i am repairing a mid-80's peugeot iseran that had a broken rear derailleur (shimano rd-z501). i happened upon a similar rd, the only difference being an attached hanger/claw (see picture).

the frame has a forged hanger by the dropouts, so the attached hanger is redundant. is it possible to remove this piece (turning the bolt rotates the whole arm about 1/4-1/2 turn)?

I'm pretty sure it can be done, but your photo isn't quite clear enough to tell me how you need to go about it. I'd like to see a photo or two of the old broken derailleur next to the new one.

You just need to unbolt the claw and use the mounting bolt and hardware from your broken derailleur in your new RD. The bolt is pretty tight on the claw so if you have a vice it would be a good idea to use it to clamp the claw in first.

Glenn

Make that a vise.

That's the second Shimano derailleur I've seen that broke at that location. I don't know why they put a spring in there; doesn't seem necessary, weakens the thing at a crucial location, and makes it harder to work on.

and becareful of the spring tension

well, when i got the frame, the rd hanger was bent, so i figure this one broke because the bike fell over at some point in it's life.

Agreed. But bikes fall over a lot, sometimes with a rider on them, more frequently when left unattended. Derailleurs should be tough enough to withstand such falls.

it was interesting to see some of the fundamental differences between the two mounting systems.



i had to use the little half washer (pictured above - left) to secure the mounting bolt.



all switched!

At the upper pivot? I recall seeing some Shimano advertizing that mentioned it back in the early 80's (I think) when Suntour still had the patent on the slant parallelogram. This particular literature specifically mentioned how Shimano's derailleurs had two springs whereas some of their competitors had only one. The implication was clearly that two springs were better.

Of course, it isn't clear whether this was Product Management dictating to Engineering what to do or Marketing trying to find a way to hype something Engineering did for their own reasons. In any case, it was marketing fluff.

thanks to all for your assistance..

FWIW, the two spring design was copied from Simplex, and it does improve shifting by allowing the upper pulley to better track the cogs. Once SunTour's patent on the slant parallelogram expired, everybody started making derailleurs with two springs and a slant parallelogram.

Shimano called the design "Servo Pantograph". It didn't work as well as Suntour's slant parallelogram, but it was the best they could until Suntour's patent expired.

That's funny!

Please excuse the Lazarus thread, but this is something I've been trying to understand, with little available information.

Can anybody explain this, in technical terms? I understand the SunTour design perfectly well, with the slant parallelogram and the B-tension screw. But I cannot, for the life of me, figure out how a sprung upper pivot improves things. As the derailleur moves rearward, as it will when a Servo-Panta is shifted to smaller cogs, the lower pivot will move downward with respect to the freewheel axis. In other words, the radial distance from the freewheel axis to the lower pivot will increase. This seems to me to be the exact opposite of what would be desired. Obviously, the offset jockey pulley compensates for this, just as it does in the classic Campy designs, but they didn't have springs in their upper pivots, so it's got to be more than just that.

The best I can come up with is that two springs might increase chain tension in the smaller cogs, but the claimed benefit, according to what little actual information I've been able to find, is improved jockey pulley tracking to the freewheel cogs, not chain tension. But how?

Does anybody have any technical literature on exactly what Shimano means by "Servo-Pantograph"?

Did a quick serach and found this: https://www.youtube.com/watch?v=fODgfqpscFQ. Hopefully that helps!

It really doesn't. What it does do is increase the total chain take-up capability by letting the entire cage move forward/backwards as necessary. Of course the actual behavior depends on the balance between the two springs, and as far as I can see that is pretty unconstrained. The moment-to-moment orientations of both the cage and the derailleur body also depend on the friction in the pivots. So in general it sort of works, which is sort of okay, just not as well as the Suntour design which constrains the motion precisely. On the Suntour design one could wish for different slant angles to match different cog sets, but even with their predetermined angle it worked better than everything else.

The Crane/Rally technique of orienting the two pulleys and the cage pivot in an "L" shape makes the jockey pulley move downward/upward in response to shifting to a larger/smaller cog. This is good. But the pulley also moves in response to shifting between chainrings. This is not so good. If the chainrings are close together it isn't so bad, but closely-spaced chainrings isn't in keeping with the desire for low gears. From experience I can say that even with a half-step crossover pattern (47/42) and a widely spaced 6-cog freewheel (14-34) a front shift can sometimes change the angle of the chain at the rear enough to cause a spontaneous rear shift if the RD isn't perfectly centered.

Thanks for the link, JHess! Pretty neat video, although it shows a current "Double Servo Panta" (is that what they called it when they went to a slant parallelogram?) model, while I'm more interested in their original "Servo Panta" mechanism, pre-slant-par (think original 600, or Arabesque).

Jim, what you wrote above echoes my own understanding as well. So either I'm not crazy, or you and I both are. By the way, I always thought the adjustable parallelogram of the first-gen Chorus derailleur was quite slick, and I've always wondered why they (or other manufacturers) didn't pursue that further.

Still interested in hearing from a few more people!

After lots of musing and visualizing, I think I understand this a little bit better. I believe it has to do with the "L" shaped cage that Jim mentions. Essentially, when shifting from larger to smaller cogs, this "L" shape will cause the jockey pulley to rotate up, but it will also cause it to rotate forwards, towards the front of the bike. So if it starts out directly underneath the freewheel axis, it will end up well ahead of that axis. The parallelogram motion contributes to this as well, albeit to a lesser degree. So the sprung upper pivot will rotate the entire derailleur towards the rear to compensate. But the issue with switching chainrings remains a problem with this design, making the SunTour still superior, IMHO.

I searched YouTube, hoping to find one of Kurt's ("headbadge") videos showing a vintage 600 derailleur shifting over a vintage freewheel, but didn't find any. When my mixte build reaches final assembly, I'll mount my Arabesque GS derailleur and my wide-range freewheel, and make my own video. Until I do, the above is just conjecture...