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Derailleur capacity mystery

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Derailleur capacity mystery

Old 05-13-14, 10:54 PM
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Derailleur capacity mystery

I have been running a 1990 semi-vintage Deore DX RD for several years with a 13-34 7-speed, and 48-42-24 triple for years, and now have a problem.

First, Yes it is old, and I know that the original specs were for a 32T max cog and 38T of chain wrap. The lower three cogs in the granny ring are useless, so I never use them and the chain goes slack in them. I ran the B-screw in backwards to get the maximum up-take on the chain to swallow the 34T cog. For years it worked fine.

I replaced the chain last weekend, and no mas.

The bike will run 42/48 to 34 cog just fine, but when I drop into the 24T granny gear, the RD binds up on the large cog. The difference seems to be that the chain tension in the larger rings pulls the cage forward such that it clears the large cog. The smaller ring drops the entire cage back with the lessor tension. The upper pulley is offset on the cage such that the it translates as the cage rotates about the lower pivot. I cannot get the cage back far enough with even an inch-long B-screw to pull it back.

I toyed with just going down to a 32T, but it won't take more than a 28T right now. I could go with a larger granny ring, but I like the low range.

Any suggestions?
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Old 05-13-14, 11:18 PM
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You don't say if you shortened the new chain.
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Old 05-13-14, 11:44 PM
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Since it worked before you need to look for the variable, and I suspect it's the chain length. Shimano and Campagnolo RDs are sensitive to chain length because the jockey wheel rises and falls based on cage angle.

Shift to Granny/34 and pull the lower loop forward and see if the clearance resolves. If so, shift to big/big and see if you can spare th same amount of chain as you pulled up earlier. I so, cut that much off.

But don't forget that you still want a chain that can loop big/big with enough length to spare to make the shift (roughly 1/2-1").
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Old 05-14-14, 12:26 AM
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Did that with my road bike. Same issue. Your problem is that your chain is too long. Take out a pair of links and see what happens. Be careful though, if you go too short then an accidental big/big shift can do bad things.
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