Source
r/bikewrench•
40ft

Short cage RD compatibility with large cassette (specifically 11psd Ultegra Di2)

Hi all. I'm about to upgrade from 10 to 11 speed and have a question. I want to use an 11-32 Ultegra 6800 cassette which officially requires the long cage Di2 RD.

But I've read that Shimano builds in a margin of error to RD specifications, and that depending on your bike's dropout geometry, the short cage works fine with the 11-32.

I've also read that a short cage RD gives faster and smoother changes and better chain tension than a long cage RD, which obviously would be good to have.

Is it worth trying to get the short cage to work on my bike, or should I just follow the specs and get the long cage? Does anyone have specific experience regarding this setup? Thanks.

edit: I have a 2013 Giant Defy Advanced.

Posted Jan 29, 2016, 9:22 AM UTC

810
nhluhr•

If you don't already have the short cage SS derailleur then absolutely go with the in-spec setup. It is complete nonsense that an out of spec system would be somehow better.

There are three key differences in the medium cage 'GS' derailleur to make it properly handle the 11-32.

First, importantly, it is spaced so that the guide pulley clears the big cog. Now that is pretty easy to simulate on the SS by simply cranking in the B Tension screw. The thing is, adjusting B Tension doesn't only change where the pulley sits in the low gear position but across the whole range. The result is the guide pulley is now further from the small cogs than would be ideal. That equals less responsive gear shifts in the small cogs and slightly increased wear rates due to less chain wrap and fewer teeth being engaged.

So how does the GS solve the above issue other than just displacing it far enough to clear the cog? It actually sweeps the derailleur at a slightly steeper slope as it moves left to right. This is thanks to a parallelogram that is a few mm longer and the knuckle being angled so slightly different. If you think of the path a guide pulley needs to follow as the hypotenuse of a right triangle, the 11-32 has a steeper hypotenuse to follow since it starts at the same 11t and moves the same lateral amount. This now means the guide pulley stays close to the cogs across the whole range without needing excess B Tension while providing crisp shifts and normal chain wrap.

https://i.imgur.com/R2u8gHn.gif

Finally, and most obviously, the tension cage is longer which allows the derailleur to take up slack for the wider range of gears. Now, the SS model is right at its upper limit with an 11-28 cassette and 50-34 crank. You mention a mid compact which could be either a 52-38 or 52-36. The 52-36 has the same capacity requirement as the 50-34. -- they are both a 16t difference. It's one thing to say you'll avoid the small-small combo but the next two cogs - the 12t and 13t cogs are still out of takeup capacity if you setup for the big-big to be non-destructive. If you're using the 52-36 then you definitely need the GS capacity in addition to its proper max cog and parallelogram.

12
40ftOP•

Great reply. Thanks very much. I agree, on the face of it, it doesn't make sense to purposely go out of spec. It's just that I read both about the margins of error, and of people running 32 on Dura Ace Di2, or even of people running GS with 34 and 36. And I didn't really understand all the differences between SS and GS.

Your point about having less wrap on the 11T is well made, and I see how that would be detrimental to performance.

2
SgtBaxter•

If the short cage was set to not tear off in big ring big cog, obviously it wouldn't work small ring small cog ( or the next few small cogs), but would it take up slack okay in big ring small cog?

I'm about to do the same thing, but will switch to a longer cage to do it properly (no di2 so its not that costly). I was simply wondering about that though, as I'm never in the small ring smaller cogs. After the 4th biggest cog I shift to big ring.

1
nhluhr•

Yep, no issue with the Big-ring and any cogs - the rear derailleur has to take up the total difference in teeth for both front and rear. That spec is "Capacity". If you're in the big ring, you're still within the takeup capacity of the derailleur. It's the small-small end of the scale or the big-big end of the scale that become problematic.

1
EVIL_JIMMY•

I have semi compact 11-32 ultegra di2, with a long cage mech, I'm not sure a short cage would be able to take up the slack in the chain needed to safely run in the 52-32. My shifting is absolutely flawless with this setup though! (Set up on a Canyon Aeroad)

1
40ftOP•

The semi-compact is exactly what I'm going for! I think it provides a pretty great range with the 11-32. It's both lower and higher than my current 10spd 50-34 with 11-28.

I've read that it can take up the slack. But even if it wasn't able to, one would just have to avoid the small ring-11T combination. But that's not something I ever use anyway. I do, however, sometimes use the big ring-28T combination (on my 10spd; I know, the horror!) and it works great.

1
EVIL_JIMMY•

I find it the perfect range of gears for me! It could work, I'm Tempted to try it myself when my chain needs replacing, as I have a short cage di2 mech. I just played it safe and went with what Shimano suggested!

1
SgtBaxter•

No horror using big ring big cog. On the newer bikes its not really a big deal, buddy with a Domane 4.5 rides in his big ring big cog a lot, still got 5K miles from his chain before it needed changing, which surprised me.

0
tomjoad773•

I bought a bike in 2014 with 10 speed 12-32 cassette, 50/34 compact front. it came from the mfr with a short cage 105 rd which was pushing the spec but still acceptable. the official spec for the ss is 12-30 max but I figure if a major manufacturer specs out that way they must have talked to shimano and gotten some sort of confirmation it was ok to do

1
HydrazineIsFire•

I have done something similar to this with 10 speed Ultegra 6780 on a Cannondale Synapse and 50-34 rings. I would go for the GS cage if you have the choice.

There is no GS cage for 6780 and the official spec is 11-28. The bike shipped with an 11-30 cassette. While some folks had success with an 11-32 on other bikes, on the Synapse I had to get "creative" to reach 11-32.

For 11-32 I changed the upper jockey wheel from 11 teeth to 10 teeth to emulate the upper jockey wheel position on the GS cage. The b-tension screw was set as long as it would go. If the chain was short enough that it did not rub at small-11, it was worryingly tight at large-32 so I left the chain a bit longer reasoning that I am slow enough to need the 32 up the hills so I was more likely to make an error at the slow end of the cassette. It worked passably but was a bit sluggish to shift at times.

I eventually modified a 6700 GS cage to fit which improved shifting significantly.

With substantial modification I now have an 11-36 working with the GS cage but this involves drilling and other scary modifications to the derailleur itself.

1