Hi there,

Currently considering switching between 12/27 and 12/29 cassettes on a mechanical Campy SR11 drive train (52/39 chain rings), based on total elevation of ride. Question is -- do I have to add 1 or 2 chain links when switching to the 12/29 due to the larger diameter of the biggest ring? I'm hoping not but would like confirmation from the aficionados. Thanks in advance.

Depends really on the length of your existing chain, and how they sized it during installation. You might be fine.

indeed does depend on chain length, but I regularly switch between 12-25, 12 - 27 and 12-29 without having to add links to the chain

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I just changed from 12/27 to 12/29 SR and my chain was fine. But like mentioned before, it also depends on your current chain length.

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Thanks for the responses -- will give it a try.

Also depends on whether you use the big/big combination. If you avoid it then its not going to be a problem even if your chain was set up at the minimum length for the 12-27 cassette. You should be avoiding that combination anyway, as it will wear your chain/drivetrain a lot and its inefficient (most power loss). Of course if you are racing then its possible to forget in the heat of the moment.

Be aware that changing into big/big on a work stand and spinning away is a little different to changing into that combination under load while riding. Look at the rear derailleur on the stand, feel the cage with your hand, can you move it up and down a little bit, does it have a little bit of play? If it does you are probably fine in that combination-you aren't going to bend or snap something as your frame flexes under power as the chain has a tiny bit more slack to take up.

Hi,
can never get my head around these things. Basically, running a 12/25 cassette at the moment but going to swap it over for a 12/27 for an upcoming trip to the alps. Will this require any links to be added to the chain?

thanks

In most cases, the chain is sized as the longest chain that will work in the small cog/small ring. Then it is up to the derailleur's chain wrap capacity to do the rest.
assuming this is how your chain was sized, it will make no difference which cassette you choose, the chain will be fine.
Just to be safe, install the larger cassette, IN THE STAND shift to the large chainring and shift to the second largest cog. Then slowly pedal while shifting to the largest cog and see if the derailleur handles it or locks up. Obviously, if it locks up due to the chain being too short, you need a longer chain.

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Use this handy calculator:

http://www.machinehead-software.co.uk/b ... hcalc.html

or the method of wrapping around big chainring and biggest sprocket on cassette bypassing derailleur - this length of chain plus two links is ideal.

All this being done, even if you're a little tight in the 27, it will generally be OK. You shouldn't cross chain on the big ring and 27 anyway.

Even though you shouldn't use the big ring/big cog combo, I think it's a really bad idea to set up a bike so the chain is too short for it to work. If you shift into it by mistake you will break something. At the very least that will end your ride that day.

I'm generally pretty aware of what gear I am in, but I have done it a couple times when I was tired, accidentally left it in the big ring and had not yet realized it.

I know that's not what you're saying Idamelio, I just want to make it clear to other people. Kind of tight is ok, too tight to fit is not. Even if it's only kind of tight you may find that the cage rubs on the 2nd cog due to the extreme angle of the cage.

Longest chain that works in small/small is a better way to do it, unless you are beyond the wrap limits of your derailleur. Then saggy in the small ring and small cogs is preferable to too tight in the big ring/big cog.