DIFFERENTIAL

The differential is a mechanical organ consisting of a 'bevel gear' of the crown-pinion type, a 'planetary cage' integral with the crown and containing one or more 'satellite' toothed wheels and two 'planetary' toothed wheels, each integral with each of the two half-shafts.
In fact, the function of the differential is to distribute the drive torque coming from the transmission shaft to the two half-shafts of the drive wheels, both in straight line and when cornering.
For this reason, the differential is very useful, because, if it is true that in straight line it is necessary for the two coaxial wheels to be able to rotate at the same speed, it is also true that in order to maintain sufficient wheel adhesion when cornering, it is necessary to ensure that the outer wheel can rotate faster than the inner wheel.
Without the differential, in fact, the two wheels tied to the same axle would slip, which would also cause considerable tread wear and a reduced ability to negotiate tight bends.
The differential is mainly used on passenger cars, but is almost always absent on go-karts and rail and tram vehicles, for which a taper wheel system is mostly used.
On cars with permanent four-wheel drive, a 'centre differential' is used that distributes torque between the front and rear wheels. Usually this type of differential is not present on cars where all-wheel drive is switchable.
If a normal differential were to be used in those cases where there is a lack of grip when the vehicle is stationary, the wheel without grip would slip, removing torque from the gripped wheel, which would then remain stationary.
For this reason, lockable or limited slip differentials are used on off-road vehicles. If necessary, these differentials allow the action of the differential itself to be excluded, distributing torque evenly over the two half-shafts. For the same reason, on racing cars or particularly high-performance vehicles, self-locking differentials of the TorSen, flap or Ferguson viscous joint type are used.
Nowadays, electronically controlled differentials are also available, which allow the slipping wheel to be braked by the braking system. Such systems are known as 'traction control' or 'anti-slip or anti-skid control' and by the acronyms TCS (Traction Control System) or ASR (Anti-Slip Regulation or Anti-Spin Regulation).