Differentials

Differentials. It's all in the name. Can't live with them, can't live without them.

The pinion gear allows the rotational motion from the drivetrain to the wheels to be deflected by 90°. Going straight, on a uniform surface, both axle shafts turn at relatively the same speed. Since both wheels must travel around the turn at the same time, the outer wheel must rotate faster than the inner wheel.

The initial idea is to first divide the common driveshaft so that each wheel has its own shaft. This ensures that the shaft does not twist when one of the two wheels rotates with a different speed.

The slowed wheel is decelerated to the same extent as the other wheel is accelerated. The speed loss on one side of the wheel is compensated by a speed gain of the same magnitude on the other side. The principle is based on the law of conservation of energy.

The spider gears are driven by a “gearbox”, consisting of a pinion (shown in yellow) and a ring gear (shown in orange). The spider gears are mounted on this orange ring gear. Since the ring gear “carries” the revolving spider gears, the orange gear assembly is also referred to as carrier.

In order to avoid bending stresses in the axle shafts to the wheels, they are usually not driven by two spider gears. The second spider gear is offset by 180 degrees. When using two spider gears, the forces compensate each other. The drive shafts of the wheels are then subjected purely to torsion, but not to bending.

Even if the speeds of the wheels differ when cornering, both wheels are always driven by the same torque. This is because with gearboxes the change in torque only results from the ratio of the number of teeth of the gears. However, the differential gear has a symmetrical design. It does not differ in the number of teeth between the left and right axle shaft. This means that the change in torque between the drivetrain and the axle shafts are always the same. Both gears therefore have the same torque.

When starting on a smooth or slippery ground, one of the wheels may lose its grip and slip, while the other wheel remains on the ground. The differential gear now transmits the entire power to the rotating wheel, while no power is at the stationary wheel. The spinning wheel now turns at double speed, while the other wheel stands still (on a single axle, worse with tandems and tridems). 

In cases of slip a differential gear can be more of an obstacle. For this reason, off-road vehicles are equipped with differential locks. A differential lock rigidly connects the two axle shafts with each other, deactivating the differential.