Half-Shaft

A half-shaft is the rotating shaft that carries drive from the differential out to a single wheel on a driven axle. Its name reflects its position: on a driven axle there are two such shafts, one for each side, so each spans roughly half the width of the vehicle between the centrally mounted differential and the wheel hub. Half-shafts are most commonly associated with independent suspension and front-wheel-drive layouts, where each wheel moves separately and must be supplied with drive through a flexible connection.

The defining feature of a half-shaft is its ability to transmit torque while accommodating constant changes in length and angle. As the suspension compresses and rebounds, and as the steered wheels turn, the distance and angle between the differential and the wheel alter continuously. To cope with this, each half-shaft is fitted with constant-velocity, or CV, joints at one or both ends. The CV joint transmits rotation smoothly at a varying angle without the speed fluctuations that a simple universal joint would introduce, which is essential for keeping power delivery even and avoiding vibration, particularly on the steered front wheels.

The importance of the half-shaft lies in the fact that without it, independent suspension and driven wheels could not coexist. By allowing each wheel to be driven independently, half-shafts enable the compact, space-efficient transverse engine and gearbox arrangements that dominate modern front-wheel-drive cars, as well as the sophisticated independent rear suspensions found on performance and luxury vehicles. They allow the wheel to rise, fall and steer freely while drive is maintained at all times.

It is worth distinguishing the half-shaft from the driveshaft, with which it is often confused. The driveshaft, or propshaft, is the long shaft that runs front to rear to carry power from the gearbox to a differential at the opposite end of the car. The half-shafts then take over to deliver that power the final distance from the differential to the individual wheels. In a rigid live axle, by contrast, the internal shafts within the axle casing perform the equivalent job but are fully enclosed and need no CV joints.

In service, the half-shaft itself is robust, but its CV joints are wear items protected by flexible rubber boots packed with grease. A split boot allows grease to escape and dirt and water to enter, after which the joint wears rapidly and produces a characteristic clicking noise on full lock. Replacing a torn boot promptly is far cheaper than replacing a worn joint or a complete half-shaft assembly, making boot inspection a routine part of servicing on any vehicle with independent driven suspension.