Traction is key when it comes to 4WDing. The more traction you have, the further you can drive without losing momentum. The ability to drive your wheels, knowing that they are pushing your 4WD forward (or backwards) makes all the difference. Once you head off the bitumen the level of available traction decreases substantially.
There are a number of different ways to improve the amount of traction your 4WD has. Whether you rely on traction control, your LSD or install Lockers is something people can discuss for hours around a camp fire. They all have their pro's and con's, which I will go into below.
What influences traction?
The amount of traction you have comes from a number of different factors:
Tyre size, pressure and tread pattern
The bigger the tyres, the more surface area that touches the ground. In other words, the more rubber that is physically sitting on the ground. If you went from 31" tyres (or 265/75/16's) to 33's (285/75/16's) you would have more rubber on the ground, which results in a greater level of traction. The more you deflate your tyres the more the surface area increases (which is why 4WD tyre pressures are critical!)
Finally, tread pattern will make a massive difference. You know the road tyres that your 4WD came with? Put them in a slippery situation and they will be virtually useless. Swap them out for some quality all terrains and your traction will improve substantially. Go the extra mile and change them to mud terrain tyres and in general your traction will improve even further.
Weight of the vehicle
The heavier the vehicle the more traction you will have. However, it also means you have more weight to push around, so don't go loading your 4WD up with unnecessary gear!
The more you can keep your wheels on the ground the better traction you will have. For those who have suspension setups that offer limited flex, your traction will not be as good as it could be. Solid axle vehicles tend to be able to flex better than independent versions, which is why they are preferred for regular hard core 4WDing. Getting a few inches of extra travel from your suspension means that your wheels stay on the ground more often, and as a result you have more traction.
The path that you put your 4WD on makes all the difference too. Keep your vehicle level, with as much rubber on the ground as possible and you will be onto a winner every time. The moment you pick a wheel off the ground, or have to stop moving you will lose traction, and have to find it again!
Possibly the most influential factor of all is the terrain that you are driving on. Slippery mud is the ultimate test for traction, with pea gravel being another shocking one (especially on hills). You can't change this though, so just use what you have to deal with the situation.
Here's where it gets fun; if you have traction control, differential locks or a limited slip differential, you will more than likely go further than a vehicle without them. All 3 of these offer a different way of increasing traction.
What's the biggest issue with 4WD's and traction?
The easiest way to lose traction when 4WDing is to have all the power sent to a wheel with no weight on it. How is that possible? It's pretty simple; a standard 4WD differential (the bit between the two wheels on the front, and rear of your vehicle) will split the power 50, 50, to the left and right wheels.
However, if you pick the right wheel up off the ground, 100% of the power will go to that wheel. Why? Because it offers the least resistance, and power will always take the line of least resistance.
Even if you don't pick a wheel completely off the ground, as soon as you have more weight on one side of the differential the other side will spin.
If you have a differential locker, traction control or a limited slip differential, the above scenario changes to give you more traction.
About traction control
These days, many modern vehicles come from the factory with traction control. This is a good thing; it can make your vehicle substantially more capable. The way traction control works across different vehicles is all very similar; when the computer senses wheel slippage on one side, it uses the brake on that side to send the power to the other axle.
Lets say you drove into an offset rut, and had your front left and rear right wheel spinning, barely touching the ground and your other 2 wheels with all the weight on them. Traction control would sense this, apply the brake to your front left and rear right, which would then transfer power to the other two wheels and (with a bit of luck) get you moving again.
Diff lockers are a bit more simple; they just lock one axle to the other through the differential, meaning that both wheels are forced to turn at the same speed. If you have twin differential locks, it means all 4 wheels are physically locked together, and will all turn at the same speed.
If you pop a wheel in the air, the other wheel (with all the weight) will continue to be driven at the normal speed, which forces the wheel that has all the traction to continue to turn, and move you in the direction you want to go.
What's the pros and cons?
Traction control is reactive. This is it's single biggest downfall; you have to physically lose traction for it to kick in. Lockers on the other hand ensure your wheels turn at the same speed, and you don't have to lose traction for them to kick in. Traction control also inevitably wears your brakes out a little. However, Lockers are expensive, and one has to wonder whether they are worth the money.
What comes from the factory?
These days, traction control is becoming more and more common as a standard feature on new 4WD's. Usually it can be disabled as required. Differential locks on the other hand are usually only offered in the rear as standard on some higher model vehicles. You can opt for factory front and rear lockers on some model 4WD's, but not everyone offers this. In general its cheaper to get them from the factory than to get aftermarket ones.
So, what should I get?
Ultimately, a 4WD with front and rear lockers (manual engage - air or electric) is the best for traction. However, many people don't need this, and considering the massive pricing difference, you might just get away with traction control on its own.
It all comes down to the sort of 4WDing you do, what you expect out of your 4WD and of course, your budget.
I was never a fan of traction control until a saw a good mates Prado easily keeping up with much more modified 4WD's on the Mundaring Powerlines Track.
Have a look at the video below, showing a range of 4WD's. The Prado has traction control, the GU's just a factory rear LSD and our 80 has twin ELockers.