Axle bushings are devices used in both the front and rear of some vehicle suspension configurations and allow the axle to pivot and move smoothly and without binding. Made of a variety of materials from rubber to polyurethane and aluminum, bushings not only provide a smooth ride, they also deliver better handling characteristics and straight-line acceleration speed when used on a rear suspension. When used on the front suspension, axle bushings are used on straight axle configurations and cushion the king pins to allow for a tight and controllable steer axle. When used on steer axles, many economy kits include plastic bushings that are often prone to premature wear and failure.
On vehicles with coil-sprung rear suspensions, a four-point suspension package is common. Using two upper control arms along with two lower control arms, the rear axle is positioned in the chassis using axle bushings to cushion the ride characteristics. The typical factory equipment-style bushings are made of a durable rubber compound that commonly lasts for the life of the vehicle. The problem with this style of bushing is that the rubber often becomes worn and loose, allowing the rear axle to shift slightly in all directions. This is rarely noticed in a street-driven vehicle, but in a performance vehicle, it can destroy the vehicle's performance and handling.
Polyurethane axle bushings have an advantage over the factory-equipped rubber bushings. The poly bushings do not deflect as much under a load as the rubber type and offer much tighter chassis tolerances, resulting in better handling. Installation of the poly bushings can be more difficult than the rubber due to the polyurethane resisting stretching like the rubber. Placing a small amount of petroleum jelly in the bushings' housing will allow the bushings to be slid into position much easier than attempting to install the bushings dry. For the ultimate in performance, solid bushings made of aluminum offer zero deflection; however, the ride can be very harsh and noisy, and they should not be used on street-driven vehicles.
On front straight-axle designs, axle bushings are used to cushion the king pins and provide a smooth operating steering system. The king pins are used to secure the steering knuckles to the axle, thereby allowing the front tires to pivot and steer the vehicle. By installing bushings, which are typically brass or aluminum, the steering action will be free of slop, vibration and sticking.