4x4 Explained

Introduction to All Wheel Drive systems - By Eliot Lim

Differentials

A differential is a mechanical set of gears which takes input torque from a driveshaft and splits it evenly to two output axles, allowing them to rotate at different speeds. A differential in a front wheel drive or rear wheel drive car allows both wheels to apply power to the road and yet be able to rotate at different speeds so that the car can turn without resistance.

A permanently engaged four wheel drive system needs to have three differentials to enable it to apply power to four wheels and be able to turn without resistance: The front, rear and center diffs. (diff = short for differential) This is because the distance traveled by the turning front wheels is not the same as the distance traveled by the non articulating rear wheels.

Power leaving the gearbox first goes to the center diff, which then splits it via the driveshafts to the front and rear diffs. Manually engaging part time four wheel drive systems in most cases do not have a center diff, so they cannot be used in the dry. When four wheel drive is engaged in such a system, the front and rear axles are locked together and will rotate at exactly the same speeds. The difference in front and rear wheel speeds have to be scrubbed off by the tires.

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