Differential on a truck (photo by Satish Krishnamurthy – CC-BY)
A differential is the set of gears between the drive shaft of a vehicle’s motor and the wheels, in rear-wheel drive vehicles. It not only changes the direction of the effort but also allows the wheels to turn at different speeds. In vehicles with front wheel drive, those with a cross-mounted engine, there must also be a differential. Why? In both cases, in a curve, the wheels travel different distances. The inner wheel follows a curve of a smaller radius, not having to turn as fast as the outer wheel, which follows a larger radius. The differential allows this, while still applying torque to both wheels.
Clever, but as many know, if one wheel slips on snow or mud, the differential would let it spin, while the other wheel remains still. This problem has been solved with the limited-slip differential.
But when was the differential gear invented? It might have been invented in China many centuries ago, since it is hypothesized that it was used in South-pointing Chariots, which carry a figure that always points south, regardless of which way the chariot turns.
Credit where credit is due, although such chariots could have been made to function without differential gears. Furthermore, the explanations for the chariot’s purpose seem illogical, since travelers could orient themselves by the sun and stars, as they always had done. The chariots sound more like a sophisticated toy—from whichever century.
Leonardo da Vinci is also credited with inventing a differential gear for a vehicle, but this is also doubted. Without question, however, a differential gear became necessary for mechanically propelled vehicles. Onésiphore Pecqueur is credited with inventing the differential gear as we know it in 1827, to power a steam driven vehicle.
Rail vehicles solve the problem a different way. Their wheels are shaped into a conic section. The explanation applies to all the wheels of the train, not just those of the locomotive that are under power. This solution also has the effect of slightly tilting the train against the centrifugal force in the curve. The conical bearing surface of the wheels is not a differential, but solves the same problem that a differential does.
What would happen without coned wheels on trains, or differentials on road vehicles?
The train would be forced to go around the curve by the flanges of the train’s wheels against the track; the vehicle’s rear wheels would be forced to follow the front wheels. As evident from Pecqueur’s (or maybe da Vinci’s) invention, it was recognized very early that this was a serious problem, as one or both of the wheels would slip on the road surface. The differential solved the problem, avoiding (most of) the tire wear that would have resulted.
This wear is still a problem for vehicles that use multiple rear axles to distribute the load onto more wheels. Most of these vehicles reduce the wear by raising the leading rear axle when the load allows, thus limiting the drag of rubber on road, and the resulting additional friction.
There would be a similar problem for trains’ wheels and the tracks in curves. Imagine a train winding up a slope, the slippage generating heat, wasting the effort of the locomotive and wearing down both wheels and track.