Modern vehicles have drive trains that really are amazing, and differentials are one of the most amazing parts of the system. What they do is allow two wheels on the same powered axle to spin at different speeds, which is pretty handy when you’re doing stuff like going round corners. If it’s not obvious why that’s a good idea just remember that the inside of a curve is a shorter distance than the outside. When you steer round it the wheel on the inside doesn’t have as far to go, so if it’s turning as fast as the outside one it’s going to slip against the surface. That can scuff the tyre and create excessive wear; it can also break the traction on that wheel and risk losing control. Obviously none of this is a good idea, so your differential is a big help. It works through a complex system of gears that shifts more of your engine’s torque to the wheel that’s turning faster. That’s great stuff on roads, but it’s not always what you want.
If you’re off road in your favourite 4wd you’re going to get in a lot of situations where one wheel’s turning faster than the others. Your differential is going to want to shift engine power to that wheel, because that’s what differentials do, but that can be bad news. If one wheel can spin faster it’s probably because it doesn’t have as much traction. Giving it more power is just going to turn it even faster than it is already, but maybe it’s the other three – struggling slowly through heavy mud or shifting sand – that can use the power to keep you moving. What you need is a way to turn off the differential when you don’t need it. Many 4x4 vehicles have a handy gadget that does exactly that; it’s called a diff locker.
With your differential locked the transmission will keep all four wheels turning at the same speed, no matter how much grip they have. That’s exactly what you need. Instead of channelling power to wheels that are spinning uselessly the ones that still have traction will get their fair share. Many modern vehicles have automatic diff locks that engage when the system senses a big difference in traction between wheels. This often includes three separate locks; one each for the front and rear axles and one in the transfer case between the axles. Other designs have a selectable diff lock, which usually locks both axles and all four wheels. This is the most common option on military vehicles and a lot of people think it’s the best solution for real heavy duty off road driving.
If you have a selectable diff lock, when should you use it? Definitely not on roads; it will wear out your tyres and affect handling. Any time the ground is very uneven or soft, however, you’re going to benefit from it. Loose surfaces like sand, rocks or deep mud are all perfect places to lock your differential. When wheels start spinning without it, use it. A selectable diff locker is a feature that many people – even experienced off-roaders – don’t use to best advantage. It’s worth learning more about how it works though, because it can really expand the places you can go without getting stuck.