Measuring chain length using old chain

mcstumpy
mcstumpy Posts: 298
edited March 2019 in Workshop
Hi,

I’m aware of methods to work out chain length for a bike build, but was wondering if I could “cheat” slightly and use my old bike and chain as a reference.

Current bike has 50t front and 36t back as biggest combo.

New bike will have 48 and 34.

So if I use chain from old bike as a reference to work out best chain length for new bike, is it is simple as saying new chain would be x links shorter than old? If that makes sense.

Thanks

Comments

  • cougie
    cougie Posts: 22,512
    I think it's easier to do it the normal way.

    You can lay both chains one the ground and measure like that but you'd need something down to keep the grit off and one chain link on an old chain will be slightly longer than a new one so it makes a difference by the end of the chain.

    Are these two different bikes or just changing the chainset ?
    Different bikes can have different lengths of chainstay...
  • whyamihere
    whyamihere Posts: 7,702
    Do they use the same frame? Part of the chain length calculation will involve the chainstay length, so unless they're the same frame, or at least have the same chainstay length, you're going to need to work it out properly.
  • Darius_Jedburgh
    Darius_Jedburgh Posts: 675
    I measure new chain alongside old chain.
    What's this about putting them on the ground in the dirt? A very wierd way of doing things. Use the garage bench/kitchen worktop covered in newspaper.
    If the old chain was right then the new one will be. Just remember to allow for old chain stretching.

    Anything else just seems like making work and introducing a margin for error.
  • keef66
    keef66 Posts: 13,123
    Just replacing a chain which was the correct length I sometimes hang up the old and new on a nail and trim to match.

    But if it's a different frame, different chainset and different cassette, I'd suggest any method except copying the length of the old chain.

    I use Big-big but not through the rear mech, then add 2 links. Complete links, ie 2 inner and 2 outers.
  • cougie
    cougie Posts: 22,512
    Franco di Banco wrote:
    I measure new chain alongside old chain.
    What's this about putting them on the ground in the dirt? A very wierd way of doing things. Use the garage bench/kitchen worktop covered in newspaper.
    If the old chain was right then the new one will be. Just remember to allow for old chain stretching.

    Anything else just seems like making work and introducing a margin for error.

    Obviously I put newspaper down when I do this...

    He's talking about using a smaller chainset so he might need a smaller length of chain.
  • navrig2
    navrig2 Posts: 1,844
    I use the measure the old chain technique but if the stretch looks longer than one link will count links. I hang the two chains with one cable tie from the garage door and let gravity straighten them.
  • mcstumpy
    mcstumpy Posts: 298
    Thanks all, different frames so of course it’s apples and pears, I should have twigged that. I’ll go with the method proposed by keef66, cheers.
  • Alejandrosdog
    Alejandrosdog Posts: 1,975
    It’s a whole lot less faff to just not bother measuring it all.

    Small small longest where there is some tension in the derraileur and its done.

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