The figure shows some ovals of constant width (black), along with their evolute (red). The evolute is traced twice, as the centers of curvatures of diametral points are identical. The evolute is a special hypotrochoid. Note that the ovals of constant width are all offsets of a curve of the same type! The latter curve corresponds to the oval of constant width 0. In order to obtain regular offset curves. The offset distance has to be chosen sufficiently large.