles with the bike going straight and fully loaded.

Rake is the angle of the steering axis as determined by the angle of
the head stock of the frame with the bike fully laden on level ground.

Trail is the distance from the center of the front tire's patch of
rubber on the ground to the point on the ground where the steering
axis center would project (an imaginary line through the center of
rotation of the steering stem extended to the ground). Trail is the
bike's "caster". Because the tire patch lies behind the steering axis,
the wheel assembly will caster behind that steering axis as the bike
moves, self centering just like a shopping cart at the market.

When my friend changed the angle of the fork legs he simply added more
trail because the tire's patch on the ground moved rearward compared
to the steering axis projected ground intersection point. Designers of
motorcycle geometry have to make compromises in any design. Their
tools are rake (steering axis angle), trail, wheel base, and weight
distribution. They can set the rake and change trail by moving the
forks legs closer or father from the steering axis or by angling the
fork legs. But the rake is set by the head stock angle on the frame.
To achieve the selected trail the designer has the choice of
positioning the fork legs forward or backward by adjusting the triple
clamps centers, angling the forks, positioning the wheel axle on the
lower portion of the fork leg (leading, centered, or trailing). But
the trail is the important thing, not how it is achieved.

There are some Triumphs (I don't remember the years) that did the same
as my friend right from the factory. The triple clamps were designed
so that the fork legs were not parallel to the steering axis. Again,
they did this to fine tune the trail. Ducati's 916's and later such
models come with eccentric steering head bearings that can be
installed one of two ways to modify the rake by actually changing the
steering axis angle.

To add some numbers to this discussion the following are some
representative values with sport bike numbers first followed by dirt
bike numbers: Rake 23 to 30 degrees; Trail 3 to 5 inches. Note that
all bikes are moving towards less rake and trail. Some of the latest
motocrossers have nearly sport bike numbers for better motocross
turning and require a hydraulic steering damper to ride in cross
country events due to excessive head shake. A recent magazine had an
article where they had modified a street bike to have zero rake but
normal trail. The author was pictured on this vertically raked bike,
at speed, with both hands off the bars. His claim was that rake is not
nearly as important as trail. He is a rather brave soul, in my
estimation, but probably correct. Rake was added to bikes years ago to
make the fork assembly strong by keeping the fork legs close to the
steering axis and still have trail. Angular set fork legs tend to
operate in and out more smoothly since a vertically positioned fork
leg would tend to bend under rather than travel up for a bump.

Another error I have seen recently is the use of offset fork legs
(sometimes called leading axle fork legs) in triple clamps designed
for straight legs. The offset is forward thereby reducing the trail.
Some such leading axle forks have one to two inches of offset thereby
reducing the tail by that much. This can make the bikes front end
twitch in rough ground or generate a high speed wobble. This is
normally called head shake. When changing to a different set of forks
it is always best to keep the triple clamps with the legs to make the
trail the same and reasonable, if the new forks are leading (or
trailing) axle type.

Motorcycle geometry is always a much debated subject. I hope this
discussion helps your understanding.