• Sajiv Shah

The Open Differential in the Automotive Industry

Differential gear trains are nothing new for us today- in fact pretty much every car from the past hundred years has one, and the mechanism is part of common instruction in many universities. But back when the differential was invented - which is a debate in itself - it changed the game for automobiles.

The first traces to the differential date back to 100 BC, when historians believed that Greek astronomers used a differential to power a mechanism that could help them track eclipses and calenders. Much later on in 1827, watchmaker Onésiphore Pecqueur received a patent for a differential that could be used to power a steam-engine vehicle.

What is a differential?

A differential drive or gear train is a type of system that allows a drive axle to power more than one joint, and in some circumstances control that joint. In the automobile industry, differentials are placed in between the transmission from the engine and the rear axles. The differential allows one wheel to speed up in comparison to the other while maintaining that both wheels are rotating as a function of the drive axle speed.

If you have ever taken a toy or remote-controlled car and turned one of the rear wheels and noticed that the wheel on the other side begins to rotate in the other direction, it is likely because a differential drive is in the car.

Why do cars need a differential?

The use and need for a differential solve a pretty common problem that engineers in the early stages of vehicle transport had to get around. When a wheeled vehicle is taking a turn, the wheel speeds simply cannot be the same if the distances between the wheels and the rigidity and control of the car want to remain sturdy.

Because of the natural drive base of a car, we see in a turn that the outside wheel has a greater distance to cover since it is further from the turn radius, meaning that although it travels at a similar angular velocity to the inner wheel, it must have a faster linear velocity. But adding more drive motors and trying to figure out the speed using the rotation of the front wheels is simply a problem that is too complex and considering that lack of microprocessors and computers, a mechanical solution was necessary.

What are the problems with a differential?

Before the introduction of differentials, many cars simply had one wheel that was powered by the engine, but this introduced many issues. Firstly, power being produced on one side meant that there might be slight veering and uneven acceleration. But even more pressing was the issue that if the powered wheel were to get stuck in a ditch or could not maintain enough traction to drive the car, it would be stuck.

Interestingly enough, the differential actually does not completely solve this problem. Because the difference in wheel speed is not controlled or limited by an external actuator, if a wheel were to lose traction it would freely spin up while the other would be left with no power, and therefore very little torque. A car stuck in a ditch with a differential would often still have to be manually pushed out, which posed the common traction differential problem. Many different solutions have arisen, such as the locking differential, various types of limited-slip differentials, and the Torsen differential, all things that we will examine in later articles. Don't worry, these examinations are going to be far more technical and will look at the mechanics of these systems.