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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 in a single housing that also contains 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 may typically be suspended with leaf springs, coil springs or air bags.
Some live axles use trailing arms, semi-trailing arms, Panhard rod, or Watt's 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.
Advantages and disadvantages
As with any beam axle, the advantages of the live axle are relative simplicity, lower manufacturing costs, lighter overall vehicle weight, and the fact that the axle and suspension systems take up little interior volume. Because the axle assembly is a fairly simple and rigid arrangement, it can easily be made strong and robust, which is an advantage for vehicles with substantial power or that are intended for use in rugged environments or off-road usage. A further advantage of a live/beam axle in off-road use is that ground clearance under the axle remains constant, even if one wheel rises over a bump and the other doesn't.
The principal disadvantage is the negative effect on ride quality and handling. The wheels cannot move independently in response to bumps. Although the overall mass of the total suspension is low, 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. Conversely, in an independent rear suspension system the differential is rigidly attached to the vehicle. The lower unsprung mass of the suspension results in a greater ability to absorb imperfections in the road. In passenger car applications, often now fitted with multi-link independent suspension, the useful ability to change toe and camber independently left to right under cornering loads is not given with a live axle.
Applications
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, buses and other heavy vehicles, owing to its greater potential robustness and relatively low maintenance requirements, but most passenger cars have now adopted independent rear suspension instead.
Examples of some passenger-vehicle types that have employed a live axle with various suspension elements are:
- Leaf springs — the early Land-Rover and Jeep models;
- Coil springs — the Range Rover 1 & 2, Rover SD1, Volvo 240;
- Air bags — the Range Rover 3.