Originally Posted by
jonathanf2
Sounds like a bent derailleur hanger issue to me. Especially since you were coming from a non shadow Ultegra 6800 RD. A non-shadow RD is more prone to bending the RD hanger especially if the bike gets dropped on the drive side. My old 105 5800 RD setup had the same issue where it'd get progressively worst as you shifted to the low gears. I went to my LBS and had them straighten the hanger out for like $15-30 bucks.
Possibly. Everything looks pretty straight, but it's something I will check closer. This is an excuse for me to buy a derailleur alignment tool. Yay! New tools!!
I reinstalled the Ultegra RD and SRAM 11-36 cassette (the parts I had just taken off). I still had the same indexing issues that I had with the GRX and 11-42 setup. Super frustrating!! Trying to retrace my steps even more, I replaced the new derailleur cable with another one. That improved things. Apparently, the cable had gotten kinked in a couple of places during installation, and that may have been a big contributor to my issues. The shifting isn't perfect (a little noisy on the 2 biggest cogs), but it's rideable. For now, I'm going to set is aside until I can spend more time with it, and start from scratch again with the GRX and 11-42 setup. I'm going out of town for a week starting tomorrow, and have decided to take my road bike instead.
Doing some research on my issues, I found out why I needed a spacer for the XT 11-42 cassette, but not the Ultegra or SRAM cassettes. On Shimano 11s MTB cassettes, the inboard end of the cassette that fits over the freehub is inset from the face of the largest cog. Shimano 11s road cassettes are not inset. Cog spacing was not my issue.