What do I need to adjust on 10 speed Campag RD going from 12-27 to 12-25 cassette?
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  1. #1
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    Question What do I need to adjust on 10 speed Campag RD going from 12-27 to 12-25 cassette?

    Hi - I'm running a 10 speed Campagnolo Chorus medium cage rear derailleur, currently with a Centaur 12-27 cassette and Veloce 50/34 compact crankset. I want to swap the 12-27 cassette out for a Chorus 12-25 one I have, as am finding I'm not really needing the 27 cog for most my riding currently, and would like a 16T cog, as the jump from 15T to 17T can be annoying at times, especially when riding in a group with my cycling club.

    As this is not a big change, I'm not anticipating too many problems, but I've read elsewhere that it sounds like I may have to adjust the "B-screw", however I'm not 100% which screw that is on my Chorus derailleur, nor which way I would need to turn it, and what would be be an indication that it would need to be adjusted? Thanks

    BTW the existing Record chain has only about 600 miles on it, so I'm hoping to able to re-use that and not to have to shorten it, especially as it's possible I may want to swap the 12-27 back in for the odd hilly ride. Should I be able to leave the chain length as it?
    Last edited by Oxford_Guy; 07-19-2020 at 09:09 AM.

  2. #2
    'brifter' is f'ing stupid
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    Honestly nothing.
    #promechaniclife

  3. #3
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    yup, nothing. Same chain no adjustments. You've already used 12 through 25 with the 12-27. Well, there may not actually be a 25 on your 12-27 but you get the point.

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    Quote Originally Posted by Jay Strongbow View Post
    yup, nothing. Same chain no adjustments. You've already used 12 through 25 with the 12-27. Well, there may not actually be a 25 on your 12-27 but you get the point.
    Okay, BTW the cassettes go like this:

    Centaur 12-27:
    12-13-14-15-17-19-21-23-25-27

    Chorus 12-25:
    12-13-14-15-16-17-19-21-23-25

    I do sometimes spin out in 50-12 going downhill, but think I'd rather have a 16T sprocket than go for an 11-25: 11-12-13-14-15-17-19-21-23-25

  5. #5
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    Don't worry. Just do it. At the worst, you might get a slight amount of noise on a gear combo or two. Then again, it might cure any noise you have now. Finally, yeah, some of the true mechanics on this website might shudder, but if you put the bicycle up on a rack and shift through the gears you'll soon suss out which screw to adjust to make the alignment absolutely perfect.
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  6. #6
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    Quote Originally Posted by Oxford_Guy View Post
    Hi - I'm running a 10 speed Campagnolo Chorus medium cage rear derailleur, currently with a Centaur 12-27 cassette and Veloce 50/34 compact crankset. I want to swap the 12-27 cassette out for a Chorus 12-25 one I have, as am finding I'm not really needing the 27 cog for most my riding currently, and would like a 16T cog, as the jump from 15T to 17T can be annoying at times, especially when riding in a group with my cycling club.

    As this is not a big change, I'm not anticipating too many problems, but I've read elsewhere that it sounds like I may have to adjust the "B-screw", however I'm not 100% which screw that is on my Chorus derailleur, nor which way I would need to turn it, and what would be be an indication that it would need to be adjusted? Thanks

    BTW the existing Record chain has only about 600 miles on it, so I'm hoping to able to re-use that and not to have to shorten it, especially as it's possible I may want to swap the 12-27 back in for the odd hilly ride. Should I be able to leave the chain length as it?
    The "B" screw equivalent on a Campy derailleur is located right next to the idler pulley cage where it attaches to the body of the derailleur. The head of the screw might be pointing down.

  7. #7
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    Quote Originally Posted by Kerry Irons View Post
    The "B" screw equivalent on a Campy derailleur is located right next to the idler pulley cage where it attaches to the body of the derailleur. The head of the screw might be pointing down.
    Ah, that screw, right! I see where you mean. What are the other two on the main derailleur plate for? I assume I should leave those two alone if all seems to be working okay at the moment!

    How would I be able to tell if the B-screw needs adjusting?

  8. #8
    'brifter' is f'ing stupid
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    Quote Originally Posted by Oxford_Guy View Post

    How would I be able to tell if the B-screw needs adjusting?
    Normally if the chain gets pinched between the upper pulley and the big cog.

    ETA: Check when in the small ring and big cog.
    #promechaniclife

  9. #9
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    Quote Originally Posted by Oxford_Guy View Post
    Ah, that screw, right! I see where you mean. What are the other two on the main derailleur plate for? I assume I should leave those two alone if all seems to be working okay at the moment!

    How would I be able to tell if the B-screw needs adjusting?
    With the bike in the lowest gear (smallest chainring/largest cassette), measure the spacing between between the top pulley and the largest cassette, it should be between 5-7mm. If not, use the screw to make this adjustment.

  10. #10
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    Quote Originally Posted by Oxford_Guy View Post
    What are the other two on the main derailleur plate for? I assume I should leave those two alone if all seems to be working okay at the moment!?
    High and low limit screws, which keep the derailleur from shifting into the spokes or into the frame. Don't start messing with them.

  11. #11
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    Quote Originally Posted by Kerry Irons View Post
    High and low limit screws, which keep the derailleur from shifting into the spokes or into the frame. Don't start messing with them.
    Thanks, assuming everything is currently fine on that front 9which it is), should I never need to adjust those, even if, say, changing to an 11-23 cassette (e.g. for a time trial)?

  12. #12
    'brifter' is f'ing stupid
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    Quote Originally Posted by Oxford_Guy View Post
    Thanks, assuming everything is currently fine on that front 9which it is), should I never need to adjust those, even if, say, changing to an 11-23 cassette (e.g. for a time trial)?
    ALL Campy 10 speed cassettes take up the same amount of space and the distance from large cog to small cog is the same. Why would you need to adjust limit screws if they're all the same?
    #promechaniclife

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