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I know of several methods of sizing a chain for a bike:

  1. As many links as the previous chain.
  2. Small-small. The RD should be barely expanded, the chain nearly rubbing against itself roller-cogs. The logic here is to be able to have the longest possible chain, that still allows shifting into any gear combination.
  3. Vertical rollers. Shift big front small rear. The rollers on the RD should be in a vertical line, below the cogs. In this position the RD is in a good working point, because it's spring is both tensioned (and thus stiff against vibrations) and not over-elongated (and suffering accelerated wear). This is the medium working point.
  4. Big-big without the derailleur. Without threading the chain through the RD, measure big-big and add 2 links.

My question is what is the motivation of the last method. To me, not threading the chain through the RD just doesn't make any sense. What physical quantity is being measured by shifting to big-big and why are 2 links being added?

As a side note, I recently tested all of those, and no two returned the same result.

  • Here is an example reference, where they say " It is not necessary to thread the chain through the rear derailleur", but I have seen the more powerful claim, don't thread through the RD. – Vorac Nov 7 '14 at 10:32
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    One thing is that a slightly too big chain doesn't matter too much. A slightly too small chain on the other hand is bad.Can you rank them by number of links? I'm partial to option 1 (size by old chain) but thats not the right way to do it if you do a chainring+cassette size change. – Batman Nov 7 '14 at 11:15
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    A full suspension bike with a pivot design which has "chain growth" will require a longer chain. In that case measure after having removed all air or coil from the rear shock and fully compressed it. – cherouvim Nov 7 '14 at 12:42
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    Needs to be at least as long as #4, unless you're intentionally "shorting" it for some odd reason. (A few riders of the old school never use the big-big combination, and sometimes the derailer simply lacks enough tooth capacity.) You also don't want it to hang loose when on small-small (with the same old-school caveat). If you've got room to work with you'd probably want to "optimize" for the situation you use most often. – Daniel R Hicks Nov 7 '14 at 13:00
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    Incidentally, you're actually only supposed to add one full link. A single chain link is two pieces: two half links, an inner half link and an outer half link. See the illustration on the linked (pun not intended) page for a better visual of the difference between a half link and a full link. – jimchristie Nov 7 '14 at 14:23
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Big-big and through the dérailleur is the most chain you'll ever need as you can't shift to any gear which takes more chain than this.

You will then need to add some links to take into account the extra chain required to go through the dérailleur. 2 links is the least amount you can add and when the rear mech is as stretched out as it would near maximum extension, the chain doesn't need to be much longer to accommodate this.

Theoretically you could put the chain on big-big and through the dérailleur and try and gauge how long the chain should be for the mech to be almost fully extended but not quite, but by doing big-big plus 2, you should get a similar result.

On your side note that each method gives different results, this could be because there is no correct answer. The big-big plus two gives you a safe chain length in that if you were to shift into big-big there would be sufficient chain to do this. If you don't cross-chain, you could get away with a shorter chain length, but risk damaging something if you were to shift into big-big.

  • Big-big with the derailleur is a PITA.You have to fight the return spring while trying to holding the ends of the chain in each hand, while trying to make the chain as straight as possible. Much easier without the derailleur. – Rider_X Nov 7 '14 at 16:53
  • Yeah, a very skilled bike mechanic I know always uses the big/big plus two links approach. I always use the "same number of links as the old one" approach. – Daniel R Hicks Nov 7 '14 at 17:35
  • "same number of links as the old one" is not a good answer. It assumes the old chain was the proper length and can produce incorrect results. – Deleted User Nov 10 '14 at 18:56

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