Swapping Campy 12/27 with 11/25

[Tony T]

I've run the chain length calculation, and it looks like I can swap an 11/25 with a 12/27 without a need to increase the chain length.

Can someone who's done this confirm that this is correct?

(In any event, I shouldn't be cross chaining the Big Ring with the Big Cog anyway)

Tony

[ultraman6970]

Have not done it but looks pretty close specially if you are not going to cross chain...

[oliver1850]

It will probably be OK, unless the chain was already on the short side. I just put a new 12 - 32 cassette on Shimano bike that had been running a 12 - 25. The chain was long enough to run large ring/large cog. I'd recommend you make sure the chain is long enough to run all combinations even if you don't intend to. It only takes once to screw things up if you mistakenly shift into a combination that won't work.

[Dave]

You can't confirm the length without knowing how it was set in the first place. If it's a new 11 speed RD and the length was set to the maximum, using the little/little method, then it will be long enough for any cassette, up to a 12-29.

Increasing the largest cog by 2 teeth only requires 1/2 inch more chain, but the minimum increment of change is 1 inch. If the length was set to the minimum for a 25T large cog, using the big/big plus 1 inch method, then it's probably too short, since adding 1 inch adds enough to cover a 29T.

[kramnnim]

If it's a new 11 speed RD and the length was set to the maximum, using the little/little method, then it will be long enough for any cassette, up to a 12-29.

Does this not apply to first gen 11sp RDs?

[Tony T]

You can't confirm the length without knowing how it was set in the first place. If it's a new 11 speed RD and the length was set to the maximum, using the little/little method, then it will be long enough for any cassette, up to a 12-29.

Increasing the largest cog by 2 teeth only requires 1/2 inch more chain, but the minimum increment of change is 1 inch. If the length was set to the minimum for a 25T large cog, using the big/big plus 1 inch method, then it's probably too short, since adding 1 inch adds enough to cover a 29T.

I don't know which method the LBS used, but they set the chain at 53in for the 11/25.

Using the equations at Park (http://www.parktool.com/blog/repair-help/chain-length-sizing) I get 53in for a 11/25 and a 12/27 (a 12/29 needs 54in).

Since my original post, I've installed the 12/27, and it shifts smoothly for all gears.

[Dave]

Does this not apply to first gen 11sp RDs?

Actually, no. The first generation 11 speed RD's won't work with a 29T cog. Campy changed the "B" screw mechanism on the later models to swing the RD back a little more for the 29T cog. The cage length was not changed.