Why is the small wheel of sewing machine round?

Whether the center of the tire is the radius of its circle (application principle: in the same circle, the distance from the center of the tire to any point on the circle is equal). And "all radii in the same circle are equal" so that the car will not bump when walking! And when the car is walking, the center of the circle moves in a flat straight line! Similarly, the small wheel of the sewing machine is round, so that the track will not deviate greatly when it runs continuously, and it is labor-saving, fast and convenient to run!