Duane Behrens

Just for spits and giggles, I installed a new Ultegra medium-cage rear derailleur on my lightest road bike - a 2009 Tarmac with a 53/39 chain ring set and 12/30 cassette.

The Shimano specifications state that the largest sprocket for both the short-cage (SS) and medium-cage (GS) RD-6700 derailleur is 28T. So I guess I'm a little out of spec with EITHER the short- or the medium cage. A friend advised me thusly:

"Where [the short- and medium-cage derailleurs] differ is in total capacity, which refers to the ability to take up chain slack. Total capacity for the short-cage unit is 33 teeth, for the medium-cage unit 39 teeth. Total capacity needed for your current drivetrain is (53-39) + (30-12) > 14 + 18 = 32 teeth."

I liked that a lot. It made sense. He continues:

"So you're a little over the largest sprocket specification (which is fine), and did not need the medium-cage derailleur to be within total capacity specification. The two specs are often confused. The determinant for "largest sprocket" is the space between the top pulley and he largest sprocket. The determinant for "total capacity" is the difference in length of unengaged ("slack") chain between the two extremes of chain placement: smallest rear and smallest front at one extreme; largest front and largest rear at the other.

"Of course, "largest sprocket" and "total capacity" sometimes go hand-in-hand. People who have large sprockets on the rear often have cranks with a large difference in size between the big and the small chainring (like a compact). So it makes sense to design a derailleur which can take a very large sprocket and also has a high total capacity. But that doesn't change the fact that these two specs are fundamentally different."

Didn't understand ALL of that . . . but I went ahead and put the new, longer-cage derailleur on anyway, along with a new chain, and went for a short ride.

Observations:

1. It all works great.
2. Shifting SEEMS to be crisper, more immediate, in the lower gears. I'm not sure why, but suspect it might have something to do with the under-stressed medium-cage being able to sweep across the top end of the sprocket with greater leverage, aka less effort.

Whatever. It works. Some here have stated that a medium-cage RD is useful only for triple sprockets. I think there may also be a place for them with the newer, wider range cassettes . . . e.g. the 12/30s.

Thoughts?

Phil_gretz

It's after midnight and you're posting about derailleur capacity. Is everything okay?

reptilezs

the geometry of the parallelogram is better for a wide range cassette

rpenmanparker 

OP, as your friend advised, if you haven't exceeded the total tooth capacity of the short cage derailleur, there is no improvement that can be realized with a medium cage model having the same largest cog specification. It is quite likely all in your head. No harm done, but no real improvement. Sell one and be done.

Duane Behrens

Duane: "Shifting SEEMS to be crisper, more immediate, in the lower gears. I'm not sure why, but suspect it might have something to do with the under-stressed medium-cage being able to sweep across the top end of the sprocket with greater leverage, aka less effort."

"reptilez" agrees: "The geometry of the [new] parallelogram is better for a wide range cassette."

"rpen etc." disagrees: "if you haven't exceeded the total tooth capacity of the short cage derailleur, there is no improvement that can be realized with a medium cage model having the same largest cog specification. It is quite likely all in your head."

ME: Or perhaps it's just brand-new parts working as they should. Although I don't discount "reptilez" suggestion. Someone else stated the same; I may simply be realizing more positive and immediate shifting action as the result of the longer shifting "lever" (cage).

It does look a tiny bit kludgy, though.

Retro Grouch 

If you had made no changes and wanted your shifting to feel more crisp and immediate, I would advise you to do a tune up on your rear derailleur. When you installed your new derailleur, that's what you did.

reptilezs

the 6700a mid cage rd is 30t max cog size. the older 6700 mid cage is the same 28t max as stated in the first post

Duane Behrens

And I misspoke. Or mis-typed. I went out and looked again at the box that the new RD came in. It's marked "6700-A," which means it accepts a 30-tooth rear cog, as also noted on the box.

And that, probably more than anything, explains the better shifting. So if you're running a 12/30 in the back and a standard (53/39) chain ring up front, this might be the RD for you. Best. DB