Because of the law of the center of gravity, it is very easy to overturn. It is strange that ancient Chinese people used it to carry people and carry people. It is a long journey and smooth and light. Therefore, its creator and the first driver must be a mechanical engineer with courage and knowledge. As for the shaft of a unicycle, its length, flatness and inclination, the height, straightness and inclination of the strut, and the square and elliptical shape of the wheel cover almost vary from place to place and from person to person.