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  1. #1
    mtbr member
    Join Date
    May 2011

    What will happen? (Rear derailleur question...)

    Already posted this in the drivetrain forum, but, what can I say? I trust you the most, Ibis geeks....!

    I know the long/short cage derailleur formula, and appreciate the sticky. I'm running a shimano 38/26 2x10 crankset with an 11-36 rear cassette. That puts me at a total capacity of 37 teeth. This suggests that for my new XT clutch rear derailleur I'll need the RD M780 SGS. (Long cage). The medium cage has a total capacity of 35, which is just a tad too small. So, this begs the question:

    What will happen if I go with the medium cage, anyway?

    For the record, this is on an Ibis Mojo HD 160. I'm also currently using the Ibis specific MRP 2x chain guide. (Does that alter the formula?) Hopefully, with the addition of the clutch derailleur, I won't need the guide. (Which would be great, because it's a flexy POS!) BUT, i might end up keeping the guide on if the clutch alone proves to be insufficient for chain retention. Thanks for your input.

  2. #2
    Reputation: derby's Avatar
    Join Date
    Jan 2004
    I'm using the same cassette, 11/36, and a 22/33/44 crankset with bash replacing the big ring, and using the medium (SG) XT cage. And my minimum chain length is checked and correct. My 2x10 combo is one tooth less than your ring set's range, and I have no chain guide adding minimum chain length. In my small ring and 11 cog the rear pulley is still below the level of the upper pulley, and tension is OK. I'd estimate yours with the MRP would require a longer minimum chain length beyond the common large/large plus 2 links while suspension is compressed and without a chain guide adding length. So with the chian guide your chain will then become much slacker than mine in the small ring when the MRP is doing less chain redirection than in the big ring, and your small small combos would allow the cage to bind.

    If you never ever accidentally shift into the small/small combo it would be OK. But one mistake, dropping to the small ring while in the small cog could instantly bind up the rear der chain and cage possibly bending the rear der hanger or much worse if the der gets sucked into the spokes.

    I make shifting mistakes. I wouldn't risk using too short of a cage.

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