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Old 05-14-18, 07:48 PM
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milktree
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Cannondale rear shifting problem

Hi everyone.

I'm new to this forum, so hopefully I won't break any unspoken rules in my first post.

My background: I worked as a professional bicycle mechanic for three summers back when dinosaurs roamed the earth ('90-'92), and I've been a semi-professional motorcycle mechanic more recently, but not currently. (the contrast between a motorcycle chain and a bicycle chain is kind of hilarious)

My wife and I have a '12 Cannondale tandem which we bought used last year.

I can't get the rear shifting to work right.

It's a 10 speed Shimano Ultegra setup. (triple front, but that shouldn't matter here)

Here's what I've done:
  • trimmed/squared/polished all the cable housing ends and replaced the endcaps with aluminum ones
  • replaced the cable
  • verified the derailleur hanger is straight
  • verified the bushings in the derailleur are tight
  • verified the derailleur cage is straight
  • chain and cassette were new shortly before we bought the bike (according to the seller, and they look/feel good)
  • shifter and derailleur work: I hooked up the shifter directly to the derailleur with only the final cable housing between them and it shifted well
  • set up the high limit, low limit, and "B-tension" screw according to years of practice (and instructions, just to be sure)
Here's the symptoms:
  • When in the highest gear (smallest cog) and I shift once (to a lower gear, second smallest cog) the derailleur barely moves, and it doesn't shift. If I pull on the cable (at the downtube) it'll jump down a couple gears, and then settle quietly in the second smallest cog. If I then up shift, it shifts cleanly to the smallest cog.
  • Once the derailleur is part way up the cassette, it'll shift normally (both up and down) all the way to the largest cog, and up cleanly all the way all the way to the smallest cog.
  • I clipped a bunch of wooden clothespins on the derailleur cable at various places to measure cable travel. At the top of the top tube it's "big" like about 1.75mm At the entry to the derailleur housing on the chainstay, it's more like .60-.75mm (so, that's wrong) between the two bottom brackets (boom tube? bottom tube?) it's much closer to 1.75mm, but not quite (hard to measure, I haven't figured out the right technique yet)
  • on either side of the rear BB pulley the distance is different.
Therefore: that pulley is doing something wonky.

This is what the setup looks like (a slightly modified extract from page 18 of the 2009 tandem suplemental)

(grrr... I'd usually do this inline with nice thumbnails, but no URLs for newbies! Sorry about the huge image at the bottom)
(edit again... apparently even that doesn't work.)

try this : (remove the spaces)

mahonkin dot com /~milktree/bicycle/tandem/ stoker-cable-guide.jpg

and our particular one:

mahonkin dot com /~milktree/bicycle/tandem/ stoker_cable_routing.JPG

The cable goes through the red thing, and that appears to be mucking it up.

So here's my questions:
  • Do these things fail like this regularly? (is it common? How long do they typically last?)
  • Is there anything else that could be causing this?
  • If not, where the hell does anyone find specialty parts like that in the US? They're easily available in the UK and Germany!
  • Is there a better way to fix this than replacing that part?
  • What tests have I screwed up? (what's wrong with my diagnosis process?)
Thanks!

Last edited by milktree; 05-17-18 at 09:37 AM.
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