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Thread: "B" Screw?

  1. #1
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    "B" Screw?

    We have a 1998 (I think) Co-Motion Co-Pilot with an 8 speed cassette & Ultegra derailllers.

    I recently replaced the rear 11/30 cassette with an 11/32. When we are in the small ring in front & the big cog in the rear, the derailler bumps along on the cassette. I turned the "B (?)" screw in all the way but it still bumps. I can install a longer screw if this helps but with most things, there will be a downside to this. Is this screw an M2.5 or M3? What is the likely downside of turning the B screw in farther? Less engagement with the small (11t) cog?

    Also, an unrelated question: most of the fasteners on a bike are hex socket. Why are the adjustment screws on the deraillers slotted-cross head? These things are a bad compromise and seem to work poorly with either a phillips head or flat screwdriver.

  2. #2
    pan y agua merlinextraligh's Avatar
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    I don't believe your Ultegra derailleur was designed to handle a rear cog over 27 teeth. (I know this is true for current ultegra derailleurs) A number of people report they have been sucessful exceeding that limit. However at 32 teeth you're way beyond what the derailleur was designed for. I think if you want it to shift smoothly on a 32 tooth cog you'll need a new rear derailleur.

  3. #3
    Co-Mo mojo
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    Quote Originally Posted by merlinextraligh
    I don't believe your Ultegra derailleur was designed to handle a rear cog over 27 teeth. (I know this is true for current ultegra derailleurs) A number of people report they have been sucessful exceeding that limit. However at 32 teeth you're way beyond what the derailleur was designed for. I think if you want it to shift smoothly on a 32 tooth cog you'll need a new rear derailleur.
    Like an XT or XTR. I think 32 is the max for the XT, 34 max for the XTR.

  4. #4
    It Takes Two BloomingCyclist's Avatar
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    If you like to tinker, you may be able to get the derailleur to work if you shorten the chain so that the idler pulley cage is rotated enough so when on the large rear cog the idler wheel cage is rotated so that it doesn't hit the cog. Of course, this may have alread been done to allow your derailleur to work with a 30 tooth rear cog. If you do shorten the chain you will likely lose the ability to be in your front large ring and your large 32 rear but I don't think you use that cross-chain combination anyway. I did this for many years on my old single to make an 1970's Campy Nuovo Record derailleur work with a 14 - 28 freewheel when it wasn't supposed to work with larger than a 26.

    I agree that an XT or XTR derailleur will work well. I don't think the official specification for the current ones is for anything larger than a 30 but they obviously exceed that - I have a 11 - 34 with the previous XTR and counselguy has a 11-34 with the current XTR.

    Bloomington, IN

  5. #5
    Senior Member Retro Grouch's Avatar
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    Mountain bike rear derailleurs are what the tandem manufacturers use. Either XT or XTR will handle a 34t cog. XT is less than half the cost of XTR but weighs 50 grams more.

    Often in threads like this people recommend shortening the chain. I feel an obligation to discourage chain shortening.

    The fact is that, since most riders avoid cross-chaining, you can get away with using a too short chain for years. If, however, you ever decide to sprint up a hill in the big ring and accidentally access the big cog even one time with a too short chain, the result can be very expensive.

  6. #6
    hors category TandemGeek's Avatar
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    Quote Originally Posted by oldacura
    We have a 1998 (I think) Co-Motion Co-Pilot with an 8 speed cassette & Ultegra derailllers. I recently replaced the rear 11/30 cassette with an 11/32.
    You need to find an older XT or XTR rear derailleur: both have had a max sprocket size of 34t for many years now. In the current '07 line, the XT actually has a slightly larger capacity than the XTR.

    As noted my MerlinXL, Ultegra have (and as best as I recall always have had) a max sprocket size of 27. While you can usually "cheat" a bit, you'll eventually hit the true max. I use a Campy long cage RD on our tandems and while it has a max sprocket size of 29, I've been able to "cheat" up to a 32t.... albeit with the index wheel kissing the 32t sprocket's teeth a bit.

    Hypothetially using a longer B screw will just make the shifting lower on the cassette sloppy.
    Last edited by TandemGeek; 07-13-07 at 08:02 AM.

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