Four Wheel Drive Concepts
   Four Wheel Drive Types
   Petrol vs. Diesel
   High & Low Range Gears
   High & Low Range Gears
   Suspension Types
   Vehicle Recovery Points
Offroad Driving Techniques
Vehicle Recovery Techniques
Four Wheel Drive Types
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.
Part Time 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.
Permanent Four Wheel Drive
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.