Differentials and Diff' Locks for Four Wheel Drives. The
differential is the sort of thing that I would love to have
invented so that it would have been named after me instead of
Mr. Differential. It was used in tricycles in the 19th century
(it still is used in some modern trikes) and also as an adding
device in mechanical analogue computers. It is used in a car
axle to allow the outer wheel to rotate more quickly when the
car turns a corner. The outer wheel travels about 10m (30 feet)
further than the inner one in a 360 degree turn. A 4WD has one
diff' in each axle. A full-time 4WD vehicle also has a centre-
diff' in the transfer-case because the front wheels rotate more
quickly than the rear ones when turning a corner and travelling
forwards. The Open Diff. The heart of the differential is the
diff' centre (right). It rotates, driven through the crown wheel
(gear) by an input shaft and pinion gear. Internally it carries
two or four planet-gears and two side-gears. These planet-gears
drive the side-gears whi