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 vehicle
An all-terrain vehicle (ATV), also known as a light utility vehicle (LUV), a quad bike or quad (if it has four wheels), as defined by the American National Standards Institute (ANSI), is a vehicle that travels on low-pressure tires, has a seat ...
s 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.
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
Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucks[steering
Steering is the control of the direction of motion or the components that enable its control. Steering is achieved through various arrangements, among them ailerons for airplanes, rudders for boats, cylic tilting of rotors for helicopters, ...]
. 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
Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...
.
Gallery
File:Eliica2.jpg, Eliica, the eight-wheeled electric car of Hiroshi Shimizu.
File:Kynos Aljaba 8x8 Ejército Español.JPG, Kynos Aljaba 8x8 tank transporter tractor with wide-footprint tires
File:Gepanzertes Transportkraftfahrzeug (GTK) Boxer in Zeithain.jpg, Boxer APC; note the single tires and near-equal spacing of the front and rear wheel pairs
File:Transport heavy equipment carries an M60 Patton Tank at the National Guard Armory in Bluffton, Ind., June 30, 2009..jpg, The Oshkosh M1070 HET is unusual in that while an 8x8 it has a single driven front axle, and a driven rear tridem
File:Tow truck04.jpg, This TATRA T815 recovery truck has dual rear wheels (12 wheels in all) but is still categorized as an 8x8
File:Wittman Regional Airport fire truck AV2005.jpeg, This 8x8 fire truck is unusual in that it steers by frame articulation
File:2nd Cavalry Regiment external evaluation - .50-caliber fire (7308719060).jpg, On most wheeled armored 8x8s the front two axles steer, although on current generation offerings it is becoming more common for the rear axle to countersteer, thus improving maneuverability
File:Faltstrassengeraet.jpg, It is not unusual for longer wheelbase trucks with four axles to have greater spacing between the front axles, as this helps shrink the turning circle
File:DAF YP 408 UNIFIL.jpg, The DAF YP 408 APC may appear to be an 8x8 but the second axle is not driven, making the configuration 8x6
See also
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H-drive
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Six-wheel drive
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Four-wheel drive
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6x4 (drivetrain)
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Driveline windup
References
External links
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{{Modern IFV and APC
Car layouts