Well, full time 4WD makes driving under certain conditions more stable. Wet or otherwise slippery roads, especially in turns are negotiated with more safety than in a 2WD. But you could of course slow down when in a 2WD - however, sometimes slippery spots sneak up on you and you don't have enough time to react, in those cases full time 4WD could be like life insurance.
1 The only 4WD concept that is truly useful on pavement is "full time 4WD".
2 AWD, or all wheel drive, is the same as full time 4WD - only difference is that AWD does not have low range.
4 Automatic 4WD is a sophisticated 2WD system offered under misleading names like "True 4WD" or TOD (torque on demand). It should be called part time AWD. It works pretty good on pavement under slippery conditions but is absolutely useless off-road. A cheap solution with questionable benefits.
Not very many vehicles offer true "full time 4WD" - check with your manufacturer what they offer you.
Now, here is a simplified answer how 4WD utilizes traction better than 2 WD (and forgive my start at "Adam & Eve"):
Each of the 4 tires on a vehicle has traction (the friction between rubber and ground). Lets assume for arguments sake that each tire on dry surface has a traction value of 100 - a total of 400. **
In a 2WD only the traction of 2 tires (traction value 200) is used to create forward movement. Torque of the engine is using the resistance (traction) of two tires to get you moving. Lets say a torque value of 140 is needed to move the car. The traction value 200 is "used up" by 140 torque units. A safety margin of 60 remains.
Lets say due to slippery ground the traction value drops to 50 per tire. That gives you 100 usable units on the two powered tires - but with 140 torque units your "traction account "is overdrawn by 40. Result is spinning tires and you can't get the car moving.
In a 4WD system the engine's power is not sent to two tires (each carries 50% of the load) - power is sent to all 4 tires (each tire carries 25% of the load) utilizing the traction of the other 2 tires.
Going back to the previous examples, on dry surface with 400 traction and 140 torque withdrawal a safety margin of 260 remains (wow) and on slippery ground with 200 traction and 140 torque withdrawal a safety margin of 60 remains.Unlike the 2WD, tires are not spinning, forward movement is guaranteed. As you see, in 4WD the traction account is twice as high as in 2WD.
This long story only explains how 4WD uses traction better and helps to get a vehicle going when the ground is slippery. This is the situation when a 2WD can't take off on snow but a 4WD can. This does not constitute safety - it is only convenience. A 4WD can go where a 2WD can't.
** all traction and torque numbers were randomly selected for easier understanding of the subject.
|Here my explanation why full time 4WD makes every day driving more stable:
There is a second force aside from torque that is eating away our traction account: Lateral force. Whenever you go into a turn traction is counteracting the lateral force (centrifugal force) in order to keep the vehicle on it's intended path.
I have to go back to "Adam&Eve" (sorry, it is necessary):
You are in a 2WD, surface is dry, traction account is 200, torque is using up 140, once you go into the turn 20 units of lateral force per tire eat away from the 200, leaving only a safety margin of 20. (200 -140 -40). Suddenly you spot a patch of sand on the surface of the turn (maybe a truck lost some sand) traction value of sand on pavement is very low (sand acts like little ball bearings under your tires), maybe 70 per tire, making it 140 for the 2 powered tires. 140 minus 140 minus 40 and the traction account is overdrawn by 40 and you slide out of the turn. Don't even think where you could end up.
Same scenario in a 4WD. Surface is dry, traction account is 400, torque is using up 140, once you go into the turn 20 units of lateral force per tire eat away from the 400, leaving a safety margin of 180 (400 -120 -80). Suddenly you spot a patch of sand on the surface of the turn, traction value maybe 70 per tire, making it 280 for the 4 powered tires. 280 minus 140 minus 80 leaves you still with a safety margin of 60. Demonstrating that with full time 4WD you round that turn like driving on rails even with occasional patches of sand, mud etc. So yes, 4WD is safer when it comes to rounding turns with suddenly changin surface conditions.
Driving straight 4WD does not offer much in terms of stability. A little but not much. Make certain, it is really "full time 4WD" (like Mercedes-Benz, BMW, Porsche, Range Rover, Toyota Landcruiser, etc.)
4WD helps you get moving and supports directional stability.
4WD does not help you to go faster.
4WD does not help you to stop the vehicle.
You all know that a 2WD vehicle uses 4 brakes to stop - so, it is a "four wheel brake" or "4WB" . Same story on all 4WD vehicles - they are also "4WB" and have no advantage over 2WD when it comes to stopping. Four brakes, four contact patches of rubber on the ground - that's it!
To improve traction on snow and ice you should seriously consider using chains. A 2WD with chains will easily outperform a 4WD with snow tires and only with chains on all 4 wheels a 4WD can deliver it's full potential on snow and ice.