Highclimber1

I am riding on 11-34T, but I would like to know if I can go from 11-34, to 12-25. I like the speed, and some climbing. In case that I can go from 11-34 to 12-25. Do I need to change/upgarde any other component.

Maelochs

I do not understand.

12-25 would give you a lower top gear (lower to- speed) and much less climbing ability.

As for the question, no, you should not have to change anything. You might want to shorten the chain or the derailleur might take up enough slack.

However, if you like speed and climbing, going to 12-25 lessens both.

In my understanding.

VegasJen

Agreed. Going in the wrong direction.

Troul 

What is the bicycle, its sub model, & specified group set that is on it now?

Upload pictures of the overall bicycle, including pics focused on the group set into your album & link it here.

jadmt

I kind of understand what he is asking. He is looking for closer ratios when riding on the flats and wants to know if when he decides he wants to go on rides with climbs if he can switch back to the larger ratio cassette without issues.....answer should be yes...

choddo

Really? Doesn't read like that to me, just a one time change.

Steve B.

Assuming an 11 spd. system, yes you can change to the 12-25. You might have too much chain and would want to lose a link or two. As well, B screw might want to be adjusted on the rear derailer. You are giving up; some hill gears but if you don't need them the 12-25 is a nice cassette, I use it on my road wheels on my gravel bike, that has a 30/46 crank.

Iride01 

If you are wanting to swap cassettes based on the terrain and goals for your ride, you might can do that. But you'll need to have a chain sized to go along with each cassette. When I go to Colorado I sometimes swap out the cassette, chain and front rings.

But you also need to look at your RD specs and know that it will be able to handle that size cassette. Min/max of both the low sprocket and high sprocket is spec'd on many Shimano RD's. So being outside that spec with a smaller sprocket might affect your shifting performance

Welcome to BF.

Jack Tone 

I have a bike that came with 11x32. I have a 11x23 I switch to for flats. I change nothing, It works great.

terrymorse 

No need to change anything else -- just make sure that the 25 cog gives you a low enough gear for the climbing you intend to do.

No need to shorten the chain, either. It will shift fine.

12-25 is a good range for some not-hilly time trialing, or a fast and flat group ride.

Iride01 

Yeah, I was wrong to say the chain had to be changed. Because obviously the chain length that works for a 11-32 cassette also works when that bike shifts to the smaller tooth cogs on that same cassette. Still, I'd want to check that the RD is spec'd to run on that low sprocket. Just because yours works well doesn't mean the OP's will. Or perhaps it works for them but their shifts in and out of that combo aren't as crisp as desired.

But there isn't anything wrong for just trying. Which if the OP has the stuff, they ought to try it instead of thinking too deeply about it as we do in these discussions.

Highclimber1

Thank you to everyone for the advice. I just made a decision, I installed and
11-25 on my trainer, and keep the 11-34 installed on the tire. Any good observation will be appreciated.

SpedFast

Sounds like you figured it out. Good on you.

SpedFast

Yeah, I think we've all been told that a time or two

aliasfox

It'll work perfectly fine with no other equipment required, though you'll want to adjust the rear derailleur, both the B-screw as well as the limit screw, as the 11-34T is designed slightly wider than any other 11-speed cassette (or 12-speed cassette, for that matter). The 34T cog is essentially cantilevered slightly over the spokes rather than all sitting on the freehub.

I traded out my 11-34T for much closer ratios when I realized I never used even the 30T gear, nor the 11T. Trading off the 11, 30T, and 34T allowed me single tooth jumps between 12-17, which is much better for terrain that's -1% to 2%, which is what I was riding at the time. I went with a Miche, which allows me to select individual cogs. I've been in northern CA for a few months now, and have switched out the top three cogs (23T, 26T, 29T) for bigger ones (24T, 28T, 32T) to help with the steeper grades out here.

Going from a 29T to a 32T meant I had to get a longer chain to accommodate, but going from 34T to 25T shouldn't require that. In fact, if you're going between the 11-34T and the 12-25T, I'd just leave the chain as-is. You'll have more slack than you need in the 12-25T, but it won't hurt anything unless you're chasing that last half watt or 6 grams from the two links.