Adjusting the Caster Angle
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Positive Caster
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Zero Caster
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Negative Caster
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Knuckle Pin (King Pin)
Fig. 1, Caster Angle
Important: Do all the preliminary checks in in the Western Star 47X and 49X Workshop Manual before checking the caster angle.
For vehicle alignment to be accurate, the shop floor must be level in every direction. The turn plates for the front wheels must rotate freely without friction, and the alignment equipment must be calibrated as directed in the alignment equipment manufacturers maintenance procedures and intervals. Western Star dealers must have proof of this calibration history.
Using the alignment equipment manufacturer's operating instructions, measure the front wheel caster.Compare the caster angles with those shown in the appropriate table in Alignment Specifications in this section. If needed, adjust the caster angle by placing wedge-shaped shims between the axle spacer and the axle beam, as follows (see Fig.):
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Axle Spring U-Bolt
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Leaf Spring Assembly
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Spacer
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Front Caster Shim
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Dowel Pin
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U-Bolt High Nut
Fig. 2, Axle Spring Installation
- Apply the parking brakes, and chock the front and rear tires.
- Back off the U-bolt nuts from the U-bolts on one side of the front axle. See Fig..
- Raise the spring away from the axle enough to allow removal of the front caster shim.
- Remove the shim, and install one that will provide the correct caster angle, as specified in the table in Alignment Specifications in this section. Install the dowel pin and check penetration.
- Lower the vehicle onto the axle.
- Coat the threaded ends of the U-bolts with chassis lube or an antiseize compound, such as Loctite® 242. Tighten the U-bolt nuts to the value in the appropriate table in of the Western Star 47X and 49X Workshop Manual for complete procedure. U-bolt nuts need periodic retightening. Refer to of this manual for recommended intervals.
- Using the steps above, replace the shim on the other side of the axle.
- Do a final caster angle check.
Important: Extreme angle shims cannot be used to correct caster angles that vary by more than 2 degrees from the values in the table. Weak or broken leaf springs, or worn shackle bushings, can cause extreme deviations to caster angles. Replace damaged parts before doing caster adjustments.
Important: Place front caster shims between the axle beam and the axle spacer, or between the axle beam and the shock absorber bracket. See Fig..
Caution: Failure to periodically retighten the U-bolt nuts could result in spring breakage and abnormal tire wear.