Eight-wheel drive



Eight-wheel drive, often notated as 8WD or 8×8, is a drivetrain configuration that allows all eight wheels of an eight-wheeled vehicle to be drive wheels simultaneously. Unlike four-wheel drive drivetrains, the configuration is largely confined to heavy-duty off-road and military vehicles, such as armored vehicles, tractor units or all-terrain vehicles such as the Argo Avenger.

Operation
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 rear axle – all are also driven but the 8×8 designation remains. Very occasionally, the two front axles are fitted with dual wheels, such as on the Sterling T26. For most military applications where traction/mobility are considered more important than payload, single wheels on each axle are the norm. On some vehicles, usually recovery trucks or heavy tractor units, the rear axles will have wider tires than the front axles.

Heavy hauler and ballast tractor 8×8s have had a long history as prime movers in both the military (as tank transports and artillery tractors), as well as commercially in logging 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 pair steering. Occasionally a single front and three rear (tridem) axles are seen, an example being the Oshkosh M1070 tank transporter. In such configurations, the frontmost and rearmost axle usually steer. Other wheel groupings include that of the ZIL-135.

Many wheeled armored 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.

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 increasing fuel efficiency.