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Old 07-31-10, 02:53 PM   #1
wearyourtruth
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gearing/chain/rear derailler issue

hey everyone, i recently built up a new touring bike and i'm running into some issue with the gearing. i have a 12-32t rear cassette with a deore long pull derailler. the front is set up for a triple, though i'm not running the granny gear right now. the large ring up front is 46t.

when i'm in the small/middle ring in the front and the 2nd to largest cog in the back, the derailler rubs against the largest cog


if i were to take a couple links out, then it would clear, however if i am in the large front chainring and the largest rear cog, the derailler just barely clears. if i were to take out any more links then it would catch the teeth in the rear. even though i would never ride in this gear ratio, i feel like this shouldn't happen.


now my understanding of the compatibility of gearing is not non-existent. even though you don't really ride in either of these gear combinations, the general rule that i've followed (which i think i got from sheldon) is that in the largest/largest chainring/cog, the chain should still have some "S" shape to it, not pulled straight... and when in the smallest/smallest, the derailler should still have a little tension to it and not allow the chain to rub against anything.

here is where my derailler sits on the largest/largest set-up.


i'm not sure how to fix this problem. as i said, i don't think i can take any links out of the chain, and adding a couple would only make matters worse. i also know that deraillers and cassettes need to be compatible, but i have yet to come across a derailler with longer reach than the one currently on the bike. i've also played with the adjustment screw on the back of the derailler that pushes against the dropout on the frame, but that hasn't seemed to help much either

any suggestions?
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Old 07-31-10, 03:00 PM   #2
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On the top back of the RD is an adjusting screw commonly called the "B" screw. Turn it in to bring the derailleur lower until it clears. If it's already in all the way, you can modify the RD a bit by filing inner cage plate a bit where it interferes. It's possible you might need to do both, but in all likelihood the "B" screw adjustment should be enough.

BTW- chain length also affects this, so double check that the chain length is OK (big/big + 1" minimum) and if you have extra length to work with pulling the lower loop forward 1" and seeing if that helps.
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Old 07-31-10, 05:44 PM   #3
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yeah the B screw is what i was referring to, i think it's all the way in, if not close to it... i'll double check. when i threaded it in further it did help, but not enough to make the problem totally go away.
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Old 07-31-10, 06:00 PM   #4
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Now it's time to try a test or two. First pull the RD back a bit (as if the "B" screw were able to go in) more and see if that solves it. Also double check whether changing chain length is possible and if it helps.

If you'd rather go the"B" screw route, you can buy a longer screw of the same thread, preferably one made of stainless steel, or remove the RD and build up the area behind the stop at the bottom with "plastic steel" to push the stop back about an 1/8 inch. Let it cure then re-assemble and you'll have more range with the same screw. When I do this I put a 1/8" bearing ball, or small nut into the plastic steel for more strength.
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Old 07-31-10, 09:28 PM   #5
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Originally Posted by wearyourtruth View Post
here is where my derailler sits on the largest/largest set-up.


any suggestions?
What the heck? Did you or someone else take the inner cage plate off the derailleur (to remove the pulleys or something)?? Because it looks like the inner plate is now on upside down. The hook on the inner plate that's running into the cogs should be on the bottom, not the top! It should sit next to the tab on the outer plate that sits under the lower pulley.
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Old 08-01-10, 06:45 AM   #6
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Originally Posted by Jeff Wills View Post
What the heck? Did you or someone else take the inner cage plate off the derailleur (to remove the pulleys or something)?? Because it looks like the inner plate is now on upside down. The hook on the inner plate that's running into the cogs should be on the bottom, not the top! It should sit next to the tab on the outer plate that sits under the lower pulley.
HA! you are a saint! i don't know how i didn't notice that it was upside down. that COMPLETELY fixes the problem. thanks!
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Old 08-01-10, 07:16 AM   #7
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Jeff, we all missed that! My first thought was that the chain is slightly too long since in big-big there is a fair amount of slack and removing a link would still allow big-big and move the rd away from the largest cog. I never looked at the rd itself. Nice catch.
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