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Short cage vs. long cage rear derailleur

Discussion in 'Bicycle Mechanics and Repairs' started by Pottsy, 13 May 2008.

  1. Pottsy

    Pottsy ...

    Location:
    SW London
    When do I need which one?

    Must be the range of the block on the back and chainrings on the front I guess?

    Anyone know?

    Thanks.
     
  2. alecstilleyedye

    alecstilleyedye nothing in moderation Staff Member

    short cage: double chainset with normal (eg 52-42, 53-39) range
    long cage: triple chainset
    i'm not sure about compact chainsets, but i think campag do an intermediate cage.
     
  3. OP
    OP
    Pottsy

    Pottsy ...

    Location:
    SW London
    It's actually on an MTB that I'm converting to a tourer, so it'll be 11-32 on the back and I'll probably just leave the single 34 on the front. I suspect short cage could handle that?!?
     
  4. alecstilleyedye

    alecstilleyedye nothing in moderation Staff Member

    not sure about that pottsy, maybe check the range of the mech with your lbs, although no doubt someone will be along soon with the maths to work it out.
     
  5. domtyler

    domtyler Über Member

    I would either go with a long cage to play it safe, or find the correct information on the internet somewhere, maybe look at the manufacturers site or Sheldon?

    Trusting in some spotty oik in Evans is not the way to go.
     
  6. Smokin Joe

    Smokin Joe Legendary Member

    Up to 28t rear sprocket on the back is ok for a shortcage, 30t if you are careful to avoid big/big combinations. There is no difference using a compact.
     
  7. OP
    OP
    Pottsy

    Pottsy ...

    Location:
    SW London
    Thanks for all your answers, it looks like I need a long cage (which was always going to be the safer bet for potential future changes to the set-up anyway).

    Pottsy
     
  8. andrew_s

    andrew_s Veteran

    Location:
    Gloucester
    Cage length is about the range of gears, not sprocket size per se.
    You need a chain that's long enough (just) to change onto the big/big gear, otherwise you may break something if you ever select that gear by mistake.
    The small/small gear would require a much shorter chain if the derailleur cage wasn't there to take up the slack. The long cage/short cage bit is about how much slack chain the mech can cope with. If the cage is too short, the chain will hang slack in small/small, and will be prone to coming off the chainring.

    A standard road setup of 53/39 & 12-25 requires the cage to wrap 53+25 - 39+12 = 27T
    A bike with a touring triple setup may have 48/38/24 & 12-32 requiring a capacity of 48+32 - 24+12 = 44T

    Typical short cage mechs have a capacity of 28-29T, long cage of 39-40T


    The other issue is maximum sprocket size. This is dictated by the slant angle on the parallelogram and how far the top jockey wheel is from the axle in the bottom gear position.
    Generally road rear mechs will cope with a sprocket up to 29T, possibly 30. If you want a bigger sprocket, you need an MTB rear mech. If you use one of these with a road cassette, the changes in the larger sprockets will be less snappy as the jockey is further from the sprockets than is ideal.