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Caster is the angle to which the steering pivot
axis is tilted forward or rearward from vertical,
as viewed from the side. If the pivot axis is
tilted backward (that is, the top pivot is
positioned farther rearward than the bottom
pivot), then the caster is positive; if it's tilted
forward, then the caster is negative.
Positive caster tends to straighten the wheel
when the vehicle is traveling forward, and
thus is used to enhance straight-line stability.
The mechanism that causes this tendency is
clearly illustrated by the castering front wheels
of a vehicle. The steering axis of a vehicle
wheel is set forward of where the wheel
contacts the ground. As the vehicle is driving
forward, the steering axis pulls the wheel
along, and since the wheel drags along the
ground, it falls directly in line behind the
steering axis. The force that causes the wheel
to follow the steering axis is proportional to
the distance between the steering axis and
the wheel-to-ground contact patch-the
greater the distance, the greater the force.
This distance is referred to as "trail."
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