Anall-wheel drive vehicle (AWD vehicle) is one with apowertrain capable of providing power to all its wheels, whether full-time or on-demand.
The most common forms of all-wheel drive are:
Vehicles may be either part-time all-wheel drive or full-time:
Particularly in North America for several decades, the designationAWD has been used and marketed – distinctly from4×4 and4WD – to apply to vehicles with drive train systems that have permanent drive, adifferential between the front and rear drive shafts, and active management of torque transfer, especially following the advent of theanti-lock braking system (ABS).[citation needed]
However, the designationsAWD[1] andall-wheel drive[2] long predated the trend, withAssociated Equipment Company (AEC) producing AWD trucks in 1929 in conjunction with the British subsidiary of the pioneering American firmFour Wheel Drive Auto Company. Additionally,General Motors began manufacturing a line as "all-wheel drive" as early as the late 1930s. This distinction in terminology is not generally used outside North America.[citation needed]
In the context ofhybrid andelectric vehicles, the North American designation of 'all-wheel-drive' may differ. In some hybrid vehicles, the combustion engine is only mechanically connected to the front wheels, while the rear wheels are powered independently by an electric motor; this may be marketed aseAWD. Furthermore, many electric vehicles have individual, unconnected motors powering each axle, or even each individual wheel, also without any center differential.
When tire grip is good during road driving, adifferential is used between the axles to avoiddriveline windup. This is not required off-road, as the limited grip allows the tires to slip. All-wheel drive vehicles designed for extensive off-road use may not have such a differential, and so they suffer from wind-up when used on-road.[3] Selectable 4WD also avoids this problem and requires only a simpledog clutch in thetransfer case, rather than a differential. For this reason, most early off-road vehicles used that system; e.g.,Jeep,Land Rover.
As vehicles became more sophisticated and tires gave better winter performance in the 1960s, there was an interest in giving the benefits of all-wheel drive to conventional cars: not for off-road use but for winter use in snow or on wet roads. Exotic vehicles such as the high-poweredJensen FF followed by theAMC Eagle,Subaru Leone andAudi Quattro series were the first to offer all-wheel drive in a high-speed road-based car. These, particularly the Quattro, would extensively develop thisdrivetrain with the use ofviscous couplings and differentials to provide a safe and drivable car. The first off-road / on-road hybrids such as theRange Rover also chose the permanent all-wheel-drive system rather than manual selection.
1929: AEC started to build AWD trucks in conjunction with FWD (UK)
By December [1939], GMC had orders for more than 4,400 all-wheel-drive military trucks.
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