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Old 05-28-08, 06:31 PM   #1
bellweatherman
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Exceeding the recommended max tooth capacity for a rear derailler. Bad idea?

Has anyone exceeded the recommended max tooth capactiy for a rear derailler? I have an Ultegra rear derailler which has a max tooth capacity of 27T. I was thinking about ordering a new cassette that has the largest cog at 30T. Just wondering if it is possible or if this is a bad idea. Anyone tried this or know if this is ok? I mean, what's the worst that could happen if it doesn't work?
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Old 05-28-08, 06:35 PM   #2
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The general experience is that Shimano road rear derailleurs will usually work with a 30T cog, sometimes with 32 and nearly never with a 34. Unless your rd hanger is oddly placed, the 30T cog should be fine.
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Old 05-28-08, 07:05 PM   #3
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Yes, there is a margin of error so a 30 will probably be fine. If it doesn't work, you'll get some rubbing between the 30t cog and the derailleur pulley.

Now, make sure you're not too far past the maximum chain wrap capacity as well. If you go too far with that, it can yank your derailleur apart (happened to me) if you cross chain in the big/big combo.
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Old 05-28-08, 07:40 PM   #4
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Yes, there is a margin of error so a 30 will probably be fine. If it doesn't work, you'll get some rubbing between the 30t cog and the derailleur pulley.

Now, make sure you're not too far past the maximum chain wrap capacity as well. If you go too far with that, it can yank your derailleur apart (happened to me) if you cross chain in the big/big combo.
What likely happened with your rear derailleur was the chain was too short for the big/big combo.

The situation that requires the rear derailleur to wrap the most chain is the small/small combo.
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Old 05-28-08, 07:53 PM   #5
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What likely happened with your rear derailleur was the chain was too short for the big/big combo.

The situation that requires the rear derailleur to wrap the most chain is the small/small combo.
Exactly. The chain was as long as it could possibly be for the small/small combo (11/34) meaning a chain any longer would go slack, but was still too short for the big/big combo (50/28). Kaboom!!!
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Old 05-28-08, 08:02 PM   #6
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Exactly. The chain was as long as it could possibly be for the small/small combo (11/34) meaning a chain any longer would go slack, but was still too short for the big/big combo (50/28). Kaboom!!!
Ah, I see......One should definitely size a chain to make sure it will cover the big/big combo, and if the drivetrain requires too much chain wrap capacity than the RD has, avoid those small/smallish combos. The more catastrophic things happen if you have a brain lapse and shift to the big/big with a chain that's too short-
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Old 05-28-08, 08:08 PM   #7
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Now you tell me
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Old 05-28-08, 08:15 PM   #8
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Now you tell me

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Old 05-28-08, 09:57 PM   #9
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You can get 27t max derailleurs to work with a 34 by removing the main pivot spring, drilling a new hole in the derailleur body that increases spring tension in the main pivot when reinstalled. This pulls the guide pulley away from the cog and lets the system run quiet - something that can't be done just by running the b-tension screw all the way in. Here's a link to a description with photos.
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Old 05-29-08, 01:38 PM   #10
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You can get 27t max derailleurs to work with a 34 by removing the main pivot spring, drilling a new hole in the derailleur body that increases spring tension in the main pivot when reinstalled. This pulls the guide pulley away from the cog and lets the system run quiet - something that can't be done just by running the b-tension screw all the way in. Here's a link to a description with photos.
I get the feeling that some folks on here are more machinists than mechanics.
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