metal fatigue, and lead to the axle breaking. Heating and bending the axle should never
be attempted as it removes the heat-tempering camber
.
1.2.0 Caster Angle
Look at a furniture caster and note that the caster can swivel around its attachment
point. This attachment point can be thought of as the center about which the caster
swivels. When the furniture is moved, the center moves before the caster wheel. If the
center is behind the caster wheel, the wheel will swivel and follow behind the center.
This occurs because the wheel has less resistance if it follows the center than if it
attempts to stay ahead of it. Even when the wheel starts out directly ahead of the
center, it will quickly move to one side or the other. This increases the friction on the
wheel because it is trying to turn at an angle to the direction of movement. The wheel
will continue to turn around the center until it reaches its point of least resistance, which
is directly the center.
The principle that makes the
caster work is applied to
vehicles to make steering
easier. The upper and lower ball
joints or the lower ball joint and
strut mounting form the steering
axis, as shown in
Figure 9-3.
The steering axis is an
imaginary line between the
upper and lower ball joints or
the lower ball joint and the strut
mounting. The caster angle is
usually referred to as caster. As
with camber, caster is usually
small. Common caster
specifications range from one
half degree to three degrees,
with a maximum of about seven
degrees. Caster can be positive
or negative.
Positive caster.
If the
imaginary line of the steering
Figure 9-3- Steering axis.
axis strikes the ground of the point at which the tire contacts the road, the tire and wheel
will follow behind the steering axis (
Figure 9-4)
. This keeps the wheel and tire assembly
from wandering, since it wants to follow behind the centerline set up by the steering
axis. This is positive caster. To get positive caster, the lower ball joint must be ahead of
the top ball joint or the strut mounting. Like the furniture caster, the tire finds the path of
least resistance by following behind the steering axis line. This tendency of the wheels
to continue to move in a straight line is called tracking.
NAVEDTRA 14050A
9-5