ok.. i'm currently riding a 12-25 shimano ultegra cog in the rear and a standard 53T in the front. my question is.. can i put a 12-27 shimano ultegra cog in the rear without having to get a new chain or adjust the deraileurs?

plz help.. it's double points this weekend.. hehe

EDIT: forgot to mention it's a 9speed ultegra group i'm riding on.

You don't need a new chain and probably will not have to adjust the derailer assuming the new cog is also 9 speed.

thanks geraldatwork... yeah i'm looking into the 9speed version. just wasn't sure if that extra 2T would create a bigger headache or not.

Agreed, most probably will not need to lengthen the chain but you should check to make sure the derailleur does not reach its limit with the chain in the big ring - big cog combination, 53 - 27. In that combination you should be able to push the upper half of the derailleur forward without bottoming out. Of course you should not ride in that combination but in case you hit it accidentally you don't want to trash the derailleur.

Al

cool.. good tip Al.. gonna have to check that when i get it.

When will you actually EVER use the 27 53T combination? Do you ever use the 25 53T now...thought not. I switched to a triple a few years back and left the short cage derailluer on the bike knowing I never would use the 53/29 combination EVER. I eventually bought a triple RD but it wasn't really necessary. Check out your shifting patterns and you will be able to answer that question yourself. For example I only use the 4 smallest cogs with the 53 and the 5 largest cogs with the 30 on the triple. Using the extremes on a 9 or 10 speed usually have radical chainline which in my opinion will wear a chain and teeth out quicker. Looking at all the graphs and tables in the world may not represent the way you actually ride.