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A little context first: I'm going to be riding in the French Alps and plan to tackle some of the classic climbs (Alpe D'Huez, Galibier, etc) next month - I currently have a Campagnolo Athena 50/34 (compact) chainset paired to an Athena 11 speed 11-27 cassette on the rear. None of the drive-train components are anywhere near worn enough for that to be an issue. Additionally, the rear derailleur cage is able to deal with the full range of the 11-29 cassette so this is also not an issue.

What I am considering is swapping out the 11-27 cassette with an 11-29 to give myself a little extra in terms of 'safety gears' - better to be overgeared than undergeared in my opinion!

My question is this: will I have to replace the chain when replacing the cassette, for example due to chain length?

My reasoning is that since I will only ever use the 29 tooth on the cassette with the 34 tooth chainring, that chain length should not be an issue. As above, the chain is not in need of replacement due to wear, so that is not part of the consideration. Perhaps I'm missing something?

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    I think this will be fine. Your reasoning is correct: because you don't use the 29T cog with the 50T chainring, the larger cog is not going to make any difference regarding chain length.
    – Nik
    Jun 20, 2015 at 18:06
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    If you understand that you cannot use the full (and somewhat illogical) range of gear options, you can use a chain that is longer or shorter than the "perfect" length. This does require a bit of care, as the chain can jam if stretched too tight, or may come off if made too loose. But in your case is makes sense.
    – Daniel R Hicks
    Jun 21, 2015 at 2:36

3 Answers 3

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I agree with @ Nik that it should not be an issue. You can verify it by shifting the chain to the big ring in the front and the large cog in the rear. With your hands fold the chain over itself to simulate it being two links shorter. Observe the rear derailleur. You should see two bends in the chains as it passes around the jockey wheels. If the chain passes thru the jockey wheels in a straight line the chain will be too short.

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  • OK - having manually simulated it being two links shorter, the rear derailleur still has two bends (although of course significantly smaller bends) and the chain is not simply running through in a straight line - can I infer from your answer that the chain will be of sufficient length in this case? (related, as an aside: looking at it I may be wrong but I think it's actually a little 'too long' for the current cassette anyway if it has 2 extra links of slack?)
    – BE77Y
    Jun 22, 2015 at 7:58
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    With the chain on the small ring gear and the smallest cog the horizontal length of chain doesn't rub the chain on the upper jockey wheel you chain isn't to long. Normally you would not want a set up with the chain very close to the jockey wheel but it may be necessary if you are at or near the tooth capacity of the derailleur.
    – mikes
    Jun 22, 2015 at 11:03
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My reasoning is that since I will only ever use the 29 tooth on the cassette with the 34 tooth chainring, that chain length should not be an issue.

I wouldn’t count on that. It just needs one sleepy, careless moment where you shift and shift and shift while being on the big chainring. I’ve tried too-short chains on a bike stand with manual pedaling and I was able to engage the gear but it does get stuck halfway through because the chain is just too short. When you are pedaling with your legs something is bound to break in such a case.

That being said, I would (and did) still use it as a temporary solution. You just have to be careful on the big chainring.

A minor effect is that a used chain will increase wear of the new cassette. There are people who use multiple chains and swap them every 1000km or so to wear down chains and cassettes evenly.

Otherwise: Go ahead.

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The only issue would be if your current chain length assumed you would not use the 27 with the 50, while now you presumably might do so now. In that case you might be one short. I would change the cassette, then put the bike on a stand, and try it. I find it hard to avoid occasional cross-chaining, but you may not. If adding a link solves the problem, that is so easy that you should do it. It is harder if the derailleur can't handle the range-now you need to buy a longer cage derailleur, which is some money.

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  • Thankfully the rear derailleur cage is equipped to deal with the 29 tooth cog (updated my question to reflect this for clarity). All of the answers here seem to reflect the same sentiment: "should be ok as long as you don't cross-chain" which is making me think, perhaps I might as well just stick a new chain/two extra teeth on there.
    – BE77Y
    Jun 21, 2015 at 7:44

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