4 Wheel Drive
Four wheel drive is a drivetrain configuration where all 4
wheels provide propulsion.  Typically cars are propelled
by the rear wheels or front wheels only.  The exception
of course, is 4 wheel drive or all wheel drive.  

In the most basic 4 wheel drive setup, power from the
engine/transmission is transferred by a spinning drive
shaft to a transfer case.  The transfer case is a power
distribution device, which transfers power from the
engine driveshaft to the front and rear driveshafts via
chain drive or meshed gear drive.   The front driveshaft
propels the front wheels through the front axle.  The
rear driveshaft propels the rear wheels via the rear axle.

The downside of this basic 4 wheel drive, is that the
front and rear wheels turn at the same rate.  Both the
front and rear wheels are dependent upon the driveshaft
speed entering the transfer case.  Gear ratios can be
changed in the transfer case and axles, but the input
speed remains the same.   An advance in 4wheel drive
came when the front and rear wheels could change their
speed independently and constantly.  This way each
wheel would receive just enough power to maintain
traction, and not too much to spin freely.
Basic 4 Wheel Drive Structure
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