Road Cycling - Changing 12/25 to 12/27 on a triple - what do I need to know?

On a bike with a standard triple crank (52/39/30), can I simply swap the existing 12/25 cassette for a 12/27 cassette and then perform some derailleur adjustments, or do I need to add chain links, or worse, change to a new derailleur? The front and rear derailleurs are Ultegras, and the rear looks to be a long cage (but I don't have specific model numbers). If I need more chain links, does anyone sell just a few links, or do I need to buy a whole new chain - or do I get a bike shop to do the work since a new chain will probably be about the same as their labor and they'll throw in a few links as part of the work?

I'm handy in general, and have been able to do all the adjustments my bikes have needed myself, but this would be my first drivetrain component swap.

TIA

On a bike with a standard triple crank (52/39/30), can I simply swap the existing 12/25 cassette for a 12/27 cassette and then perform some derailleur adjustments, or do I need to add chain links, or worse, change to a new derailleur? The front and rear derailleurs are Ultegras, and the rear looks to be a long cage (but I don't have specific model numbers). If I need more chain links, does anyone sell just a few links, or do I need to buy a whole new chain - or do I get a bike shop to do the work since a new chain will probably be about the same as their labor and they'll throw in a few links as part of the work?

I'm handy in general, and have been able to do all the adjustments my bikes have needed myself, but this would be my first drivetrain component swap.

TIA

You should be ok with the simple cassette swap. Try it out. Worst case is you pick up another chain and then you'll have one for each cassette.

No derailleur adjustments would be necessary.

The chain might be a bit short (though unlikely). Try mounting the new cassette, then switch into the big chainring-big cassette combo. If the chain makes no visible kink while going through the rear derailleur, you'd need something longer (that's a new chain, not just extra links). If not, you're fine.
If in doubt, consult this page: http://www.parktool.com/repair/readhowto.asp?id=26

I swapped out my 12/25 for a 12/27 last summer. Nothing else needed.

If the present chain is sized properly, you could likely use one more link. You would have a long cage derailleur on your bike already so should be set.

don't be in the 52-27.

problem solved

You may needto add 1 additioanl link or you may be OK.

The difference between the 25 & 27 cog results in an additional 0.5" calculated for Chain Length, using the url to the Park Tool site (posted by EZOONS) which shows the Simple Equation for Chain Length,
L in inches = 2*C + (F/4) + (R/4) +1
You are only changing R from 25 to 27 so the difference in the 2 (R/4) terms is 0.5". Alll other terms remain the same. The equation rounds the chain length up or down depending on which side of 0.5 the answer is (56.4 becomes 56", 56.6 becomes 57"). IF your current calculated C is >X.5, then the new answer rounds to the same value. If you current calculated C is <X.5, then the new C rounds up to 1 added link.

You can do:
1. The calculation

or

2. Use existing chain since you probably have enough margin. Just avoid the Large Ring & Large Cog combination, which all good cyclists avoid.

I split the difference between an 11-34 on a touring/cross-ish wheel, and a 12-25 on the lighter roadier wheels. No adjustments that I've needed... 12-25 to 12-27 should be no problem, no adjustments on the derailleur, and probably no change to the chain if you don't have it super tightly set.

I switch frequently between an Ultegra 12-25 and 12-27 cassettes. No additional chainlinks are needed, provided your chain was properly sized for the 12-25 cassette.

Are you swapping the cassette cause the old one is worn, or for the extra two teeth which won't make much of a difference except in the wallet?:D

If you take it to the shop, I doubt they will throw in free parts as part of the work.

Incorrect.

25-27 IS a difference climbing.

Incorrect.

25-27 IS a difference climbing.

Uh, okay!

I switch frequently between an Ultegra 12-25 and 12-27 cassettes. No additional chainlinks are needed, provided your chain was properly sized for the 12-25 cassette.

+1 - At most you'd have to drop 1 link, but unlikely if you bike wasn't built by a imbecile.

at most you'd have to ajust the body angle screw to accomodate the larger cog. if so, it's all covered at the park tools site.

I just swapped the cassette to the 12-27 and it worked fine. I agree that the 27 makes a difference on climbs. With 50 lbs of cargo I never encountered a hill that could not be climbed with the 27. Having used the smaller cog before, I would encourage you to make the switch if you ride hills.