Which rear derailleur? Medium Cage or Short Cage

nochekmate
nochekmate Posts: 3,460
edited February 2010 in Workshop
Just about to buy some Ultegra from CRC. I would like the 12-25 cassette. Which rear derailleur do I need to buy, will it be ok top operate on a short cage derailleur or will I need the medium cage version.

Apologies if it should all be so simple - new venture into bike builds!

Comments

  • Wheelspinner
    Wheelspinner Posts: 6,710
    Short cage fine for 12-25.
    Open One+ BMC TE29 Seven 622SL On One Scandal Cervelo RS
  • gaspode
    gaspode Posts: 110
    I thought it was dependant upon the front rings? - a short cage for a double and a medium/long for a triple....
  • Ben6899
    Ben6899 Posts: 9,686
    Depends on the front rings (double/triple) and the range on the cassette as far as I recall.

    12-25 cassette with double chainrings will be okay with a short cage.
    Ben

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  • Mister W
    Mister W Posts: 791
    Rear mechs have a "total capacity" which is the amount of chain slack they can take up, measured in teeth. So add the number of teeth on your smallest chainring to the number of teeth on your smallest cog. Then add the number of teeth on your largest chainring to the number of teeth on your largest cog. The different between these two figures is the amount of chain your mech has to be able to take up.

    I think a short cage Shimano mech can take up 29 teeth and a long cage mech will take up 37 teeth.
  • nochekmate
    nochekmate Posts: 3,460
    Many thanks for the replies - short cage one on order!

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