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I am upgrading a vintage 10speed, new chain was purchased. The new chain was a couple of links shorter than the original that I removed. I currently have the new chain connected with a piece of wire. See picture below:

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It seems that it fits, I had the chain on both lower sprockets, front and rear and it seems to function fine when I move the chain on different gear combinations. Once I remove the piece of wire it will be a little tighter but it seems there is some play in the length. The rear derailleur just moves if I increase or decrease the chain length slightly. Just wondering if this is correct, is there some degree of flexibility in terms of how long the chain can be ? Does the new chain have to be the exact same number of chain links as the old one ?

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  • There are prescribed chain-lengths for new systems, but the most important thing is that the rear shifter has no problem moving about. Since this is a vintage I am assuming that you also can't know for sure if the previous chain was the "correct" length. – Sebastiaan Jan 16 '20 at 21:54
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    It's really hard to believe that the new chain was too short -- usually they are about 10 links too long. – Daniel R Hicks Jan 16 '20 at 22:26
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    On a conventional derailer setup, the rear derailer arm should be pulled to an almost horizontal position when front and rear are both on the largest rings. – Daniel R Hicks Jan 16 '20 at 22:46
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    There is a definite correct length of chain determined by the size of the largest chainring, largest sprocket and length of the chain stay. The old chain may not have been the correct length, so definitely go through the process for sizing the new one correctly youtube.com/watch?v=LC8_ndnXJUY – Argenti Apparatus Jan 16 '20 at 22:51
  • Video was helpful, thanks – user2197774 Jan 17 '20 at 10:50
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Chain length can be obtained using different methods.
On my vintage bike I used the maximum cog length + 2 chain links. It can be a little tight so for start rather add 4 additional links. Because it is better to shorten the chain than lengthen. Those pins between normal links aren't made to be used multiple times!
Biggest cog method
Source: https://si.shimano.com/pdfs/dm/DM-RD0003-08-ENG.pdf

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Your chain needs to be long enough to move freely when it is shifted to the largest cog and the largest ring at the same time.*

To check the length, shift to these positions and move the chain by hand there. Then pull the chain together. You have to pull against the springs in the derailleur that requires a bit of force. As D. R. Hicks mentioned in the comments, the derailleur ought to be almost flat.

  • If you manage to do that, the chain is long enough.

  • If you can push up the derailleur even more, the chain is too long.

  • If the derailleur is nearly horizontal and does not move without a lot of force your chain is too short indeed.†

* It is not a good idea to ride an old bike cross chained that much. But one doesn't want things to break when doing this.

† This would be somewhat unusual, given the largest cog on an old bike is not all that big. Unusually long chain stays could lead to it. Some chains meant for internal hub gears and single speeds are short though. These are mostly 1/8" wide chains rather than 3/32" derailleur chains.

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