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.
Four: IIII, IV, 100 (base 2), 3+1
Wheel: big round thing
Drive: to make go
Part Time Four Wheel Drives
A 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 Drives
A 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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