A rear axle shaft is a component of the rear axle housing on a rear-wheel drive vehicle. Manufactured of solid steel, the rear axle shaft provides the power to the tires to drive the vehicle. The inner end of the rear axle shaft is splined to fit snugly and securely inside of the rear axle differential, while the outer end of the axle makes up the axle flange, or wheel flange. The axle is designed in one of two styles: a full floating design or a simple axle design. The full floating version will provide the ultimate in strength, while the simple axle design is the most common design found in automotive use around the world.
The most common design for a simple rear axle shaft is a solid, one-piece shaft, although there are some two-piece axle shafts that are used in automobiles and light-duty trucks and off-road vehicles. The difference between a one-piece and a two-piece rear axle shaft is in the area of the wheel flange. The one-piece axle has the flange machined into the actual axle shaft, while the two-piece uses a separate flange. The flange fits onto a machined spline on the outside end of the axle shaft and is held in place with a large axle nut.
A full-floating rear axle shaft gets its strength by not supporting any of the vehicle weight. The vehicle is supported by the outer end of the rear axle housing through the use of two wheel bearings inside of the wheel hub assembly. The rear axle shaft is connected to the wheel hub through the use of a drive flange and does nothing but drive the wheel. In the case of a broken axle, the wheel will remain fastened to the axle housing. With a simple axle, a broken axle is often followed by the rear wheel and tire coming off of the vehicle. This can result is severe body and chassis damage to the vehicle.
A rear axle shaft is hardened to give the axle strength to withstand the forces of the engine's torque. Racing axle shafts are hardened much more than a typical street-driven axle, however, this does not make for a superior product on the street. The harder makeup of the racing rear axle shaft will often result in a broken axle if operated for prolonged periods on a street. The hardened axle is not designed to withstand the bumps and pot holes, so the racing axle will commonly break instead of bending and flexing when a bump is encountered.