I would like to know what methods there are to size chain length on a derailleur geared bike. There should be more than one answer, so there are effectively no wrong answers - don’t be shy.

What are the advantages and disadvantages of, and reasons for, your preferred method?

  • I would be very surprised if there was more than one method. Most drivetrain manufacturers suggest the same protocol (see Argenti Apparatus’ answer). – Rider_X Aug 9 at 23:29
  • If you are replacing an existing chain that served you well, you can cut the new chain to be the same length as the old. But you have to account for stretch, so you need to figure the length in links rather than inches/cm. What I do is lay the old and new out on a table and get them lined up link-for-link. – Daniel R Hicks Aug 9 at 23:39
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    Possible duplicate of Setting chain length – whatsisname Aug 10 at 6:08
  • What do you mean by "size"? There are all kinds of possible dimensions. Also "There are effectively no wrong answers" is a discussion, not a Q&A. – David Richerby Aug 10 at 10:11
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    @DavidRicherby y'know what, I'll do I should have done in the first place - add an answer to the dupe and vote to close this. – Argenti Apparatus Aug 10 at 15:56

Starting from a new split chain:

Thread chain onto largest chain ring and largest cassette sprocket. Either thread through front derailleur or move it out of the way. Do not thread chain through rear derailleur.

On the chainring, find the links that will join to form the shortest possible chain that can be connected. Add 1 inch of chain to that (two rivets or 1 inner and 1 outer plate link).

If using a quick-link, remember that takes the place of one outer plate link.

Reading between the lines, what you are actually asking is 'what's the correct length of chain for a modern derailleur' bike?'

Given that modern derailleurs can accommodate the chain running from the chainring to sprockets in almost a straight line, the shortest possible chain that fits over the largest chainring and sprocket, that can run through the derailleur is desired. This is because:

  • Eliminates rick of breaking chain or derailleur if largest ring and sprocket are selected.
  • Maximizes chain tension and the slack that the derailleur can take up, enabling the largest spread of gear ratios.

If one had a system where the derailleurs could be prevented from selecting the largest chainring and n largest cassette sprockets at the same time - which could be programmed into Di2 or eTap systems - then the chain could be sized for the large chainring and n-1th sprocket.

  • That’s great, one on the board. But I want to know the reasons why it’s a good method – Swifty Aug 9 at 19:54
  • @Swifty see edit – Argenti Apparatus Aug 9 at 20:13
  • @Swifty because anything shorter risks ripping off the derailleur and damaging the wheel when big-big cross-chaining. Anything longer and you can get a loose chain and chain slap on small-small combos. – Rider_X Aug 9 at 23:26

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