Positive caster causes the wheels
to travel straight ahead and
increases steering ability. Positive
caster also assists in recovery.
Recovery is the tendency of the
wheels to turn back to the straight-
ahead position after the driver
completes a turn.
There are some disadvantages to
positive caster. Positive caster
makes it hard to turn the wheels
from the straight-ahead position
when entering a turn. On vehicles
with power steering, this is not
usually a problem. Positive caster
also contributes to body roll. During
a right turn, positive right wheel
caster will cause the right steering
knuckle to rise slightly, while the
left side knuckle will drop slightly.
This causes the vehicle to rise on
the inside of the turn, reducing
cornering ability. When the wheels
Figure 9-4 - Positive caster.
are turned to the left, the left steering knuckle rises and the right knuckle drops.
Negative Caster
. To ease turning, vehicles today have negative caster (
Figure 9- 5
).
Negative caster is obtained by placing the upper ball joint or the strut mounting ahead of
the lower ball joint. With the steering axis behind the tire contact point, there is no caster
force causing the wheels to track
forward. Other alignment angles
compensate for the lack of
tracking to ensure steering stability
is not compromised. Negative
caster also helps compensate for
body roll when the vehicle is
turned. During turns, negative
caster causes the outside steering
knuckle to raise and the inner
steering knuckle to lower. This
compensates somewhat for body
roll, making the vehicle more
stable.
Caster has almost no effect on tire
wear, but improper caster can
cause wheel pull and other
handling problems. Wheel pull is
the tendency of the vehicle to drift
to one side when the driver
releases the steering wheel when
the vehicle is moving. A large
difference in caster between the
Figure 9-5 - Negative caster.
NAVEDTRA 14050A
9-6