powerglide

shouldn't have a problem, my lbs says the same fwiw

sogood

But how did you determine your chain length in the first place? If they are long in the first place, then obviously there's no problem.

I note that the RD is already pretty well stretched when I am on 50/25.

xfimpg

My LBS checked the chain, stretching, and they were right that i didnt need a new chain. That's the most I can tell you.

FatguyRacer

I'm loving the 12-27 combined with my new compact crank with the 46/36 rings. I got the crank to put on a cross bike later. I'm loving it for climbing big hills and long grades. On the group hammerfest the other night, not so much. I need to get a 50T or 52T 110 BCD big ring.

Hocam

I always cut my chains with the chain in small-small combo, when the RD is close to rubbing the chain together.

FIVE ONE SIX

even though LI for the majority is flat, there are still climbs if your willing to find them. the 55 mile Gold Coast route has almost 4000 feet of elevation, which may not be much for some of the other riders here in other locations, but for LI that's the hilliest sub 60 mile ride i've ever done. if you could do that entire ride in a 52/53, than your my hero, because even the top riders i know that did the 100 mile version eventually had to resort to the small ring on climbs, especially going up Avery at the 90 mile mark...

Epiphreddy

I have a Shimano compact 34/50 crank and a Dura Ace rear derailleur (10 speed). I currently am running a 12/27 Ultegra Cassette and was thinking I would try the SRAM OG 1070 11/28 cassette. Will this work?

jameyj

I went from a 12-25 to a 12-27 and loved it. Now I have a SRAM Red 11-26 and find it the best of both cassettes for me.

s4one

I rode a compact for a little while, 34/50 with 12-25. It was a pretty nice ride up hills and my cadence was pretty stable for the most part. I swapped back to a standard 39/53 with 12-25, its pretty damn harsh on the hills when I am on 39/25.

terrymorse 

FYI, IRD makes several wide range 10-speed cassettes: 11-32, 12-32, 11-34.

These are too big to be used with a Shimano road rear derailleur. You will need a MTB derailleur.

We just swapped out cassettes and rear derailleurs on our rental bikes, installing IRD 11-34 cassettes and XT long cage derailleurs. Those bikes can climb trees now.

Should work okay, just make sure to adjust the B-tension screw to keep the pulley from touching the cog.

SpongeDad

Agreed - unless the chain was sized for an 11-23, going to 12-27 generally shouldn't be a problem. It's a relatively cheap fix.

I prefer having the 27 over the 25, but I prefer to spin up 6% grades, not mash.

xfimpg

I did this upgrade last year and the difference was negligable. It wasn't money well-spent IMO.

grahny

While we're on the topic of cassettes.... what's the largest cog anyone's run with a campy (chorus) short cage RD?

humboldt'sroads

When I put my wheel with a 12-27 on, the top sprocket on the rear derailleur doesn't leave enough room for the chain to fit smoothly between it and the 27t cog. Is this easy to fix?

SpongeDad

I have DA10 w/27 and it fits fine - it's the B tension adjustment screw, not the Hi Lo adj screws or the barrel adjustment. You can download a .pdf from Shimano that shows which screws do what.

Also, as to what the real difference is - it comes down to this. If you're pushing up a hill at 9mph, 39/25 is 74 rpm and 39/27 is 80 rpm.

grahny

Yes, you're B-tension screw will fix this.... read below, good stuff:

https://www.sheldonbrown.com/derailer-adjustment.html

GernBlanston

The difference between a 25 and a 27 is something you can definitely feel, but IMHO, it's mostly a psychological gain. If you think you feel more comfortable, you do.

You shouldn't need to worry about chain length, but I would avoid shifting into the 53/27. If you can even do it, you'll get a lot of drag from the extra tension in the rear derailer.

GB

buelito

The only difference in these two cogsets are the two lowest gears... the rest is the same. So, with the 27, your shifting goes in 3 tooth increments to 24 and then to 27. With the 25, your shift stays in two tooth increments, from 21 to 23 to 25.

Therefore, given that there is no difference in the higher geats (12-21), I would say go with the 27-- it gives you a lower bailout gear, and you can crossover to the 24 with the bigchainring without too much trouble--and that gives you a little more flexibility in choosing your gears-- although I would probably not use the big chain ring with anything bigger than the 21.

On the other hand, you could go fixed...

train safe-

Bontrager

Wow blast from the past.

I've switched from a regular to compact ultegra with the same 12-25 cassette and it's awesome. i seem to use more of the cassette. I never changed the length of my chain and notice if I'm cross chained 30 front 12 rear then my chain is a wee bit too long for my RD to pick up the slack. I'm still considering changing to 27 or 28 tooth rear for those times I need something lower. Nice to know you have it in case you need it.

Bontrager

Are you kidding? It might not make a difference on the 1st hill of the day but 2 extra teeth definitely make for a easier climb on the way back home

Dubbayoo

I have an 11-23 on my trainer wheel, a 12-25 on my Zipp 404 and a 12-27 on my Ksyrium Elite. If I lose 20 lbs I want an SRAM Red 11-23 for the Zipp.

humboldt'sroads

B-tension, nice. Thanks grahny & spongedad. I definitely notice the difference in a big way between 25 & 27, especially after 30-50 miles of hills. We've got a lot of 15-20% grades here, many of which extend for miles with little let up. I can get through any of them with my 34x25, but I can get through them and enjoy myself a little more with a 27. I like to climb out of the saddle (trying to break myself of that) so I very rarely hit my 27, but it's nice to have.

rbart4506

My wife and I have been debating this change ever since we got our new Tarmacs. They came with a 12x25 mated to a standard crank. Our Roubaix's have a 12x27 mated to a compact crank. We were finding the Tarmac just a bit tough on the main climb out of town. We wondered how the 12x27 would work and how much of a difference it would make.

After reading this thread I decided to make the swap. Instead of swapping cassettes I just swapped the wheels. The Tarmac came stock with Fulcrum Evolution 5's and the Roubaix has Mavic Ksyrium Elites. The swap was easy and only require some adjustments to the brakes. The Fulcrum rims must be a tad wider then the Mavics.

We both noticed a difference with the 12x27. The climb was easier and I really think the main difference for me was having the 24. All I know is that I think the 12x27 will stay on the Tarmac and the Roubaix will get the 12x25 with the Fulcrum wheels.

BTW I must say the black Mavic Ksyrium Elite wheels look so good on the Quickstep coloured Tarmac.

Fox Farm

Well, depends on how long the chain is sized for the 25. You are not really into a chain wrap issue there.

I am a Campy user and have the compact 50/34 and an 11-25 on the rear. I can get my 195 pounds up pretty steep hills but for long climbs (like a Mount Mitchell ride) I would put on something with more bail out gearing. I first did Mitchell when I was 32 and I used a 39 inner chain ring with a 26 big gear and was very sore. The next year I did it with a 28 (Seven Speed Days) and it really helped on the very steep stuff but the jumps were greater. Now with the 10 speed stuff, you can put on a 29 and really have some bail out gearing but I would probably only go for a 27 with the compact. Again, it all depends on how close you are to 10% grades, how long the hill is, and what your body size is.

neospazzy

Switching to 12/27 from a 12/25 made climbing a bit easier for me as well as putting less strain on my knees.