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I've put a new chain on my bike using method 3 from Zinn's art of road bike maintenance to determine length:

"Wrap the chain around the big chainring and biggest cog without going through either dérailleur. Bring the two ends together until the ends overlap; one full link (Fig. 4.7) should be the amount of overlap (Fig. 4.10)..."

However, I didn't notice that my fingers must have slipped at some point because I remeasured the size after breaking the surplus pieces off & the overlap was gone (facepalmed myself with a greasy hand at this point).

How big of an issue is this likely to be? What are the problems associated with an overshortened chain? Realistically, should I be running down to the bike shop and buying a second chain in as many days (whilst trying to pretend I hadn't)?

I rarely (if ever, I dislike cross-chaining) end up on both the big cogs and chainring at the same time therefore the chain & rear dérailleur would rarely be under maximum possible tension. I'm aware it's not a good idea to re-add links and given I'm about to head to the Alps I don't want a weak chain!

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    Fear from re adding links to a chain is overrated. If done properly with a good chain tool. If you are on a budget, just do it. If not or your fear is too much, buy another chain and cycle with peace of mind. I never install my chains with chain connectors, but that's another option for you. Nothing is wrong with using more than one in a single chain. Just add together the piece you removed first, then re-shorten the chain in the other end of the longest uncut part. – Jahaziel Aug 23 '14 at 0:36
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    @Jahaziel fear of doing anything is overrated, if you're going to do it properly. By and large, my reluctance to mess with chains increases with number of gears. – PeteH Aug 23 '14 at 10:12
  • @PeteH Your reluctance isn't unfounded either though, the higher number of gears requires a thinner chain and tighter tolerances. – BPugh Aug 27 '14 at 16:59
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To quote Sheldon Brown: "If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination. Never run with a chain that is too short, except in an emergency." (I will say that depending on if your chain is short enough, it can happen in combos other than large-large.)

Here is a good article on sizing chains.

If its a new chain, I wouldn't hesitate in breaking the chain at a different link and adding some of the extra links back to it. But you really should get the chain to be of the right length before riding.

Also, for what its worth, the easiest way to size a new chain for a bike which hasn't had a cassette/chainring size change occur is to start with the old (properly sized) chain, and use that to size out the new chain. Much less annoying than sizing the chain from scratch.

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    Thanks, the old chain was very worn (beyond 1%) so very stretched so I didn't trust that for measurements. So I'll go with adding back some of the links. – rg255 Aug 22 '14 at 14:25
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    It doesnt matter how stretched it is - you can still use the old chain to size up the new chain by counting links. – Batman Aug 22 '14 at 14:27
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    Yep, when sizing the new chain from the old you must count links rather than just matching length. I hang them side-by-side on a piece of wire and sort of match up the links. – Daniel R Hicks Aug 22 '14 at 15:46
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    Its easier to do by pinching the corresponding links together while laying the chains parallel on the floor in my experience, but you need to put some newspaper or something underneath so the new chain wax doesn't get on your floor (and less so dirt from your floor on the chain). – Batman Aug 22 '14 at 15:56
  • What if at some point the old chain was broken and repaired by removing a couple of links and you don't remember by now? Personally I would prefer to do the large/large measurement method. – Jahaziel Aug 23 '14 at 0:32
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Had just this problem to the extent where when the chain was over the large cogs, front and rear, the derailleur was excessively stretched. The changes operated OK, but noticed that the changes weren't smooth when on the larger front cog. Changes on the rear derailleur operated with a bit of clicking and hesitation not dissimilar to an indexing problem.

I added four links from a spare chain to lengthen it, and the changes become smooth again.

Although technically chain tension should not affect gear changes, it does when the derailleur is excessively stretched.

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