Axles

An axle is a central shaft or housing that carries wheels and supports the load of a vehicle or machine. In wheeled vehicles, axles serve two main purposes: supporting the vehicle’s weight and providing a mounting point for wheels; and transmitting torque from the drivetrain to the wheels in drive axles. In many designs, the axle also houses bearings and a differential, which lets the wheels rotate at different speeds when turning.

In automobiles, axles are classified by how they handle load and how the wheels are connected to the suspension. A rigid (live) axle is a single rigid housing that holds both wheels and may carry the drivetrain; it is simple and strong but can reduce ride quality. Independent suspension uses individual uprights or half-shafts for each wheel, allowing more wheel travel. Drive axles contain a differential; non-drive axles, or dead axles, do not transmit power. Axles may be described as semi-floating or fully-floating, terms that describe how the axle shaft and bearings support load versus wheel mounting. In four-wheel-drive systems, multiple axles may be driven, including front and/or rear axles, sometimes with limited-slip or locking differentials.

Beyond cars, axles appear in bicycles (the axle passes through the hub), rail vehicles (wheelsets with rigid axles and axle boxes), and various machines where a shaft supports rotating elements or transmits torque.