What rear mech?

[deanosabby]

Hi forum,

I am in the middle of buying Ultegra groupset parts and have already the gear lever, brakes, 12-30 cassette, bb and chainset but was wondering if I need a short cage rear mech or a medium one.

Ta,
Deano.

 

[Rob3rt]

With a 30T cassette you will need a long cage rear mech I believe. Not 100% sure, as I have never seen the need for such a massive cassette so haven't looked in detail but fairly comfortable saying that the spec of a short cage (SS designated mech) ultegra rear mech is good for up to 28t then a long cage (GS designated mech) should be fitted, this should be good up to 32t.

 

[deanosabby]

I have a 28t just now but I am doing the etape du tour next July and would like a 30t to help with the big climbs.
My lbs said that I would be fine with the rear mech that is fitted but I am unsure what size it is.

 

[deanosabby]

This is what is fitted just now. I am not sure if it is easy to tell which size of rear mech it is from the photos

 

[tiermat]

Looks like an SS to me, judging by the distance between the jockey wheel bolts

SS on left, GS on right:

 

[Cubist]

This is what is fitted just now. I am not sure if it is easy to tell which size of rear mech it is from the photos View attachment 61431 View attachment 61432

Your B tension screw is wound too far out. The chain should clear the bottom of the biggest cog by about 6mm.

 

[Cubist]

Your B tension screw is wound too far out. The chain should clear the bottom of the biggest cog by about 6mm.

My Ultegra is a GS, on a 11-32. It's not necessarily just about cogs, but how much capacity the mech is designed to handle. The difference between the front rings also has a bearing.

Chain reaction shows Ultegra SS as having a max capacity of 33t and max cog sizes of 11-28. The GS has a max capacity of 41t and max cog size of 32.

To find capacity required in your bike, subtract the smallest front ring from the biggest front ring, and then add that figure to the difference between your biggest and smallest cogs on the cassette. E.g a 12-30 cassette on a 50/34 compact has a capacity of 18+ 16= 34.

 

[T.M.H.N.E.T]

That's a short cage and it's screaming to be put out of it's misery

 

[deanosabby]

This is all way above me guys. Just now I have a 12-28 rear cassette fitted and my LBS reckoned that I could just change the cassette to a 12-30 and it would all be fine. As I have decided to change the whole group set I just want to make sure I buy the right rear mech.
I know plenty about car mechanics but put me in front of a bike and my knowledge is limited to changing the rear cassette and chainset ;-)

 

[lpretro1]

Looks like the rear mech is going to get ripped off there in that gear. You will have to fit a long cage mech

 

[Arjimlad]

I'd never ride that combination - surely you'd be on the smaller front ring if you were on your biggest rear cog ?

 

[deanosabby]

Cheers for your help fellas. I think I'll double check with the LBS tomorrow and see what he has to say. :-)
Ta,
Deano.

 

[TheDoctor]

FFS don't use the big-big combination! It's bringing me out in a rash just looking at that!

 

[slowmotion]

@deanosabby, how many teeth are there on your chainrings at the front?

 

[TheDoctor]

I'd say that's a compact - 50/34? Can't be sure though.