HOW DOES IT WORK?
A CONVENTIONAL diff splits the torque equally to left and right (minus some small frictional losses) at all times. It does this regardless of whether both wheels are driving you down the road at 100km/h, or whether one wheel is in the air spinning madly on a low range track.
TRICKY SITUATION
CHECK out the traffic going around the roundabout. Notice the front wheels of larger vehicles will be near the outside of the roundabout and the rear wheels will be near the inside. This tells you that when heading around a corner, the front wheels take a larger arc than the rear wheels.
When we lock the centre diff on a full-time 4WD with a conventional diff (or engage 4WD on a part-time 4WD), we remove the ability of the centre differential to compensate for the speed differential between the front and rear when negotiating corners.
This is only an issue on high traction surfaces, like bitumen and slick rock. On loose surfaces such as dirt or gravel the speed differential doesn’t pose an issue, as the surface allows the wheels to slip a little and relieve the tension.
On high traction surfaces the speed differential going through corners will build up a twisting force between the front and rear diffs, leading to an eventual driveline failure.
The first indication of failure is that the car won’t come out of 4WD. Or perhaps you’re driving a part-time 4WD and you can’t push the lever back into 2WD. This is because you’re being prevented from disengaging 4WD by the twisting force in the driveline binding up the mechanism.
To get the car out of 4WD you have two options. First, you can reverse in an arc. This will get the front wheels travelling faster than the rears and will relieve the tension to a point where the centre diff light with stop flashing, or you’re able to push the lever to engage 2WD.
The second option is to head over to the side of the road, put two wheels into the gravel, and continue driving forward. The slippage allowed by the loose surface will dissipate the twisting force, allowing you to get the car back into road mode.
WHY LOCK THE CENTRE DIFF?
A LOCKED centre diff will guarantee power is sent to the front and rear diffs.
Another reason to lock the centre diff is to avoid the problems associated with getting stuck in a vehicle with an automatic transmission. If you’re going uphill and get stuck you’ll have to perform a reverse recovery. If you try this without locking the centre differential, the front wheels will stay locked all the way to the bottom of the hill, due to the weight bias on the rear and the fact all vehicles have a braking bias to the front.