Far from wanting to get bogged down in mechanical engineering, understanding the basics of how a four wheel-drive works will complement your ability to read the terrain.
How to... drive your 4x4
Last column, we looked at negotiating sand with momentum and gravity on our side - now a note on the technical aspect of how your 4x4 vehicle behaves in soft sand. Far from wanting to get bogged down in mechanical engineering, understanding the basics of how a four wheel-drive works will complement your ability to read the terrain. As an overview, the turning power of the engine (torque) goes through a series of gears before reaching the wheel: they are the transmission, the transfer case (central differential and gear reducer) and the spider gears at the centre of the axles (in the differentials).
The main concern in off-roading lies within the transfer case and axle differentials. A normal differential, or what off-roaders call an open diff, is responsible for transferring power between left and right wheels along the same axle while at the same time allowing each wheel to spin independently. They are made specifically for driving on a hard surface. Imagine your car driving around a roundabout. The outside wheel takes a circular path with a larger circumference than the inner wheel, so the transfer of power to the wheels must allow for the outside wheel to turn at a faster rate than the inner wheel. The spider gears must transfer the torque from the engine to both wheels equally, but allow for the wheels to spin independently of each other.
What happens on soft sand, however, is a dramatic negative effect. Imagine your car with the wheel on the passenger's side of the front axle hanging off a dune with no contact to the ground. In contrast, your left side front wheel is bearing the car's weight and has full traction. What happens with an open diff is that the torque leaves the left wheel and transfers to the uselessly-spinning right one.
When the vehicle is moving, with momentum on your side, the effect of torque transferring left-right along the axle is negligible, but when you are stopped it is disastrous and will result in you revving and one wheel spinning and digging in deeper, and the other wheel just sitting there doing nothing. So how do you defeat this? The moment you stop moving forward, get off the throttle. Get out and find out which wheel is spinning - that is where all the power is going. Now, dig under the opposite wheel. Sometimes, just a few handfuls is sufficient, and then gently apply throttle, hoping to re-establish the 50-50 split of power across both wheels - and you will come free in most cases.
A real 4x4 is one that has locking differentials - which force both wheels on an axle to turn in unison. Nowadays, with so many soft SUVs designed for on-road use, there are few cars that come with lockers. Some vehicles have what are called limited slip differentials, which help this problem but are still not as good as fully locking diffs. A real 4x4 also has a locking transfer case. If you have an all-wheel-drive vehicle, where the transfer case also shifts torque front-to-rear, then stay off the sand.