Foolprrof way of getting correct chain length

jimcameron
jimcameron Posts: 199
edited October 2009 in Workshop
I'm a very amateur home mechanic and i can do most things except from get the chain length correct when installing a new chain.

I know what you might say why don't you just use the last chain as a guide but i've also changed the cassette so i don't think that method will really work and the last chain wasn't quite the right size either.

I've tried going through derailers largest sprocket front smallest sprocket back with a 90 degree angle between the derailler arm and chain going to the crank but when i swith it to small cog front, large cog back it's far to large.

So can anybody tell me a foolproof method for getting the correct size?

Cheers
Jim

Comments

  • DaSy
    DaSy Posts: 599
    Without going through the dérailleurs, wrap the chain around the big chain-ring and the biggest rear sprocket, mark where the chain meets and add a link. It's as simple as that.
    Complicating matters since 1965
  • bompington
    bompington Posts: 7,674
    DaSy wrote:
    Without going through the dérailleurs, wrap the chain around the big chain-ring and the biggest rear sprocket, mark where the chain meets and add a link. It's as simple as that.
    Does this work equally well for different size mechs?
  • DaSy
    DaSy Posts: 599
    Yes, it works for short, medium or long cage, assuming you are running the correct cage length for your setup.

    Just to be clear, you mark the closest point at which the chain could be joined, then add a full link, so that is an inner link and an outer link, (an inch in length, not just an inner or an outer link which are 1/2 inch each)
    Complicating matters since 1965
  • Cheers Dasy,

    Just a thought, how do you know if you are running the correct set-up. I've got a basic 53/39 and 11/23 at the back, but a switch the rear when i go on tour for 11-28 so i've got a medium cage to allow for the extra teeth. Is that incorrect. Cheers for the info on the chain!
  • DaSy
    DaSy Posts: 599
    Jim, it depends on what short cage mech you have, but basically it depends on what the stated maximum capacity of the mech is.

    The total capacity is calculated by the sum of the front big ring minus the front smallest ring, plus the sum of the rear big sprocket minus the rear smallest sprocket. In your example it is

    53 - 39 (=14) + 23 - 11 (=12) so total tooth difference is 26

    53-29 (=14) + 28 - 11 (=17) total of 31 tooth.

    Some short cage mechs can cope with as much as 33T (ie the Shimano Ultegra 6700) but the 105 rear only copes with 29T, so it will depend on your individual components.
    Complicating matters since 1965