The terms rake and trial relate to the steering geometry
of a two-wheeler. On a motorc ycle,
at the front of the frame is a tubular lug in which the steering
stem is carried on bearings to allow the front wheel to be steered.
On a typical motorcycle the steering head assembly comprises
the tubular steering head, upper and lower bearings and the
upper and lower fork yokes. The lower yoke incorporates the
steerinng stem, a short shaft extending up from the centre which
passes through the bearings. The upper yoke fits over the top
of the steering stem, and both yokes include holes and pinch
bolts to clamp the fork stanchions rigidly in place.
The steering head is set at an angle, and this means that the
forks extend forward of the steering head, rather than vertically
downwards. This angle is known as the 'rake' of the forks. The
rake angle determines an important feature of the steering geometry
- trial. This can be calculated by drawing a line through the
steering head to the ground (rake). If a verticle line is now
drawn through the wheel spindle axis to the ground, where the
tyre touches the road surface, it will be noted that this contact
path lies some way behind the rake axis. This difference is
known as the trial of the fork, and is responsible for the self-centring
or 'castor' action.
Without this degree of castor, the front wheel would flutter
uncontrollably, and this is easily seen in the behaviour of
castors on say an airport trolley or an office chair. If, on
the other hand, the rake angle were too great, the steering
would be impossibly heavy, so the correct angle must be chosen
very carefully. On a road bike it is more important to have
stable steering which is not easily deflected by road surface
irregularities, so a greater degree of trial is often
built in.
Source: Overdrive
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