A differential is a gear train with three drive shafts that have the property that the rotational speed of one shaft is the average of the speeds of the others. A common use of differentials is in motor vehicles, to allow the wheels at each end of a drive axle to rotate at different speeds while cornering. A differential is used to distribute the torque at a 900 angle that was coming from the propeller shaft and send the torque to the wheels, the differential is made of several parts including; the pinion gear, sun gears, ring gear (grown gear) and a spider gear (planet pinion). During cornering, the outer wheels of a vehicle must travel further than the inner wheels (since they are in a larger radius). This is easily accommodated when the wheels are not connected; however, it becomes more difficult for the drive wheels, since both wheels are connected to the engine (usually via a transmission). Some vehicles (for example go-karts and trams) use axles without a differential, thus relying on wheel slip when cornering. However, for improved cornering abilities, many vehicles use a differential, which allows the two wheels to rotate at different speeds. The purpose of a differential is to transfer the engine's power to the wheels while still allowing the wheels to rotate at different speeds when required.