Axle blocks are devices used to raise or lower a vehicle equipped with a leaf-type spring suspension. Positioned between the spring and axle, axle blocks are held in place through the use of two U-bolts per side. Typically made of aluminum or a very hard plastic and nylon material, the amount of lift or drop that the axle blocks provide is relative to the actual size of the blocks. The use of blocks to alter the ride-height of a vehicle is restricted to the rear axle only and should not be used on the front axle for any reason. Most blocks are machined flat on the axle side, however, some block kits include a machined block to correct or maintain the pinion angle on the rear axle.
One relatively easy vehicle customization trick is to alter the ride height of the vehicle. While this can become a rather complex procedure with a coil spring suspension system, the use of axle blocks makes the modification very simple on a leaf-spring style suspension. The ability to raise or lower the vehicle through the installation of axle blocks is defined by the location of the axle to the spring. An axle that is mounted on top of the spring can be lowered by the addition of axle blocks, while an axle mounted under the spring can be raised by the installation of a block kit.
Two vehicle types that rely heavily on the use of axle blocks to provide the desired stance of the vehicle are the low-rider and the four-wheel drive truck. For the most part, the use of the blocks in a low-rider application is limited to the rear axle since most vehicles do not use a leaf spring on the front suspension. This makes lowered coil springs and modified spring perches a requirement for the front suspension of the lowered vehicle. Four-wheel drive trucks provide a different set of requirements, even though many of the raised and lifted trucks include leaf-spring front suspension systems.
On four-wheel drive trucks that use axle blocks to raise the ride height of the rear suspension, the use of the blocks is prohibited on the front leaf-springs. This is due to safety issues focused on the leverage of the axle blocks. This configuration would allow the front-steer axle to rip away from the leaf-springs. These vehicles rely on re-arched leaf springs in the front, as well as relocated mounting points or shackles, a type of lifting mechanism for the spring, on the front springs.