Peacewiz

Hello!

I just found out my rear derailleur had a broken arm but thankfully they were working while on trail. it's a cannondale topstone with ultegra components 46/30 crankset with 11-34t rear. I like to upgrade to 11-40t cassettes and new grx 11 speed rear derailleur, leaving front alone. Any suggestions on this upgrade if I'll have any issues with front 2x crankset and chain sag/short?

Thanks in advance~

Badger6

The GRX RDs by specification won't do this. The RDs that take up to a 42T large cog cassette are spec'd for 1x systems, because they don't have enough capacity to handle the chainring shift in the front. The RDs for the 2x systems do have enough capacity but they are spec'd to take only up to a 34T large cog. It bears noting that Shimano design specs are notoriously conservative, and their stuff can regularly be made to work outside the published specs...the issue always is that iff you exceed spec, how much can you exceed it, and if there's ever an issue you're not going to get tech or warranty support.

That said, some people have made the 2x RDs work by cranking the b-screw all the way in (supposedly 40T is a little more reliable than 42T if you go this route). Using the 1x RD is not an option, the cage swing is not sufficient to deal with the total capacity (Large cog-small cog + chainset difference). There are a few older threads here, fall of 2020 IIRC, and some YouTube videos of folks who got their 2x (mechanical) GRX RDs to work way outside the design spec. Also, here's a link to a guy who got an 11-40 cassette and a 48/31 crank working with GRX components out of spec: https://www.gravelcyclist.com/videos...-xtr-cassette/.

Peacewiz

Thank you for the info. I'll try it out and tinker along the way~