Full time four wheel drive, also called permanent 4WD,
(not to be confused with: part time 4WD
) is a system that powers all four wheels at all times and can be used full time on all surfaces including pavement
. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque
when the ground is level with a consistant surface
. Driver has a choice of a "4-high" and "4-low".
When "4-low" is selected the wheels create substantially more torque
(on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).
Important: "4-low" does not create more traction - it creates more torque and that can be detrimental when the ground is slippery. Slipping tires are more likely in "low" than in "high"! On snowy, icy roads "low" would be a bad choice - some really deep snow, however, puts up so much resistance that "low" is needed to push forward. You see, using 4WD is not an easy task.
The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed.
All wheel drive (AWD)
is a system that powers all four wheels of a vehicle at all times as well. Full time symmetric AWD would be the best term to be used. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range" AWD vehicles are much less capable
in off-road settings than full time 4WD vehicles, but work perfectly well on-road.
Recently some new "automatic" AWD
systems have emerged. Fancy names like "Real Time 4WD" or "active AWD" are hiding the fact that they are essentially sophisticated 2WD systems. Automatic asymmetric AWD
would be the best term for them. Unfortunately, since they offer AWD only part of the time, some confused magazine writers have called it "part time 4WD" - but that term has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD
- the name coming from the fact that 4WD can only be used part of the time (when off-road), most of the time they have to operate in 2WD (on-road).
Here is how they work:
Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes some of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first
the other axle will be added and try
to keep the car moving and stable. Once the primary driven axle regains
traction and both axles rotate at the same speed again, the system
reverts back to 2WD. So, for a moment you had AWD.
Automatic asymmetric AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. However, automatic asymmetrical AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.
must see video:
automatic AWD vs "real" AWD
Examples of automatic AWD: Honda CRV, Toyota RAV4, LandRover Freelander, Isuzu Trooper (TOD), Volvo V70, 1999 and later Jeep Grand Cherokee (in high range)
A frequent consumer complaint about some automatic AWD is, that all 4 tires need to be replaced even if only 1 or 2 are bad.
Confusing but true: some vehicles have a combination of part time and full time 4WD systems, or even a combination of 2WD, automatic asymmetric AWD and part time 4WD (low range). Impossible to give those custom mixes a name..