taken on line A--A of FIG. 2A;FIG. 3A is a longitudinal sectional view illustrating an embodiment of the present invention in the disconnected condition of the hub clutch;FIG. 3B shows the connected condition of the hub clutch shown in FIG. 3A.FIGS. 4A and 4B are a perspective view and a diagramatic explanatory view of the cam means shown in FIGS. 3A and 3B, respectively; andFIG. 5 is a perspectiv