Postby Duck! » Thu Apr 04, 2019 11:54 pm
OK, so I finally got the chance to play around and find out just how far the capacity limits of the new-generation "Shadow"-type road derailleurs can be pushed.
Last week a customer wheeled his wife's bike in, a 2016-ish Giant Defy, specced with 6800 mechanial Ultegra, with 32T cassette and stock shortie derailleur, and a bag of bits including a XT 11-40 cassette, and a Road Link hanger extender. I took one look and advised that the link might get the sprocket clearance, but there was no way the derailleur would carry the necessary chain for that gear range, and a new derailleur would be required. It so happened that I had an R7000 shortie on the shelf, which even for a "short" derailleur has a lot more play room than any traditional derailleur (R8000 Ultegra and R9100 Dura-Ace share the same geometry, in case you're wondering).
As a matter of routine, I tried the 6800 with the Road link, and completely as expected, it got the clearance but couldn't carry anywhere near enough chain. So off they came and placed aside, and the R7000 came out to play. Officially rated to 30T in short form, I thought I'd test just how much clearance this baby can muster. Well guess what, with the B-adjuster screw cranked right in, it just gets enough clearance for the 40T, although it does crash sprocket & jockey wheel teeth a bit while shifting on or off that gear. so realistically a 38T, if such a thing existed would be its maximum sprocket allowance.
So that's the first test, but could it handle the two chainrings as well? The "short" cage on these new models is damn close in length to a previous-generation "medium" or "long" (they're the same thing) cage, but as it turns out, not long enough. Running between the 11T top and 35T second gear over the two rings got pretty marginal, so really the biggest cassette you can run on a shortie with double rings is 34T, but you could stretch it to 36T with a close-ratio CX-style ring set.
As a matter of elimination, I tried it with the Road Link, but it made no difference, it still couldn't wrap enough chain slack.
Nothing for it then but to order in a "long" aka "medium" R8000 derailleur. with that in hand, first thing was to size it up next to the shortie. shimano claim a bit more clearance with this version, and it's immediately obvious that the upper body is about 5mm longer. Fitting it proved it to be considerably happier getting under the 40, with a bit of room left on the B-screw. I have doubts as to whether it has enough in reserve to play nicely with a 42T.
So with clearance sorted, that just left chain length, and I am pleased to report that it does the 16T difference plus the huge cassette comfortably, but again, even if you can get the clearance under a 42T, it won't do wide-range doubles, but will be OK with CX-type rings or 1x.
So, condensed version: R7000/8000/9100 short will fit up to 36T with a 1x, possibly 40 depending on the frame's hanger length, or 34T with a 2x.
Longer version will accommodate 40T with any combination 2x, and may fit 42T with a 1x, again, hanger length may make it more of a possibility on some frames than others.
With that bike sorted, the customer returned with his own bike, a 2018 Defy, specced from the box with R8050 Ultegra Di2, 34T cassette and long derailleur, and the same model cassette. Went together an absolute charm, as the Di2 variant has exactly the same derailleur geometry as the mechanical version.
I had a thought, but it got run over as it crossed my mind.