5.3 Front fork geometry: Fork angle and Trail
5.3.1 Contribution:
Small differences in front fork angle, trail and tire width give distinctive characters to motorcycles quite similar
in terms of weight, height, wheelbase… Lots of supporting examples can be found in motorcycle magazines.
5.3.2 Limits:
But these differences in front fork angle, trail and tire width do not explain the major difference in steering ease
and precision at all speeds, between the cases WITH and WITHOUT hands on the handlebars.
5.3.3 Better understanding some front fork geometry effects:
 a) Stability of the front wheel by itself:
A 2-Wheeler front wheel is often compared to a 'caster' such as found under an office chair or the front of a grocery
cart. In all of these cases, the fork axis combined with a positive 'Trail' lets the wheel-ground contact patch trail
behind this fork axis and stay aligned with the 2-Wheeler longitudinal axis.
So the front wheel of a 2-Wheeler is stable, but nothing insures whether the 2-Wheeler itself will be stable or not.
b) Combination of the front fork geometry with the gyroscopic reactions to stabilize the 2-Wheeler:
 In a top view, a 2-Wheeler leaning to its right can be seen
as in this figure, where the dark black line represents the contact path of the wheels on the ground.
If the tire contact point to the ground (1) does not slip laterally and the 2-Wheeler plus rider weight push down on
the front wheel center (2), a lever arm (3) exists to orient the front wheel towards its right (4). This front wheel
orientation towards the right brings the 2-Wheeler in a curve to the right, which generates a centrifugal force towards
the left that brings the 2-Wheeler back to the vertical. The front fork geometry thus combines with the gyroscopic
reactions to stabilize the 2-Wheeler.
This mechanism does not depend on the vehicle speed and can be seen at stand-still: A bicycle (a motorcycle may be
too heavy) can be held up by the saddle without touching the handlebar. When leaning the 2-Wheeler right or left, the
front wheel is oriented towards the same side. But it becomes obvious when pushing the bicycle over a certain distance
at low speed, that the front wheel swings to the right and left and that the bicycle goes a little erratically. On the
contrary as speed increases, the gyroscopic reactions increase with the speed squared and thus become much more
important to stabilize a 2-Wheeler.
c) Lateral shift of the CG at very low speed while turning the handlebar:
 If a 2-Wheeler is held up at stand-still and the handlebar
is suddenly turned towards its left for example, the wheel-ground contact patch axis will keep the same
orientation. But the 2-Wheeler chassis axis and center of gravity (CG) will be shifted laterally towards the left
due to the front fork geometry. The center of gravity (CG) may thus force the 2-Wheeler to lean to its left or towards
the handlebar orientation.
This supports the impression that a 2-Wheeler can be steered at slow speed by turning the handlebar towards where we
want to go, without considering the countersteering 'principle'.
But it's easy to check that it's not the case. You only have to ride a 2-Wheeler, bicycle or motorcycle, at such slow
speed and suddenly turn the handlebar towards where you want to go, for instance to the left. This test has to be
done very carefully because the countersteering 'principle' will apply and the 2-Wheeler will be thrown by the
centrifugal force towards the right.
Sure, the center of gravity (CG) will have been slightly shifted to the left. But the front wheel orientation towards
the left and the vehicle speed will be such that the front wheel contact patch to the ground will also have been shifted
to the left. So in a fraction of a second, the front wheel contact point will have shifted much more than the center of
gravity (CG), which will make the 2-Wheeler fall to the right.
[ Next steering theory ]
[ 2-Wheeler Index ] [ Up one level ] [ 5.1 Keep balance ] [ 5.2 Gyroscope ] [ 5.3 Fork geom. ] [ 5.4 Countersteer ] [ 5.5 Foot pegs ] [ 5.6 Lateral shift ] [ 5.7 Target fixation ] [ 5.8 Camber steer ] [ 5.9 Straighten up ]
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