Live axle

A live axle is a type of beam axle suspension system that uses the driveshafts that transmit power to the wheels to connect the wheels laterally so that they move together as a unit.

A live axle consists of a central differential mated to tubes containing the driveshafts that connect the differential to the driven wheels. The differential is connected to the engine via a swinging propellor shaft and a universal joint. The complete assembly is suspended on coil springs or leaf springs.

Some live axles use trailing arms, semi-trailing arms, Panhard rod, and/or Watts linkage to control the vertical and lateral movements of the axle. Others, particularly older vehicles, use Hotchkiss drive, in which the leaf springs provide axle location as well as suspension.

As with any beam axle, the advantages of the live axle are relative simplicity, lower manufacturing costs, and the fact that the axle and suspension systems take up little or no interior volume. Because the axle assembly is a fairly simple and rigid arrangement, it can easily be made stronger and more robust, which is an advantage for vehicles with substantial power or that are intended for rugged and/or off-road usage.

The principal disadvantage is the negative effect on ride quality and handling. The wheels cannot move independently in response to bumps. Also, the mass of the differential and driveshafts are part of the vehicle's unsprung weight, so the greater unsprung mass transmits larger forces to the body of the vehicle and its occupants. Until the 1980s the live axle was the most common rear suspension system on rear-wheel drive cars in the United States. It remains common on trucks and other heavy vehicles, owing to its greater potential robustness, but many passenger cars have now adopted independent rear suspension instead.