Originally Posted by
bigredgrad01
I got the Ultegra RX on there today, and it's perfect, resolved my one outstanding issue with 11-40, which was cassette inertia chain bounce when stopping pedaling. I also dialed in the B-screw a little bit and took off 2 links of chain to tension it in small small, there is still a little bit of slack in big-big. I am fairly sure you can do 11-42 as well with uncut chain, based on how much adjustment is still available on B-screw, but I haven't tried it myself, since 11-40 meets my needs. I would probably go to subcompact double before going any bigger on the cassette. XT RD + Tanpan seems like a fine set up, but ultimately you are introducing an extra adapter and I think Ultegra RX obsoletes it for new builds with road brifters, unless you want to go 1x with bigger than 42 teeth in the back, or already have an DynaSys11 RD sitting around.
Introducing an adapter into the chain isn't a problem with the Tanpan. From my experience it does not degrade the shifting. The SGS dereailleurs have a max capacity that is 8 teeth greater. With a front double and 11-40 you are already exceeding the max capacity of the RX by 6T. Yeah, Shimano is conservative with their capacity ratings, but it is true that an SGS will be more capable in taking up chain slack. Another advantage when using an MTB Rd with a wide range is that the upper jockey wheel moves much closer to the sprockets in the smaller sized cogs compared to a road RD. This extra distance make for sloppier shifts, and sometimes won't shift at all unless you overshift. It's a common problem when using Roadlink+road RD with a wide range cassette. The sloppiness resulting from this increased distance is greater than any(if any) sloppiness resulting from adding a Tanpan. If one is going to buy a new RD anyway and total range in the setup exceeds 39T, there is no reason to go RX over an MTB RD other than a few grams.