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When determining correct chain size, one common technique is to install the chain (without passing it through any derailleur) onto the largest front and rear sprockets. Then, determine the shortest length that the chain could be cut to, and then add 1 complete link to that length. One complete link corresponds to one inner and one outer pair, which corresponds to 1 inches.

I have noticed that in recent times, some resources recommend adding 2 links (2 inches) instead of only 1, specifically when the bicycle is set up with a single chainring in the front. This seems to be a "tribal knowledge" practice, but fairly common in my area.

Is there any evidence or data to support this practice of adding an additional link to 1X systems, and are any major component manufacturers recommending the practice?

See for example :

https://bike.bikegremlin.com/614/chain-length-sizing-for-bicycles-with-derailleurs/

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    A complete link is 1” long. The pitch is 1/2” per pin.
    – MaplePanda
    Feb 3, 2021 at 18:48
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    I updated my question accordingly
    – BetterSense
    Feb 3, 2021 at 20:41
  • I don't think MaplePanda is correct. The big manufacturers refer to a single link as a half inch. For the same reason, we don't think of a 48tooth ring as a 24 link ring.
    – Noise
    Feb 4, 2021 at 16:52
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    But it's impossible to add a single 1/2" link to a chain. You have to add both an inner and outer link to extend the chain by one unit, unless you can add a "half link" which is used for single-speed. So we can assume any instruction to add "one link" means to add one inner and one outer link, or 1 inch total length.
    – BetterSense
    Feb 4, 2021 at 17:05
  • @JoeK The answer originally stated “One complete link corresponds to one inner and one outer pair, which corresponds to 2 inches”, which is definitely wrong. It’s all very confusing!
    – MaplePanda
    Feb 4, 2021 at 19:52

2 Answers 2

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When pulled tight around your chainring and largest cog without the derailleur, add the following to your chain: 4 pins worth of chain for 1X drivertain 2 pins worth of chain for 2X drivetrain

If in doubt, make the chain 1/2 link longer not shorter

You can always shorten the chain later if you are getting too much chain slap, but making it longer is more of a hassle.

As an aside, I've seen a few folks who thought they had a chainring spacing problem because pedaling backwards caused the chain to drop off the largest cog. Adding a longer chain, taking tension of the clutched derailleur, solved this problem.

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Current gen Shimano mountain rear derailleurs (dealers' manual) require at least that much chain by their own instructions: 4 to 5 links plus quick link for hard tails and 5 to six links plus quick link for full suspension bikes. Note that Shimano refers to 1/2" section as a link, as is quite common.

I recently set up a chain on a 1x full suspension with RD-M5100 with the 'standard' method, following the instructions from the Shimano MTB chain manual and the chain was much too short putting the derailleur in a peculiar position in all gears and not enough to wrap around the larger cogs, it was unworkable. Following the instructions in the derailleur manual instead, with 5 links plus quick link gave the expected result.

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  • In this case, the amount of "extra" chain is a function of the derailleur used, and not presence or absence of multiple front sprockets. We would agree that these derailleurs require the same amount of "extra" chain, even if they were being used on a 2X system? Would running 2X vs. 1X make any difference?
    – BetterSense
    Feb 3, 2021 at 20:34
  • @BetterSense yep good point, in the instructions, the chain goes around the 'largest chainring' so not limited to 1x in that sense. however current RDs are based around the large capacity rear cassettes which go hand-in-glove with 1x set ups. RD-M5100 has max front difference of 0T so not intended for use on 2x
    – Swifty
    Feb 3, 2021 at 21:07

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