Trying to put a Shimano Ultegra R.D. on a CAAD 9. The R.D. came off of a Motobecane Le Champion. The R.D. has a "notch" in the area that mounts up to the hanger. It's a lip sort of, that captures the edge of the hanger. It mounts fine on the Le champion, but the notch is too small to fit on the hanger of the CAAD 9. Is it necessary to have the hanger in that notch? It seems to slide around if it isn't.