The ornamental design for a rotor for a motorcycle, as shown and described.
FIG. 1 is a front view of a rotor for a motorcycle showing one embodiment of the design of the present invention, the back view being the same;
FIG. 2 is a side view of the rotor of FIG. 1, the other side view and the top and bottom views being the same;
FIG. 3 is a front view of a rotor for a motorcycle having a second embodiment of the design of the present invention shown in FIG. 1, the back view being the same, the side, top and bottom views being the same as FIG. 2; and,
FIG. 4 is a front view of a rotor for a motorcycle having a third embodiment of the design of the present invention shown in FIG. 1, the back view being the same, the side, top and bottom views being the same as FIG. 2.