Axle shims are thin pieces of metal that are placed between an axle and a leaf spring to adjust the pinion angle of the vehicle's axle. Commonly offered in 2- to 8-degree angles, axle shims are designed to be bolted to the bottom of the leaf spring pack and held in place both by an interference fit between the spring and spring pad on the axle, as well as the center bolt of the spring pack. When using axle shims on a front axle, as in a four-wheel drive application, special care must be taken to correct the front wheels' caster and camber settings after the shims are put into place.
A vehicle's rear suspension is designed to give optimal performance with a predetermined pinion angle. The pinion angle reflects the angle in degrees that the rear axle's pinion is positioned relevant to the ground or road. Typically, the pinion being set at a slight downward angle at the drive shaft yoke is best. This is the angle that gives the tires the best bite into the road's surface, providing the best traction and handling for the vehicle. While some degree of pinion angle is factored into the placement of the spring-mounting pads, certain alterations such as tire size, ride height and spring wear can change the angle, requiring axle shims to correct the problem.
This method of pinion angle is accomplished only with a leaf spring suspension. Coil spring suspensions use a four-link suspension setup with two upper and two lower suspension bars positioning the rear axle. Pinion angle is not set with axle shims, but rather through the adjustment of the upper suspension bars. In either spring system, a pinion angle gauge is used to determine the amount of pinion angle that is adjusted into the suspension. This is typically a magnetic gauge that is placed against the snout of the rear end or against the rear-axle pinion-yoke.
In a four-wheel drive application, axle shims are often used to combat severe drive shaft angle stemming from excessive suspension lift. With excessive lift, the drive shaft can often run on a severe angle to the axle, creating bind in the universal joint. Placing axle shims in the spring pack can often ease the problem by shifting the axle pinion upward towards the drive shaft. The use of axle shims can only remove a portion of the problem before the increased negative or upward angle begins to cause other problems within the suspension. In the front axle, axle shims can severely alter caster and camber angles to the negative, creating a dangerously ill-handling vehicle.