Having a double chainring and 11-42 cassette on a GRX 11-speed: goat link/tanpan vs cage swap

Question

For some niche cases, gravel riders would appreciate to use wider range cassettes (11-40, 11-42 or 11-46) on their double chainring gravel setup, which would allow have ranges close to 600% (vs 507% for the latest GRX 2x12 or 510% for the latest 1x12).

The GRX RD-RX810 (or RD-RX815 for the Di2 version) is the official 11-speed double chainring GRX rear derailleur and is derived from road derailleurs (its specs are very close to the 105/Ultegra rear derailleurs), with similar max sprockets size and capacities. When searching how to fit a larger cassette, the answer that can be found is to replace the derailleur attachment by a goat link/tan pan, that would move the derailleur away from the cassette and give the necessary clearance to use such cassette. But that is done at the detriment of shifting performance on small sprockets, as the jockey wheel is too far from the cassette to do its work properly. From my experience on another bike, the decrease in shifting performance is significant, and I would not repeat the experience.

I recently came across another "hack", which is to start from the 1x version of the GRX rear derailleur (RD-RX812 or RD-RX817 for Di2), and swap the cage for the one of a XT RD-M8000-SGS rear derailleur. The body of the RD-RX812 seems to be indeed closer to the one of the RD-M8000, both are designed around 11-42 11-speed cassettes ...and it looks like the cages of the RD-RX812 has the same part reference as the one of RD-M8000-GS (short cage). Considering that the RD-M8000-SGS (long cage) has a capacity of 47, that is enough for a 46-30/11-42 setup, it seems like a more elegant solution: the parallelogram works within their design specs, only genuine Shimano parts are used. The drawbacks are: the rear derailleur must be replaced, and the cage replacement is not as easy as fitting a goat link. This video shows this hack, but pushed further with an 11-46 cassette and an Ultegra 50/34 crankset. This other video (thanks DoNut) applies the same hack to a 1x drivetrain, it shows how to replace the cage but the shifting should be seriously compromised as the parallelogram is not designed for this kind of cassette.

Given the second hack seems much more elegant, I'm surprised to not see more occurrences of it online — the price doesn't seem to a valid argument for me, as people able to afford RX810 equipped bikes can afford swapping a rear derailleur and cage plates are easy to find online (for around 25€). So is there a catch I didn't think about?

Answer 1

I have a bike set up with

• CS-M8000 11-42 11-speed cassette
• RD-RX817 rear derailleur (GRX Di2, 42tooth max)
• FD-RX815 front derailleur (GRX Di2)
• FC-RX600 crankset (46/30)

This set of equipment has a 37-tooth difference between small-small and big-big, which exceeds the 31-tooth capacity of the RD-RX817. The Di2 programming, however, allows me to lock out problematic combinations, keeping the gears I can actually access within the bounds of the derailleur capacity.

The specific software setup I use is “front derailleur auto-shifts to track position within the cassette”, with the right auxiliary button set to downshift the front if I need more torque in a hurry. I’ve got the non-Di2 brake/dropper lever on the left (and thus the only way I could end up in a big-big combination would be a software fault that bypassed the software bad-ratio lockouts and a second fault that (software, mechanical or a low battery shift-lock) that kept the bike on the big chain ring.

Answer 2

I finally made the modification, and have tested it now on more than 500km involving road and gravel (medium mountain). To answer the question, I don't see any catch on the proposed hack, except "the relatively advanced nature of it", to quote NathanKnutson's comment.

Unlike RLH answer, my setup is mechanical, but very close:

• RD-RX812 rear derailleur, with the cage of a RD-M8000-SGS XT derailleur.
• FD-RX810 front derailleur
• Sunrace CSMX8 cassette 11-speed, 11/42
• FC-RX600 crankset (46/30)
• CN-LG500 LinkGlide chain

Changes in gear are crisp on the whole range of the cassette (unlike solutions based on goat links, that penalize shifting performance on the smallest sprocket). I went a few times inadvertently to the big-big combination without any particular issue, and I haven't seen any chain mark on the chainstay, so I can conclude that chain slap is limited.

For a mixed road/gravel use, it should also be noted that the 6 smallest sprocket are identical between 11-34 and 11-42 cassettes, the difference only appears from the 7th sprocket, where being on the right sprocket is typically less important.

On other possible uses of this hack:

• the GRX810 is in the same family as the R7000/R8000, so using a RX812 rear derailleur and RD-M8000-SGS long cage is potentially applicable to these groupsets as well.
• the GRX400/Tiagra 4700 uses an 11-speed pull ratio, and an 11-speed chain is only 0.2mm thinner than a 10-speed chain, so there's little reason that should prevent it to work for this groupset as well.
• 11-speed road pull-ratios are also identical to 12-speed road groupsets, so that could work in 12-speed as well, the only problem being the lack of compatible cassettes (the closest would be 10-45, but that would deviate too much from the M8000-SGS spec to my taste).

For reference, here are pictures showing the rear derailleur in the most extreme positions (big-big, and small-small).