Installing Caster Adjusting Leaf Spring Wedges aka Shims. A common practice while installing a lift kit on a leaf spring equipped CJ is the installation of degree shims. These shims correct the caster angle by rotating the axle either clockwise or counter-clockwise. Installation is rather easy, these shims are placed between the axle mounting perch and the leaf spring. They are typically made of aluminum or steel. A downside of these shims is they can deform over time and/or be the cause of suspension noise.
typical degree shim
Usually a Jeep needs an increase in positive caster. (see diagram) As stated before positive caster allows a wheel to tend to return to center and reduces the Jeep from wandering down the road.
In most cases the shim will be installed into the Jeep by putting the thicker part of the shim toward the front of the axle, in effect you are tipping the pinion yoke downward.
Support the Jeep securely on jack stands under the frame allowing the front suspension to hang. Making sure the differential is still supported by the jack to prevent the differential from tipping backward. Remove all the u-bolts, insert the degree shim between the leaf spring and the axle mount perch. Reinstall all the u-bolts loosely and then tighten them evenly. Set the Jeep back on all 4 wheels and double check the bolt tightness. Test drive the Jeep to see if the caster correction improved or worsened the condition. Check the u-bolts again after driving a few miles.
We recently installed a set of Rubicon Express Steel Degree Shims on the jeepfan.com CJ, although they were in the rear the process is the same.