The car’s differential is yet another one of those systems that has existed for so long that we rarely wonder how it works exactly. Virtually every car that uses a combustion engine has a differential because it is simply the part that allows your car to make turns. And I’m not talking about being agile on those winding roads that you like on the weekend, but making any turns.
The need for a differential is due to the fact that mounted on the same axis have different speeds in curves. This is because, as the axle begins to rotate, the wheels begin an arc-shaped trajectory whose length is determined by its radius. As the outer wheel is furthest from the pivot point of the rotation, the radius of that arc will be larger, thus increasing its length.
Therein lies the problem: in order to run this longer arc at the same time, it needs a higher speed, but without a device that allows a different speed between the wheels, its speed will always be the same as the inner wheel. As a result, the outer wheel would be dragged along the shaft by not rotating fast enough.
With the differential each of the wheels can rotate independently, while still receiving the torque of the engine. For this, it uses a set of four opposing gears, of the pinion type, also called “planetary and satellites”. Two of them, the satellites, are connected to the gears that transmit the motor torque, and are positioned between the other two (planetary), each connected to a semi-tree.
When the car is in a straight line, with the wheels at the same speed, the gears are stopped between them, but they turn together, following the transmission crown. When the car makes a turn, the inner wheel turns slower causing its planetary to rotate at a different speed from the rest of the set, which continues its movement without interference due to the arrangement of the satellites. It seems complicated like that without visual reference, but this video produced by General Motors in 1937 (!) Is still the best explanation:
Although it was the device that helped the cars to make turns, the open differential has its side effects: the amount of torque that results in the wheels also depends on the tire’s adhesion. Therefore, in situations where the unlocking of one of the wheels occurs, as the open differential distributes the force equally to the two wheels, the torque sent to the undrawn wheel will be limited by the wheel being unlocked. That’s why rookie and off-roader cars use differential locking.
It is for this reason also that sports cars adopt differentials with limited slip, which balances the functions of the open differential and the differential with lock. In ideal conditions the system works as an open differential, but when one of the wheels loses traction, the differential can send extra torque to the other wheel with more traction smoothly and without total blockage. But that part – the exact functioning of the differential with limited slip and the different existing types – is a chat for a next post.