GAPIS4WD

Remember when 4WD is engage all the wheels are being forced to turn at the same time and revolution. Driving dead straight on a dry, firm surface will cause no problem. The moment the steering wheel is turned, say to left, the wheels on the right-hand side of the vehicle are having to travel a greater distance around the arc than those on the left.

This will tighten up the transmission considerably as both axles and all the wheels are forced to rotate at differing speeds. That is transmission wind-up. Long term, this will lead to severe tyre wear and eventually damage to the gearbox. Changing gear is difficult with transmission wind-up. This situation does not arise off road because of the type terrain. Muddy, wet or stony surfaces will allow wheel slip without the driver knowing and this will cancel out any potential wind-up.

The design of permanent 4WD vehicles therefore needed careful consideration of transmission wind-up. The problem was delat with installing a third differential. Whilst the other two are located, respectively, on front and rear axles this additional one is inside the gearbox. It allow both axle to rotate at different speeds.

However, if one axle's wheel revolves faster than the other while driving off-road in slippery conditions this center differential will permit the drive to go the easiest route - the power will go to the axle that can spin its wheels most freely. This is the last thing you want because if you have one wheel on a slippery surface and the other three wheels on firmer ground everything transfer to that one wheel.

Thus, all permanent 4WD vehicles are equipped with a centre differential lock on that central differential. This has the effect of stopping the centre differential working so that drive will be equalised between the front and rear axles. This lock should be in the 'lock' position when on slippery surfaces and off-road but unlocked whenever driving on dry, hard surfaces.