Bicycle Mechanics - changing cassette

I'm considering changing my rear cassette from a 12-23 to a 12-27 so big hills won't seem quite so tough. :eek:
Everything is Ultegra.
Will I have to change my rear derailleur also?
Thanks.

I changed my 11-23 for a 13-28 because I hade a very hilly event to do.
the gruppo is Campag Centaur, but the rear changer was just fine with the bigger cassette. There is a possibility that the top jockey night be a wee bit close on the big cog, but the LBS guy said that if that was the case, rotating the derailleur a bit and stretching it out would compensate.

I advise go for the change. Its 95% likely to work fine. on the 5% off chance tho' worst case is you either go without the biggest cog temporarily, (you'll still be better off than now for those hills), or tweak the derailler, or, worst case, you shell out another 50 bucks or so for a new derailleur.

I recon it'll be ok. Give me a slap if I'm wrong

Stew

The answer is NO.The 9 speed ultegra rear mech is speced for a 27 tooth large cog and will handle more.Make sure your chain is long enouth to handle the big/big,just in case.And, you may have to play with the 'b' tension screw on the derailer if the top pulley interfeers with the cassett cogs.You do not rotate the derailer and stretch it out a bit.

You will have to lengthen your chain, however.......

Originally posted by D*Alex
You will have to lengthen your chain, however....... , I will go for that, longer chain

Longer chain is what you need and you will probably need to adjust the B-Screw tension.