Axle spacers are thin pieces of steel machined to a specific thickness to compensate for lost axle length due to the installation of a vehicle suspension lift. Installed between the axle shaft and the front differential on independently-suspended, front suspension, four-wheel drive trucks, the axle spacers maintain adequate axle travel and suspension movement once the lift kit has been installed. The thickness of the axle spacers is dependent upon the amount of lift altered in the vehicle's chassis. The use of the spacers prevents the destruction of the constant velocity (CV) joint used in the independently-suspended front suspensions, as well as the binding and breaking of the axle itself.
When installing a suspension lift kit in an independently-suspended front suspension application, there are two methods of compensating axle length for the added height of the suspension lift. One method is to lower the front axle differential the same amount as the rest of the suspension was lifted. This will maintain factory geometry of the front axle and allow proper operation for the most part of the front axle. The problem with this type of axle length compensation is that the added ground clearance that was achieved through the installation of the suspension lift kit is negated by lowering the front differential.
A common compensation method for most owners of a lifted four-wheel drive truck is through the use of axle spacers. Once the suspension is lifted, the axle becomes too short to reach the differential on the front drive train, the installation of axle spacers closes the gap and allows the axles to be reconnected to the differential. The axle spacers are commonly machined from steel or a strong alloy that allow the axles to retain all of the factory strength designed into the suspension system. Longer mounting bolts are sold with the axle spacers to solve any mounting problems created by the addition of the thick spacer.
It is common for some extremely radical suspension lift kits to use both a dropped differential as well as axle spacers to accommodate the lift. Some four-wheel, independently-suspended vehicles use axle spacers in all differential connections to allow both the front and rear axles to be retained. This saves on the added cost of including longer axles with the lift kit. These kits are, however, found on a limited basis. The front suspension spacers are a much more common addition to lifted suspensions used on pick-up trucks.