I’m slapping together an 11-speed Di2 re-build for my 10-speed mech gruppo P5. I’ve got all the components chosen/ordered except for the RD. The Ultegra RD-6870 is the logical choice for $$/performance balance in a Di2 RD, but I’m waffling between the -SS (short cage version) versus the -GS (medium cage version) which differ by allowing a 11-28T vs. 11-32T cassette. Where I live, where climbs are at most 10%, I’ve never needed more than an 11-26 with the 52-38 Q-Ring chainrings I use, but I’m left wondering whether there are any races I might one day go to that would require a 32T cog. Incidentally, I also have a 36T inner Q-Ring I can swap out for the 38T if need be.
The question I ask the ST masses is whether there are any (major) drawbacks to installing a medium cage -GS version despite likely never needing anything more than an 11-28 cassette? I don’t consider a 10 gram difference a major drawback, by the way! Is there something I’m overlooking?
Thanks,
Dave
I believe shorter cages shift just a little quicker and are lighter… and i suppose slightly more aero too. But I can’t think of mechanically any other drawback.
…trying to revive this 
DJ,
Did you end up with the RD-6870-GS or the SS?
I’m especially curious if you’ve run the GS mid-cage with a cassette with a smaller then 28t minimum (like the 11-26 that you mentioned), and what your results were. I recently ordered a kit with a GS mid-cage but I’d like the option of using some of the tighter spaced cassettes.
Technically, Shimano specs a Minimum Low Sprocket of 28T.
No drawbacks. If you want, you can adjust the b-screw when you change out cassettes to make the upper pulley snug against the largest cog.
Oh and in addition to the extra 10 grams for the derailleur, you will be lugging around 2 extra chain links! The horror.
I have the GS on my road bike so I can run a 32 when in the mountains. Most of the time though I have a 28 on there. However, I’ve used a 25 once or twice without any issues.*
*Only used it because I noticed a flat tire on the road bike just as I was about to leave the house. I was already late and in danger of missing the group so I grabbed my training wheel that’s on my P3 that has a 25 on it.
The -GS. Ironically, I just walked in the door from a ride on the bike in question! I’m running/have run 53/38 and 52/38 QRing front end paired with 11/28 and 11/32 without any issues, ergo 11/26 will work too. I don’t recall if I ran 52/36 QRing front or not in 11-speed. I simply took enough chain links off to just clear the derailleur pulleys in my 38T front ring/11T rear cog (since I’d rarely, if ever spend any time cross-chained like that), leaving lots of room for the derailleur to spread out on the bigger combos. On the more realistic gear combos, it’s heart-of-the-envelope for angle of dangle.
Dave