Why are wheels round? Of course, wheels are not necessarily round, but round wheels are used the most.

People use an important property of the circle to make the wheel into a circle: put a circle between two parallel lines to make it tangent to these two parallel lines. Yes, no matter how the circle moves, it is still in these two parallel lines and always tangent to them. This is the fixed-width property of a circle, and a curve with a fixed-width property similar to a circle is called a fixed-width curve.

There are more than one fixed-width curve. For example, make an equilateral triangle ABC, then make an arc to connect BC, then make a vertex B as a circle and a triangle side length as a radius, make an arc to connect AC, then make a vertex C as a circle and a triangle side length as a radius, and make an arc to connect AB, then the curve ABC is also a fixed width curve.

Using a circle as a wheel is the result of the expansion of human civilization, not only because the width of the circle is fixed, but also because the circle is one of the most common figures, such as the sun and the moon. It is also the simplest of all fixed-width curves. Bullets are easier to machine. Moreover, the stability of fixed width is good, and even if the circle is abnormal, it will maintain a good fixed width.

In addition, the circle also has an important property, the stability of the geometric center, and the height of the central axis of the circle (the axis passing through the center of the circle) remains unchanged when the circle rotates, which is always the height of the upward radius of the ground.

Imagine using another fixed-width curve given above, whose geometric center is unstable and jumps up and down with the rotation of the figure, which is not suitable for making wheels.

Based on the characteristics of appeal, round wheels are the most widely used.