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12-25 to 12-27?

Swordfish84Swordfish84 Posts: 5
edited May 2012 in Workshop

Im thinking of changing my Ultegra 10spd 12-25 cassette for 12-27 Sram 1070 for the Dartmoor classic in June. Atm im running a Dura-ace chain, my main question is will I need to replace the chain in order to accommodate the extra teeth or do you think it will be ok with this sort of ratio??

The other option I have is to run an Ultegra 11-28 to match my current drive train, would I need to change the chain in this case as well?

Not really sure about this sort of thing, the Dura Ace chain is pretty much new, so I didn't really want to replace it unless I completely need to.



  • EyoreroxEyorerox Posts: 43
    You do not need to change your chain, it may not be long enough for 27-53, but you would never
    cross chain like that would you, I don't know about your rear derailleur
    I would not go 11-28, I doubt you need "11"
    for mountains I have 12-29 Campag 11 and 50/34 at the front
  • mamba80mamba80 Posts: 5,032
    No, just swop the rear cassette for whatever you decide to get and make sure you are happy well b4 the event.
  • Never Mentioned the cranks, running a 34/50 compact. As Far as I'm aware the rear derail is a standard Ultegra SS which can cope up to 33teeth according to the specs sheet!
  • jomojjomoj Posts: 777
    the 33 you're referring to is the maximum difference in the number of teeth that the derailleur can wrap chain for, the largest sprocket it will cope with is most likely 27 or 28 at a push.
  • pdwpdw Posts: 315
    It rather depends on how the chain is set up for your current cassette. If it's already a bit on the short side, you may have trouble. You're adding two teeth, so that's the equivalent of one full link. Put the chain on big-big. Now look at the derailleur and see if it could cope with the chain being a full link shorter - you should be able to test this by grabbing a full link and holding it vertically. If the chain is now close to straight where it passes through the derailleur then it needs to be longer.

    Whilst it's true that you shouldn't use the big-big combination in normal cycling, I wouldn't rely on not doing so if your chain is too short, as you've only got to make the mistake once and stuff will break.
  • Thanks for the replies and help, looks like I would need a new chain :(
  • rolf_frolf_f Posts: 16,015
    You need spare chains anyway - rotating them gives a longer life to the mechanism so there's no harm having another chain specifically for a cassette you only use on particuarly hard rides.

    And you do want to be able to cross chains - not because it isn't a bad habit but just because there are times when it just happens though more commonly small small rather than big big.
    Faster than a tent.......
  • mamba80mamba80 Posts: 5,032
    You could try downloading this for peace of mind. ... lator.html
  • antonyfromozantonyfromoz Posts: 482
    I changed from a 12-25 to a 12-27 with a compact chainset and 105 derailleur and had no problems with the length of the chain - and the 27 made a big difference in the hills too. If you have the time perhaps you can fit the cassette and then assess if a new chain is needed or not?
  • I went with the 11-28 in the end, as wanted to keep it ultegra and they were cheaper. Did get a new chain as I needed one!!
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