There are two types of four wheel drive, part-time and permanent. Apart from
exceptions such as the Pajero (Shogun in some countries), which neatly combines
the two, most vehicles employ either the one type or the other, irrespective of
what the marketing people have called it. These systems have very different
handling properties and you should know which system your vehicle uses.
|Part-Time Four Wheel Drive|
Part-time four wheel drive, as in the Jeep Wrangler, allows the driver to
manually engage and disengage the four wheel drive system. This allows the
vehicle to be driven in either two wheel drive or four wheel drive.
No center differential, front propshaft
engages for four wheel drive.
Part-time four wheel drive doesn't have a center
differential, so when four
wheel drive is engaged the front and rear propshafts are locked together,
forcing them to turn at the same speed. For this reason four wheel drive
should never be engaged on a hard, dry surface such as tarmac. The resulting
diff wind-up causes unstable cornering, tyre wear and places excessive stress
on the drive train. However, engaging four wheel drive on wet, slippery
surfaces, dirt roads or when off-road can considerably improve driving
safety, handling and aid traction.
Some vehicles, like the Honda CR-V, do not allow the driver to select four
wheel drive. A viscous coupling automatically locks the rear propshaft to the
front when the wheels under power start slipping. The result is an
automatically engaged part-time four wheel drive system.
|Permanent Four Wheel Drive|
Permanent four wheel drive, as found in the Jeep Grand Cherokee, is just what
its name suggests - four wheel drive that is always (permanently) engaged.
Center differential, front and rear
propshafts always engaged.
As both front and rear propshafts are always under power, a center
differential is employed
to compensate for the wind-up problem encountered when both front and rear
proshafts are forced to turn at the same speed. However, when off-road this
can cause problems, so many manufacturers allow the driver to manually lock
the center differential, thus ensuring that both front and rea axles are
always under power.
Just as with part-time four wheel drive, manufacturers have invented numerous
variations of the theme, each which has its application. The Jeep Grand
Cherokee, for instance, uses viscous couplings which automatically lock up when
wheel-spin is detected. Some vehicles, like the Land Rover Freelander, do not
allow the center differential
to be locked at all, thereby limiting its off-road abilities.