A bran new all terrain 4WD military vehicle has been unveiled that can drive on water, snow, swamp, snow, ice and land. Four-wheel drive, 4×4 ("four by four"), 4WD, and AWD is a form of drivetrain capable of providing power to all wheel ends of a two-axled vehicle simultaneously. It may be full-time, or on-demand, and may be linked via a transfer case to provide multiple gear ranges. A four-wheeled vehicle with power supplied to both axles may be described as "all-wheel drive". However, not all "four-wheel drive" vehicles are "all-wheel drive", as vehicles with more than two axles may also be described as "four-wheel drive" regardless of how many axles, so long as two axles (of two wheel ends apiece) are powered.[1] Definitions[edit] 4WD/AWD systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology to describe the various architectures and functions.[2] The terms used by various manufactures often reflect marketing rather than engineering considerations or significant technical differences between systems.[3][4] 4×4[edit] Four-by-four (4×4) refers to the general class of vehicles. The first figure represents the total wheels (more precisely, axle ends, which may have multiple wheels), and the second, the number that are powered. Syntactically, 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive.[5] Alternatively, a 6x4 vehicle has three axles, any two of which provide power to two wheel ends each. The number of wheels may be greater than six, as on ubiquitous ten-wheel tractor units, but the designation stays the same.[1] 4WD[edit] Four wheel drive (4WD) refers to vehicles with two or more axles providing power to four wheel ends.[1] In the North American market the term generally refers to a system that is optimized for severe off-road driving conditions.[6] Four-wheel drive vehicles typically have a transfer case, which locks the front and rear axles, meaning that the front and rear drive shafts will be locked together when engaged. This provides maximum torque transfer to the axle with the most traction, but can cause binding when turning in high traction turning situations.[7] AWD[edit] Main article: AWD (vehicle) All wheel drive (AWD) historically was synonymous with "four-wheel drive" on four-wheeled vehicles, and six-wheel drive on 6x6s, and so on, being used in that fashion at least as early as the 1920s.[8][9] Today in the United States the term is applied to both heavy vehicles as well as light passenger vehicles. When referring to heavy vehicles the term is increasingly applied to mean "permanent multiple-wheel drive" on 2×2, 4×4, 6×6 or 8×8 drive train systems that include a differential between the front and rear drive shafts.[10] This is often coupled with some sort of anti-slip technology, increasingly hydraulic-based, that allows differentials to spin at different speeds but still be capable of transferring torque from a wheel with poor traction to one with better. Typical AWD systems work well on all surfaces, but are not intended for more extreme off-road use.[10] When used to describe AWD systems in light passenger vehicles it describes a system that either applies power to all four wheels (permanently or on demand) and targeted as improving on road traction and performance, particularly in inclement conditions, rather than for off road applications.[6] Some all wheel drive electric vehicles solve this challenge using one motor for each axle, thereby eliminating a mechanical differential between the front and rear axles. An example of this is the dual motor variant of the Tesla Model S, which on a millisecond scale can control the power distribution electronically between its two motors.[11] IWD[edit] Individual-wheel drive (IWD) was coined to identify those electric vehicles whereby each wheel is driven by its own individual electric motor. This system essentially has inherent characteristics that would be generally attributed to four-wheel drive systems like the distribution of the available power to the wheels. However, because of the inherent characteristics of electric motors, torque can be negative, as seen in the Rimac Concept One and SLS AMG Electric. This can have drastic effects, as in better handling in tight corners.[12] The term IWD can refer to a vehicle with any number of wheels. For example, the mars rovers are 6-wheel IWD. Design Differentials[edit] Main article: Differential The Lamborghini Murciélago is an AWD that powers the front via a viscous coupling unit if the rear slips. The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a manually lockable center differential, with Torsen differentials for both front and rear. Two wheels fixed to the same axle turn at the same speed as a vehicle goes around curves.

• published: 11 Feb 2016
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