4WD and AWD systems explained
A word of caution first. Manufacturers tend to obscure the true nature of their 4WD/AWD systems behind phantasy names like "Active 4WD", "Quadra-Drive", etc. - others might call their automatic AWD "Real Time 4WD".
There is plenty of confusion about what is what. Does it matter? Can't you just call "four-wheel-drive" "all-wheel drive"? Yes you could. If all eight wheels of your big truck are driven, isn't it all wheel drive? Yes it is.
However, there are so many different 4WD systems on the market now that it is important to be precise and specific. It is important to call AWD when it is AWD and full time 4WD when it is full time 4WD. Just to say 4x4 is not sufficient any longer.
Wrong terms lead to misinformation - intentional or not. Wrong terms might make you buy something you neither want nor need! Do your homework before buying a 4WD - to get what you really need.
There is no sanctioning body whoever established the definition of terms regarding 4x4. The terms I use below are the ones used internationally by engineers and competent magazines.
4WD has been invented a very long time ago and various concepts have been tried. Here is a simplified synopsis of what is what. Links within this page provide detailed insight.
#1 Part time 4WD is a system that can only be used part of the time in four wheel drive.
This 4WD system was created to provide a vehicle with more traction to either carry higher loads and/or to travel in adverse terrain conditions. Clearly purpose built to do hard work. It can only be used for adverse terrain conditions - not for dry pavement.
Typical lever settings are 2WD, 4WD Hi, 4WD Lo (4WD Lo is missing on some newer cars like Dodge Nitro).
Very good off-road. Most competent when combined with axle differential locks.
• 2WD means better fuel economy for normal driving conditions.
• If 4WD is selected, all 4 wheels are permanently powered.
Prominent examples: 1942 Willys, Jeep Wrangler, Toyota Tundra, etc etc.
Torque Wind up
It is imperative that you never drive on the bitumen in Four wheel drive mode. Each wheel travels at its own speed, based on the distance to go around the corner. The Differentials job is to take the speed differential from the slow wheel and transfer to the fast wheel. In 4WD and All Wheel Drive drive trains, this is compounded by the front and rear wheels being driving. All with different speeds based on the variables.
If you do not take your 4WD and put it into 2WD, all those shafts cannot continue to absorb these forces, either the tyres have to slip on the surface or the shafts will twist up like a chinese burn.
Here it is a hollow drive shaft, however it can represent solid shafts in gearboxes, differentials, transfer cases etc.
Always engage 4WD in the straight where possible, of course if required it will engage in a variety of circumstances including downright, "Im really in trouble mode"
What we learned on our course is to anticipate in all conditions, be prepared for all challenges and use the good moments to prepare yourself and the car for the bad ones. The goal is always not to get stuck.
#2 Full time 4WD - also called permanent 4WD, can be used full time on all surfaces including bitumen and concrete.
Full time 4WD was created to provide a vehicle more useful for everyday life. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is no longer available. It can still be a strong workhorse. Some rough terrain competence is retained - the priority is added stability as a safety gain for everyday driving.
Typical lever or switch settings are 4WD Hi, 4WD Lo. Very good off-road when center diff is lockable. Even better when combined with axle differential locks.
• All 4 wheels are permanently powered.
Prominent examples: pre 2006 Mercedes M-Class, Mercedes G500, LandRover, RangeRover, Toyota Prado, Lexus GX470.
#3 Full time symmetric AWD is similar to full time 4WD - only it lacks the slow speed torque enhancing low range feature. Can be used full time on all surfaces including pavement.
Symmetric AWD was created as a safety feature for modern day vehicles. Not designed for hard work. Clearly a safer, more comfortable vehicle. Rough terrain competence has almost vanished - focus is on added stability and performance.
Limited use for off-road.
• All 4 wheels are permanently powered.
Prominent examples: 2006 and later Mercedes M-Class, Audi Quattro, most Subaru, pre 2006 RAV4
#4 Automatic asymmetric AWD. Auto AWD can be used full time on all surfaces including pavement. AWD only briefly activates automatically when speed differential between the front and rear wheels occurs. Eg, getting bogged on the beach, slippery gravel or mud road etc. Essentially a 2WD car with 2WD handling characteristics. Absolutely no adverse terrain capabilities. Clearly built for providing off road capability in smaller, more urban style vehicles.
• 2 wheels are powered most of the time
• 4 wheels are only temporarily powered.
Prominent examples: Volvo AWD, 2006 and later RAV4, Honda CRV.
A word of caution!
4WD, AWD, traction control and stability control are very useful features. However, none of them can defy the laws of physics.
If you are driving too fast on slippery ground none of the systems are able to save you. And even if you feel you are already going very slow - you might be too fast for the conditions.
The best safety feature of every car is still the driver. Be safe! Be Careful! Slow down!