If the caster difference from side to side exceeds
�
degree, the axle is probably twisted.
Also, remember that the right side should not have less positive caster than the left. If
the right side has less, it will tend to lead the vehicle to the right, particularly on crowned
roads.
Caster shims.
Caster angle is changed by using caster shims, which are angled metal
wedges inserted between the spring seat and axle pad. Shims are typically
manufactured in half-degree increments, from
�
degree to about 4 degrees, and in a
choice of 3-, 3
�
-, or 4-inch widths to accommodate typical spring pack widths.
When inserting caster shims under spring seats, you should observe the following:
•
Use only one shim on each side. Do not stack them.
•
The width of the shim should equal that of the spring.
•
Do not use shims of more than 1 degree difference from side to side.
Changing caster.
After determining the caster desired for each side of the axle, use the
following procedure to install the caster shims:
CAUTION
When changing caster, be sure to block the wheels so that the vehicle will not roll. Also,
use safety stands to support the springs when changing the caster shims.
1.
Loosen all spring U-bolts on each side just enough to relieve clamping tension.
2.
On one side, fully back off the U-bolt nuts without completely removing them from
the U-bolt threads.
3.
With a frame jack, raise the springs from the axle seat. Do not lift the wheel from
the floor.
4.
If the caster shim has been previously installed, remove it and make sure that the
angle is factored in the calculation of desired caster angle. In the interest of
safety, use a tool to remove the shim. Never insert your fingers between the axle
and the springs.
5.
Remove dirt and debris from the exposed axle pad.
6.
Ensure that the spring center bolt cap protrudes sufficiently to pass through the
caster shim and locate to the axle alignment recess.
7.
Insert the shim, aligning the hole with the center bolt, and slowly lower the spring
onto the axle, making sure the center bolt seats the axle alignment recess. The
caster shim must be installed with the thick part of the wedge positioned to
produce the correct caster reading.
8.
Apply initial torque to the U-bolt nuts.
9.
Repeat steps 2 through 8 on the other side of the axle.
10.
Torque the U-bolt nuts to the OEM specifications in a crisscross pattern.
11.
Measure the caster again. Changing shims does not always produce the exact
caster change desired.
12.
In most cases, caster angle should not change significantly when a load is added
to the vehicle. If steering difficulties persist, caster should be rechecked with the
vehicle loaded.
NAVEDTRA 14050A
9-29
