I would classify most rear derailleurs since the 60's into three groups:
1) Drop parallelograms: Campy NR and SR, Simplex Prestige, Criterium and SLJ 5000, and many other RDs. These pointed downward and the front faceplate is in an almost vertical plane. Looking down from directly above you see mostly the top pivot bolt. Since the front plate is almost vertical, it's difficult to read the name on it.
2) What I'd call a "horizonal parallelogram": Shimano Crane, Simplex SLJ 6600, some of the last Hurets. The body makes a 90deg bend below the top pivot, so the other end of the body points forward, but the front faceplate is still in a vertical plane. Looking down from above you see the top pivot bolt and the top edge of the faceplate, but you still can't read the name on the faceplate.
3) The slant parallelogram patented by SunTour: The body makes the 90deg bend and points forward, but the front faceplate is not in a vertical plane. Instead, its top edge slants inward by nearly 45deg. This is the "slant". Looking down from above the faceplate is slanted inwardward on top, so you can easily read the name on the faceplate.
The slant also causes the upper jockey wheel and the chain to move on a similar slant as the chain travels from cog to cog. This causes the jockey wheel to keep an almost constant clearance from the freewheel. Most users, including myself, believe this resulted in clearly superior shifting. I think the "horizonal" parallelogram was an attempt to copy the Suntour design as much as possible without infringing the patented slant. Whether this design was any better than the old drop parallelogram is a matter of opinion. As soon as the SunTour patent expired, essentially everybody copied the design, so modern derailleurs are virtually all slant parallelograms. New cyclists wonder what was so special about it because they have probably never used anything else.
Regards,
Jerry Moos
garth libre wrote:
> At the risk of seeming very foolish here, I am going to speak the unspeakable: What is slant that is refered to in the slant parallelogram design rear deraileur? I would imagine that the slant must be that one which causes the downward line of the rd to point more forward. If this is the case, then how was this an invention? Many rd's had at least some forward pointing slant...no? This would be a degree of forward slant then and not a whole new concept. In those beautiful old campy derailleurs, the design did seem to make them hang down straight, but the jockey wheels were still somehow very close to the cluster. The old campy rd's looked so delicate and elegant hanging like women's pendant earrings. Did the full slant design really improve things, or was it just that aluminum casting, chain design and cluster teeth just evolved so that rear shifting vastly improved? So there are three questions that are bugging me: 1) Which is the slant that is refered to in slant parallelogram? 2) Did it vastly improve things all by itself? 3) Don't high quality downward hanging derailleurs work as well or almost as well? Garth