A rear axle is a component of a vehicle that allows the wheels to rotate freely. This axle is mounted at the rear of the vehicle, and it can serve several purposes: sometimes a rear axle is meant simply to support the weight of the vehicle, while at other times it may be a part of the drivetrain that propels the vehicle forward. In other instances, this component may be part of a suspension system. When the axle simply rotates and does not play a part in the drivetrain system, it is sometimes known as a dead axle.
On many vehicles, the rear axle is part of the drivetrain. This means torque is transferred from the driveshaft to the axle, which in turn transfers that power to the wheels so the vehicle can be propelled forward. The driveshaft meets with the axle at a component known as the differential, which is essentially a series of gears that allows the two perpendicular rotating items — the axle and the drivetrain — to work in conjunction with each other. If the vehicle features front-wheel drive, the rear axle will not have a differential and will instead most likely be a dead axle.
The purpose of a dead rear axle can vary. The axle will spin freely within the axle housing, but it will not be driven in any way. The axle may support the weight of the vehicle's body, and it may also be part of the suspension system that absorbs shock. It will in most cases help stabilize the vehicle laterally, having a direct effect on the steering ability of the vehicle as well. The rear axle is very rarely a direct component of the steering system, unless the vehicle features a four wheel steering system. This is not common, though such systems do exist.
Sometimes a vehicle can have more than one rear axle. The presence of more than one axle may mean that the vehicle has more than four wheels, and two drive axles are mounted at the rear. It may also mean that a dead axle is present in addition to a drive axle. This system takes much of the strain of supporting the body of the vehicle off the drive axle and instead places it solely on the dead axle. Especially large vehicles and trucks meant for towing or hauling are likely to feature such systems, as the extra force on the drive axle can lead to failure.