The humble differential, this collection of gears is most likely the most important piece of your 4WD's drivetrain. After all, no diff, no drive. But how many of us know the differences between the variety of diff centres and how they work? For someone who is new to 4WDing, the whole locked vs limited slip vs open centre discussion can be a bit confusing. What does my rig have? How does it work? How can I make it better? These are the questions most of us have asked at one point or another.

First off, let's go through just how a differential works. As your tailshaft spins, it drives the pinion shaft and pinion gear, which in turn spins the crown wheel. Attached to the crown wheel is a housing called the hemisphere. Inside this are the gears that transmit the power to the axles.

Inside the hemisphere of a typical open diff, you'll find a pair of axle gears. These are splined to suit the axle shafts. The hemisphere is also home to a pair of planetary gears that run at 90° to the axle gears. As your truck moves forward, the crown wheel turns the hemisphere, which in turn drives the planetary gears and axle gears. When you are travelling straight ahead with both tyres on a solid surface, the axle gears are moving at the same speed, so the planetary gears remain stationary, simply following the hemisphere as it rotates.

Now comes the tricky bit. When you start to turn a corner, the outside wheel is forced to turn faster than the inside wheel as it takes a wider radius. If the planetary gears remained stationary, both wheels would be forced to turn at the same speed, making it very hard for the vehicle to turn (essentially this is what a locked diff does, but more on that later). So when you're taking a corner, the spider gears start to rotate slightly, allowing both wheels to turn at different speeds.

What we just described is your standard differential operation or basically an 'open diff'. An open diff is fine in most everyday driving situations, but problems start to occur when more traction is needed, and here's why. If you're a long-time reader, you've probably heard Roothy explain the 'path of least resistance' when talking about lockers. With an open differential, when you're trying to power out of a corner or you've got a wheel up in the air while getting dirty in some ruts, the wheel with the least force acting against it (whether it be tarmac, dirt, mud etc) will get all the torque and start spinning. Not much help if you're trying to drive a heavily rutted track.

To help overcome this, most manufacturers fit limited-slip differentials (LSDs) to the rear of their various 4WDs. In some respect, the LSD does its best to act in a similar way to an automatic locked differential and will sense when one wheel is getting too much torque and will send some to the other wheel in an effort to gain traction. A locking differential does basically the same job, but only works when you want it to and when it locks in it drives both axles at exactly the same speed. These various types of diff centres aren't operated by black magic, so let's take a closer look at how each centre works, how they can help you driving further, and how you can help keep yours in tip-top condition so that it keeps delivering traction when you need it most.

OPEN CENTRES

As we just discussed, the open centre is the most common of all diffs. Pretty much every vehicle on the road has (or had at some point) an open centre. In terms of 4WDs, nearly all will have an open front differential.