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derailleur hitting cogs(5 posts)

derailleur hitting cogsDougSloan
Sep 23, 2002 1:32 PM
On my Cervelo, the rear derailleur (Dura Ace) hits the cogs when I use a 23 cog; it's ok with a 21 large cog, but I replaced the 12-21 with an 11-23 and when in the large cog, the upper pulley rides all they way up against the cog, making an awful noise. I tried adjusting the B adjustment screw all the way both ways, but this made no difference. I also tried a longer and shorter chain length, but no difference.

On other bikes, the DA derailleur works fine up to a 27 large cog. I can't understand why it would not work with even a 23 cog on the P3. I might even want to use a 25 or 27 some day, so this concerns me. Short of putting an XTR derailleur on, anyone have any ideas? Thanks.

Doug
couple of possibilities...C-40
Sep 23, 2002 2:21 PM
The stop for the B-screw may not be positioned properly. Could also be an error in the position of the threaded derailleur mounting hole on the drop-out.

Campy gives some specific dimensions that insures compatibility with their derailleurs.

Adjusting the chain length to the shortest possible length can move the upper pulley forward enough to clear the large cog, but a change to a 27 would require another inch of chain and even the 1/2 inch of extra chain required by a 25 might cause the derailleur to over-extend. Risky to say the least.

I just worked on a recumbent trike that use a short cage 105 derailleur to shift an 11-34 cassette. The bike had only one chainring, with an internal two-speed transmission built into the BB. This bike used the short chain technique to barely clear the 34T cog.
hmmDougSloan
Sep 23, 2002 2:26 PM
Do you think the axle being all the way forward in the horizontal dropouts is the culprit? It seems like it would need to be pretty far back before it would make a difference, though. I'll take a look again at the position, and maybe post a picture. Thanks.

Doug
re: derailleur hitting cogsJohnG
Sep 24, 2002 6:04 AM
Maybe you didn't try a short enough chain. ????

I.e. A shortish chain will help "pull" the upper jockey wheel away from the big cog. You can verify this by temporarily "overlapping" a couple of links on the small crankset ring and looking at how the derailleur responds.

** Once you find the magic 'short length', check to see if everything is OK on the BIG-BIG combo. If not.......... long cage derailleur time.

JohnG
Derailleurs are cheap, XTR is good.scottfree
Sep 24, 2002 9:45 AM
Since you've fiddled with the B screw and chain length to no avail, you can save yourself a lot of frustration by throwing one on there. I've done it to solve a similar problem.