3dsteve

if i changed from a 12/25 to a 12/27 would i have to change the chain and would i need to readjust the derailer ,i have a century coming up with lots of climbs and was thinking about changing the cassette to the 12/27 just to make things easier

barba

It will probably be fine (stay out of the big ring/cog combo, but you shouldn't do that one way or another). If the chain has many miles on it, I might consider replacing it with the new cassette anyway.

rog

I swap between my 11-23 and 12-27 with no ill effects, same settings except for the cassette.

s4one

I cross my chain, when im on 53/25, there is no rubbing or clicking. The Centaur front derailleur has 3 positions. So that I can position it where it does not rub.

I try not to use my 39, unless its for climbing.

3dsteve

thanks for the info looks like i will be changing it out to make things easier

xfimpg

I did this exact change last year. Did not notice any difference in ease of climbing - 2 teeth equals 1 gear - and i did not have to reajust the derailleurs.

3dsteve

so are you saying that i should just leave it the way i have it now with a 12/25

xfimpg

Dont leave it at 25 if you need more gears to climb; just make a more substantial change, like go to a 32 or 34.
Keep in mind with that type of soultion you'll need to change to a long cage derailleur, if it isn't one already.

Camilo

If it were me, I'd go to the SRAM 12-26 rather than a 25. You're going to make the change, why not go one tooth more? The SRAM cassettes are a much better deal than Shimano anyway, cost per weight wise.

xfimpg

The OP needs more climbing power, 1/2 a gear more isn't going to really help ... still think the OP needs to go to a 32t.

idcruiserman

Maybe try a smaller inner chain ring.

Sapling

+1

Little Darwin

What is this gear of which you speak???

cpb406

If your "B screw" adjustment was on the edge with the 12/25 you might have to tweak it a little.

Deuce Magruder

if you had any guts youd ride a fixie. just saying.

Coyote2

If you are undergeared on steep climbs, a 12-27 will give a small advantage over a 12-25, but not a huge difference. Just go 12-27 if you need to replace your cassette anyway.

Coyote2

You're in the wrong forum. Go away.

kwrides

I swap between my 12-25 and 12-27 with no ill effects (haven't gotten around to buying a 12-27 for the trainer wheel).

I do feel a difference fwiw

stapfam

If you have hills- then a compact and 12/27 is the way to go. That is what I have for my hills- But the new bike came with a standard crankset- and 12/25. I did manage the 12%'s with it but the 15%ers had me sweating a lot. And I walked the steep one.

12/25 to 12/27 will probably not require a new chain- unless it is worn but check it out by trying to engage the big ring on the crank and the 27 on the rear. When you check it out- do it gently- just in case the chain is too short.

xfimpg

Gear as in smaller gears for climbing, call them cogs if you prefer.