Chain length on new setup 53/39 to 27/12 St Cage Ultegra GS?

dazzawazza
dazzawazza Posts: 462
edited July 2008 in Workshop
Can I use the usual test of chain around both largest sprockets plus 2 links?
It's a completely different set-up from my old Sora triple so I can't copy the old chain.
I'm concerned because I know that I'm near the limit of a short cage GS derailleur with a 27/12T cassette and a double 53/39T chainset.
Thanks

Comments

  • Monty Dog
    Monty Dog Posts: 20,614
    Try the usual big ring to big sprocket - if you get too much chain slap or the rear mech wraps back round the cassette in the small ring, then try taking out another link - might mean that you can't use the big ring and sprocket combo - but should be avoided anyway.
    Make mine an Italian, with Campagnolo on the side..
  • aracer
    aracer Posts: 1,649
    I disagree on the suggestion to shorten the chain to less than is needed to select big/big. It might be a combo you shouldn't use (though who says - I certainly use mine occasionally to avoid a front shift and it doesn't make lots of noise or seem to work poorly), but you still might end up in it accidentally, and if you've not allowed enough chain then you could break something.

    A short cage mech (that's an SS - a GS is a medium cage) might theoretically be right on the limit with 12-27 and 53-39, but in reality Shimano are a bit conservative with their specs and it shouldn't be a problem at all - plenty of people using an SS mech with 50-34 and 12-27, which is theoretically outside the spec.
  • dazzawazza
    dazzawazza Posts: 462
    Thanks for the info, I'll install the chain tonight.
    It's a project that’s taking a while as I'm doing it in stages, when I can find the time. Meanwhile I'm riding the MTB everywhere.
  • enormous
    enormous Posts: 3
    Hi, can I ask how you went with your install? I have the same combo using an Ultegra SS derailleur and my chain seems to be hitting the metal bit just below the top jockey wheel when I'm in the small-small combo (39-12). I know you're not meant to ride in these combinations but I agree that sometimes you just end up in these gears.

    How long should the chain be for a 53/39 12-27 setup?
  • John.T
    John.T Posts: 3,698
    enormous. Just long enough so that does not happen. Probably 1 link less.
    My Fleet.
  • enormous
    enormous Posts: 3
    Yes, I've found this too if anyone is interested in a general formula: http://www.parktool.com/repair/readhowto.asp?id=26

    From my calculations, I used to run an 11-23 and I've now got a 12-27 and the difference should only be 1 inch (1 link) longer. However the stupid bike shop didn't give me back my chain (when I specifically asked for ALL my old parts) so I can't measure it against the old one.... I swear my new chain looks longer though.

    Oh well, I guess I'll just have to buy a chain breaker and do some trial and error on the weekend myself... :(
  • redddraggon
    redddraggon Posts: 10,862
    enormous wrote:
    Oh well, I guess I'll just have to buy a chain breaker and do some trial and error on the weekend myself... :(

    I hope you've got a chain with a quick-link-thingymabob, I wouldn't like to have to keep rejoining a chain with new joining pins everytime.
    I like bikes...

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  • enormous
    enormous Posts: 3
    Yer, I do have a connex link. Which is why I have to buy the chain tool to break the chain! :)

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