New cassette and or mech upsizing from 25 to 27 teeth

bunnerscj

Dudes,
I curently have a compact running 11-25 on the rear.
However, to increase climbing capacity I would like to change the rear cass to a 12-27.
But.....will the extra 2 teeth on the top ring of the cassette at the rear fit with my current short cage rear mech ?. If I do change, will I then have to change the rear mech for a long cage version (than becomes a little expensive !)
Thanks

alfablue

Depends on the mech, most Shimano short road mechs will go to 27 (officially) and probably 28 (unofficially). You may need an extra link in the chain, but that will depend on the chain length at the moment. I swapped my 12-25 to a 12-27 with a Dura Ace mech (short cage), no problem. The B screw just needed a couple of turns.

flasher

I swapped my 9 speed 11-25 (HG50) to a 12-27 (Ultegra) with no adjustments at all

mostly harmless

Changing from 25 to 27 won't present any problems at all.

My hack bike uses a short cage mech to shift across a 11-32 without any issues.

edindevon

With a compact chainset and a short cage ultegra rear mech, I could not get my set up right with the 27 ring. It worked fine with a big ring of 25, but it rubbed on the 27 and I couldn't sort it out. I'm no good with the more complicated mechanical stuff, so I'm sure it was my lack of skill, but contrary to the responses above, I don't think it is necessarily a totally straightforward conversion.

Ultimately my solution was to get used to riding a slightly higher low gear and I now firmly believe that the average club rider should be able to get up 90% of the hills in this country with a 34/25 gear no problem.

Edindevon

alfablue

Edindevon wrote:

With a compact chainset and a short cage ultegra rear mech, I could not get my set up right with the 27 ring. It worked fine with a big ring of 25, but it rubbed on the 27 and I couldn't sort it out. I'm no good with the more complicated mechanical stuff, so I'm sure it was my lack of skill, but contrary to the responses above, I don't think it is necessarily a totally straightforward conversion.

Ultimately my solution was to get used to riding a slightly higher low gear and I now firmly believe that the average club rider should be able to get up 90% of the hills in this country with a 34/25 gear no problem.

Edindevon

If the rubbing was with the jockey (guide) wheel was against the cassette then the chain may have been too long. When I went from 25 to 27 I put on a longer chain but it was initially too long (I overestimated), removing links and adjusting the B screw solved it. Or was your rubbing on the front mech?

I think you are right about the average club rider though, unfortunately I ain't that good yet :oops:

edindevon

alfablue wrote:

If the rubbing was with the jockey (guide) wheel was against the cassette then the chain may have been too long. When I went from 25 to 27 I put on a longer chain but it was initially too long (I overestimated), removing links and adjusting the B screw solved it. Or was your rubbing on the front mech?:

It wasn't rubbing on the from mech. I'm sure you're right that adjustments to the chain length and B screw would have solved the problem, but as I say, my mechanical skills (and patience) weren't up to it.

Edindevon

John.T

You can run 50/34 and 12/27 with a short cage Shimano mech with no trouble. Chain length is fairly critical. For best operartion the chain should be as long as possible. When on small/small gears the chain should still be in tension and just clear the small tag on the rear mech cage. The B screw should be set so the jockey just clears the cogs in all gears.
If set up like this you can use all gears with no danger of damage. Extreme cross over is still not recommended.

alfablue

John.T wrote:

You can run 50/34 and 12/27 with a short cage Shimano mech with no trouble. Chain length is fairly critical. For best operartion the chain should be as long as possible. When on small/small gears the chain should still be in tension and just clear the small tag on the rear mech cage. The B screw should be set so the jockey just clears the cogs in all gears.
If set up like this you can use all gears with no danger of damage. Extreme cross over is still not recommended.

That's what I did, also Shimano says chain length should be such that the jockey wheels are in vertical alignment when on big ring and smallest sprocket, my setup is within 2mm of this. Works fine.

John.T

Alfablue. Both ways give much the same chain length. I just prefer to go long rather than short if the vertical method does not quite work out. See below, they are nearly vertical and the changes are as near perfect as I could wish for.


Just found out how to post pics really.