The differential was a milestone development in the motor car. Without it, vehicles would be fine on straight roads, but they’d struggle as soon as they came to a corner. That’s because although wheels on the left and right of a car rotate at the same speed on a straight road, they do not in a corner.
The inside wheel in a bend takes a shorter route as it is rotating around a tighter radius: it needs to be slowed down relative to straight-road running. The outside wheel, however, takes a longer route and so needs to travel more quickly than its opposite number: it is covering a longer distance.
When wheels can rotate freely, as on an undriven axle, this is not a problem. When they are both connected to the same engine and transmission, though, a few complications arise. How do you absorb this speed difference? Well, with a differential. This essentially allows wheels to turn at different speeds. Without it, the only way to absorb any speed difference would be through skidding a tyre on the ground – the wheels would be locked and so any difference in speed would not be possible.
The differential is based on the principle of sun and planet gears. This pinion gear set is mounted within a carrier, and is free to rotate. The road wheels are connected to the pinion gears and the drive is sent via the carrier, which rotates when the car starts moving.
On a straight road, the planet gears within are stationary, although the gears themselves experience radial motion within the carrier they are connected to. They begin to rotate on their individual axes in corners, while still also moving around within the differential carrier.
This ‘dual’ rotation accounts for the speed difference in corners: one axle can ‘slow’ and the other can ‘speed up’ – the resultant driving force is the same, but it’s now split unequally between the two wheels.
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