monk

I recently puchased a new Fuji Roubaix RC; has a 53/39 and 12-25 (10 speed Shimano Ultegra). If I buy a new 12-27 cassette, is it basically just removing one cassette and putting on the new one? Will I need a longer chain (I doubt it) and/or will adjustments be required to ensure the chain gets up on the 27 cog in the back? I'd rather not go with a compact crank; I think adding the 27 will be enough help with the hills we have around here - - not too bad.

Proximo

The only thing you may have to do is add a link to the chain. Other than that, it's just a swap of parts.

grom_met

yep just plug and play no worries

Barabaika

Why don't you just buy a 28t cog and put in the cassette instead of a rarely used cog?

urbanknight

^ Aren't individual cogs on most cassettes expensive? Otherwise, it's not a bad idea as that's what I did with my cassette, but it was a Miche brand where individual cogs cost $6 (added a 29 to a 13-29).

erader

i used to use a 26T on my RB. i tried a 28T cog but found it actually slowed me down because 26T was all i needed.

i suppose the OP picked 27T for a reason. maybe not tho .

ed rader

Retro Grouch 

Going from a 25 to a 27 tooth cog isn't going to give you a whole lot of difference. You might want to compare the rest of the ratios in the two cassettes to be sure that you aren't losing a favorite cog.

There is a slight chance that you'll need a longer chain but I doubt it. Try shifting into the big/big with your current 25 tooth big cog. If your derailleur isn't at the limit of it's slack take up capability, you can go up two teeth and not worry.

capwater

I've ripped the 28 out of a cheapo MTB cassette with great results for hill climbs. It doesn't need to be great quality since you're not going to to be in it 24x7, but it's great when you need it! Shimano specs short cage RD at a max of 27, but I've seen folks get away with 29s.

HillRider

You can't just swap cogs at will on 10-speed (and 9-speed for that matter) cassettes. Most of them have the largest three or four cogs mounted on a spider and they can't be removed or replaced individually. In fact, some of the intermediate cogs are also mounted in pairs on a spider and can't be exchanged except as a unit.

yes

You might have to adjust the b screw on the deraileur, it adjusts the height of the rear pulley wheel. If it is too high, just tighten the b-screw https://www.parktool.com/repair/readhowto.asp?id=64

San Rensho 

Why would you want to do that? The difference is 3 gear inches between a 25 and a 27, which under most conditions is going to be completely imperceptible.

The difference in gearing is fairly great with small cogs. For example, the difference in gearing between a 12 and 13 cog is about ten gear inches, but once you are on cogs that have teeth in the 20s, the difference in gear inches between cogs that have one tooth difference goes down progressively, so that the difference between say a 27 and 28 is one or two gear inches.

So to get a ten inch gear difference from your 25 (which will feel just like the difference between a 13 and 12) you would have to get a 32/33 tooth cog.

Halfast

Monk,

I put a 12-27 on my new wheel for my Fuji Team. Old one was 12-25 and was left on the other wheel.
I did move the "B" screw a little, and had to make a minor adj. to the der due to different wheels (which you will not have to do).

As you probably already know, the only difference between these is 21-24-27 instead of 21-23-25, all other cogs are identical. BUT, i really like th 27 because I rarely get off my 39 even in the moderate hills we have here in central Texas. I have a Triple, and when the going REALLY gets tough, the 30 ring and the 27 cog are great!!!

urbanknight

I have to disagree with you on that. It may be a small difference in numbers, but when you're travelling that slow, the ratios become wider than you think, often a matter of 10-20 rpm for the same speed. According to Sheldon's gear calculator, at 10 mph a 39/25 would be spinning 80 while the 27 would get 90. That gap gets larger as you get slower, so the climbing in the 27 would make a noticeable difference on your knees (if you're a spinner).

Barabaika

I don't know about 10-speed cassettes. Usually cogs are connected using small bolts that can be unsrewed.
I see Miche has different 10-speed cogs both for Shimano and Campagnolo.
https://aebike.com/page.cfm?PageID=30...and=572&type=T

Certainly, I would prefer to buy a Miche cassette that can be customized for sure.

Zouf

Ultegra 10 indeed uses a spider for the last 3 (4?) cogs, so no swapping of largest cogs possible. Also the diff between 25 and 27 is not going to be earth-shattering, and will be at the expense of another cog somewhere, so make sure you don't lose something more useful than what you gain. Finally, expect you may have to readjust the RD cable tension a little, since the cassette may sit a little off from the original one - in theory not, in practice often the case. I would do it at the next cassette replacement, no chuck out a not-worn-yet cassette just for this.

monk

I really appreciate the input . . . I've decided I'll probably do nothing for now. But if I do go 12-27, I'll just purchase the entire cassette (maybe on Ebay where it will be cheaper). I had a 12-27 on my other bike and seemed to notice a difference than with my 12-25. But frankly, I haven't given the 12-25 a fair shake yet . . . need to ride it more. Thanks again!

Al1943

You can most likely change from a 12-25 to a 12-27 with no other changes. Simply check to see that when in the large chainring - large cassette cog combination that the rear derailleur has not reached its maximum forward travel, unlikely. Of course you should never use that combination but just in case, you don't want the derailleur to hits its limit, that can cause serious damage to the derailleur and even the frame.

A Shimano 9-speed 12-25 can easily be converted to a 13-28 or 13-29 if you can find the correct cogs. Don't know if those cogs are available for 10-speed yet.

The larger cogs in most Shimano cassettes are revited to a common carrier and cannot be separated, but it is possible to add a cog to the back by removing some other cog.

Al

Hermes

+1 When I purchased my Trek Madone 5.2 with a triple, I had the bike shop swap the 12/25 for a 12/27. I test rode both cassettes on the hills and the 30 front 27 rear was noticeably better on steep grades. The swap was plug and play.