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Four Wheel Drive.Four wheel drive (4WD) can mean nothing more or less than that a vehicle is driven by four wheels. The term invariably also means that the vehicle in fact has exactly four wheels in total, dual wheels counting as one. It could in principle be taken to include vehicles with more than four wheels, only four driving, but this is not common usage. There is some little confusion about 4WD, fuelled to some extent by manufacturers who would like to appropriate this or related terms to their own marque. For example, Subaru is trying to redefine all wheel drive to apply only to its "cars" to distinguish them from heavier duty four wheel drives which it seeks to cast as heavy and unwieldly.
Part Time Four Wheel DrivesA part-time four wheel drive vehicle is normally in two wheel drive - the engine drives the front or the rear wheels only - in normal circumstances. There is some means (usually a dog-clutch) of engaging drive to the other axle for slippery surfaces. When the vehicle goes round a corner, the front wheels travel farther than the rear ones and this places strain on the transmission (wind-up) which is why 4WD should not be engaged on firm surfaces for long periods.
Full Time Four Wheel DrivesA full time four wheel drive vehicle delivers power to all four wheels at all times. To prevent transmission wind-up a center differential is required between the front and rear axles (in addition to the usual diff' in each axle). If the center differential is of the open type the vehicle will loose traction if any one wheel slips (not good), unless the centre diff' is locked by a differential lock, a viscous coupling or other limited slip device.
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