Full time four wheel drive (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times. Each tire gets about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low".
When "4-low" is selected the wheels receive substancially more torque than in "4-high" - at the same time the vehicle moves at substancially slower speeds.
The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer and for drivers who at one point or another may want to negotiate difficult off-road terrain.
All wheel drive (AWD) is a system that powers all four wheels of a vehicle at all times as well. Difference to full time 4WD is that a "4-low" setting is not available. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than 4WD vehicles.
Part Time 4WD
Main mode is 2 wheel drive for everyday pavement use. Usually only rear wheels are powered (In some cases front wheels are powered instead).
When needed (usually beyond pavement) 4WD can be engaged. 4WD is used part of the time. When 4WD is engaged front wheels are powered as well. There are two different settings for 4WD - "high" and "low". 4WD "high", sometimes called high range, cannot be used on dry pavement with a "part time system. For extreme situations 4WD "low"is available, it cannot be used on dry pavement either. 4WD "low", also called low range, does not provide more traction, however, it provides two to three times more torque at about half or a third of the speeds in high range. For off-road use part time 4WD system is perfect.
Part Time 4WD requires:
· a transfer case
· two drive shafts
· two axle differentials
· two powered axles
· has NO center differential
Full Time 4WD
Also called permanent 4WD and constant 4WD. Main mode is 4WD. 4WD is used full time. No 2WD mode available. All four wheels are powered at all times. Operates well on dry pavement due to a center differential or equivalent device (planetary gears, etc). Part Time 4WD does not have a center differential.
A center differential is essential for on-road use but can be detrimental for off-pavement use. When leaving pavement the center differential needs to be disabled (locked). It either locks automatically or it has to be locked (disabled) manually.
Normal setting for on-road use and light duty off-road use is 4WD "high", also called high range. For more torque 4WD "low", also called low range, is available. Low range provides substantially more torque to the wheels and allows slower speeds than in high range. 4 low does not provide more traction - it only provides more torque.
Full Time 4WD requires:
· a transfer case
· a center differential
· two drive shafts
· two axle differentials
· two powered axles
All Wheel Drive (AWD)
Not to be confused with automatic 4WD. Only mode is 4WD. Full time. No 2WD available. No 4WD "low" available. AWD is almost useless beyond pavement.
Requires:
· transfer case
· center differential
· two drive shafts
· two axle differentials
· two powered axles
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