What is the advantage of four-wheel drive over normal drive?
The magnitude of the torque — the turning force applied to the wheel — is determined by what is called traction. Traction is the maximum amount of force the tire can apply against the ground (or that the ground can apply against the tire — they're the same thing). In low-traction situations, the maximum amount of torque that can be created is determined by the amount of traction, not by the engine. If the tires won't stick to the ground there is simply no way to harness that power.
One of the factors that affect traction is the weight of the tyre.
The more weight on a tyre, the more traction it has. Weight can shift as a car drives. For instance, when a car makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels.
Another factor affecting traction is the coefficient of friction — this factor relates the amount of friction force between two surfaces to the force holding the two surfaces together. In our case, it relates the amount of traction between the tyres and the road to the weight resting on each tyre. The coefficient of friction is mostly a function of the kind of tyres on the vehicle and the type of surface the vehicle is driving on.
For instance, one of the reasons a race car can corner at such high speeds is that its tyre has a very high coefficient of friction when it is driving on a dry, concrete track. The coefficient of friction for that same tyre in mud would be almost zero. By contrast, huge, knobby, off-road tyres would not have as high a coefficient of friction on a dry track, but in the mud, their coefficient of friction is extremely high.
Wheel slip — Static and dynamic are the two kinds of contact that tyres can make with the road.
Static contact — the tyre and the road (or ground) are not slipping relative to each other. The coefficient of friction for static contact is higher than for dynamic contact, so static contact provides better traction.
Dynamic contact — the tyre is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have less traction.
Quite simply, wheel slip occurs when the force applied to a tyre exceeds the traction available to that tyre. If you are driving four wheels instead of two, you've got the potential to double the amount of longitudinal force (the force that makes you go) that the tyres apply to the ground.
This can help in a variety of situations. For instance:
In snow — it takes a lot of force to push a car through the snow. The amount of force available is limited by the available traction. Most two-wheel-drive cars can't move if there is more than a few inches of snow on the road, because in the snow, each tyre has only a small amount of traction. A four-wheel-drive car can utilize the traction of all four tyres.
Off road — in off-road conditions, it is fairly common for at least one set of tires to be in a low-traction situation, such as when crossing a stream or mud puddle. With four-wheel drive, the other set of tyres still has traction, so they can pull you out. Climbing slippery hills — this task requires a lot of traction. A four-wheel-drive car can utilize the traction of all four tyres to pull the car up the hill. — Our Bureau