Eight-wheel drive, often notated as8WD or8×8, is adrivetrain configuration that allows all eight wheels of an eight-wheeled vehicle to bedrive wheels simultaneously. Unlikefour-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such asarmored vehicles,tractor units orall-terrain vehicles such as theArgo Avenger.
When such a vehicle only has eight wheels by definition all are driven. When it has twelve – with two pairs of dual wheels on each rearaxle – all are also driven but the 8×8 designation remains. Very occasionally, the two front axles are fitted with dual wheels, such as on theSterling T26.[1] For most military applications where traction/mobility are considered more important than payload, single wheels on each axle are the norm. On some vehicles, usuallyrecovery trucks orheavy tractor units, the rear axles will have wider tires than the front axles.[2]
Heavy hauler andballast tractor 8×8s have had a long history as prime movers in both the military (as tank transports andartillery tractors), as well as commercially inlogging and heavy equipment hauling both on- and off-road. Most eight-wheel drive trucks have two front and two rear axles, with only the front pairsteering. Occasionally a single front and three rear (tridem) axles are seen, an example being theOshkosh M1070tank transporter. In such configurations, the frontmost and rearmost axle usually steer.[3] Other wheel groupings include that of theZIL-135.
Many wheeledarmored vehicles have an 8x8 drivetrain, with the axles (which usually have independent suspension) more evenly spaced. Latest generation 8x8 wheeled armored vehicles have steering on the rearmost axle to improve maneuverability.[4][unreliable source?]
In the case of both truck and armored vehicle applications, drive may be limited to the rear two axles for on-road use, thus reducing drivetrain stress and tire wear, and increasingfuel efficiency.
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