The differential has two main functions: 1. Change the transmission direction of engine force; 2. Transmit power to the wheels at different speeds to make the wheels on both sides of the car rotate at different speeds. Working principle of differential: when rotating, two wheels generate torque in opposite directions, which is reflected on the half shaft teeth through the half shaft, forcing the planetary gear to rotate, slowing down the speed of the inner half shaft teeth, accelerating the speed of the outer half shaft, making the speed of the wheels on both sides different, so as to achieve differential speed.

　　Why differences are needed:

　　When the car turns, the wheels turn at different speeds. When turning, the distance traveled by each wheel is not equal, that is, the distance traveled by the inner wheel is shorter than that by the outer wheel. Because the speed is equal to the distance traveled by the car divided by the time required to pass this distance, the wheels with a short distance turn slowly. At the same time, it should be noted that the distance traveled by the front wheels is different from that by the rear wheels.

　　There is no such problem with the driven wheels of rear wheel drive vehicles or the driven wheels of front wheel drive vehicles. Because they are not connected, they rotate independently. But the two drive wheels are interrelated. So an engine or a gearbox can drive two wheels at the same time. If there is no differential in the car, the two wheels must be fixed and connected together to drive them to rotate at the same speed. This will make it difficult for the car to turn. At this time, in order to make the car turn, one tire must slip. For modern tires and concrete pavement, it requires a lot of external force to make the tires skid. This force is transmitted from one tire to another through the axle, causing great stress on axle parts.