surface, measure the frame angle with a protractor or inclinometer placed on the frame
rail. Positive frame angle is defined as forward tilt (front end down) and negative angle
as tilt to rear (front end high).
The frame tilt angle should be added or subtracted, as required, from the caster
specification in the service manual, noting the following:
•
Positive frame angle should be subtracted from the caster specification.
•
Negative frame angle should be added to the caster specification.
For instance, if the specified caster setting is a positive 1 degree and you measure the
vehicle frame angle to be positive 1 degree, the measured caster is 0 ±
�
degree. This
calculates to the desired 1 degree ±
�
degree caster angle when the chassis settles to
level the frame under load.
Caster should be measured with the vehicle on a level floor. If the vehicle is equipped
with a manually controlled air-lift axle, adjust it so that the frame is as normal operating
height. Replacement of worn suspension, frame, and axle components should be done
before caster is measured.
Measuring caster.
Caster can be measured in a number of ways. One way is to use a
protractor. Another is to use a radius gauge Modern computerized alignment
equipment is the fastest, most accurate method measuring caster (and other equipment
angles) and these are increasingly being used.
Protractor.
A protractor measures the angle between its base and true horizontal, as
determined by a bubble cylinder on one side of the dial.
To measure caster angle:
1.
Place the protractor on the
machined surface of the
front axle pad, as shown in
Figure 9-18
. The axle pad
must be free of dirt and
debris. Make sure that the
protractor contacts both U-
bolts so that it is parallel
with the vehicle frame.
2.
Center the bubble in the
cylinder by rotating the dial
of the protractor. Lock the
dial and remove the
protractor to record the
angle.
3.
Make sure you determine
whether the caster angle
you measure is positive or
negative. The original
orientation of the protractor
must be remembered
when determining the
direction of the caster
Figure 9-18 -
Placement of a protractor to
measure caster.
NAVEDTRA 14050A
9-26
