[slamy]

I have a gravel bike with di2 dura ace and it’s fine for gravel around here. But I’m signed up for BWR in Asheville and I’m going to need something bigger than a 34x28. I was wondering if I can put a grx di2 rear derailleur on my setup and an 11-40 rear cassette and keep rolling. It looks like since they are both 11 speeds they should be compatible. Does anyone know?

 

[JSR]

The Shimano compatibility chart indicates there is no problem driving the GRX RDs with a DA shifter. The issue is that the 40t capacity RD-RX817 is spec’d for max capacity of 31 teeth. Presuming you’re running 2X in front you’re out of spec If you have more than a 3-tooth difference between big and small rings with an 11/40 cassette.

The question becomes what is the real capacity of the the RD when installed in a 2X setup.

https://productinfo.shimano.com/download/pdf/com/3.4/en

 

[Finx]

Generally speaking, the only compatibility concern with GRX is the crankset/front derailleur. Because the chainrings on GRX is ~2mm further outboard than other Shimano cranks, you should pair GRX Cranks and FD.

Other components are no problem. Just keep an eye on the chain length.

 

[slamy]

So looking at it I probably should get an ultegra long cage and go with a 34X34 as my bail out gear. New cassette, derailleur and chain should be all I need?

 

[cxwrench]

There is no Ultegra 'long' cage, only medium...but that's the one you're talking about. Yes, derailleur/cass/chain and you'll be good.

 

[Marc]

Last I heard from people who did frankenbike it....the GRX FD will work with the 817 RD, in spite of the 817 being labeled 1x. Of course, you're at the mercy of Shimano to continue to allow it.

 

[JSR]

So looking at it I probably should get an ultegra long cage and go with a 34X34 as my bail out gear. New cassette, derailleur and chain should be all I need?

Well, since it’s for off road use I’d probably go with the GRX derailleur. It’s got the clutch thing.

 

[slamy]

Generally speaking, the only compatibility concern with GRX is the crankset/front derailleur. Because the chainrings on GRX is ~2mm further outboard than other Shimano cranks, you should pair GRX Cranks and FD.

Other components are no problem. Just keep an eye on the chain length.

I think the wider cranks are to accommodate wider chainstays to fit the wider tires. With all the different bottom brackets styles out today 2mm difference in bb clearance should not effect the grx rear derailleur. Otherwise internal bb and external bb on road bikes would have similar issues. It seems to me that I should be able to use the grx rear. However how big a cassette with a 50/34 is the next issue.

 

[Finx]

I think the wider cranks are to accommodate wider chainstays to fit the wider tires. With all the different bottom brackets styles out today 2mm difference in bb clearance should not effect the grx rear derailleur. Otherwise internal bb and external bb on road bikes would have similar issues. It seems to me that I should be able to use the grx rear. However how big a cassette with a 50/34 is the next issue.

Yes, the only 'dependency' (per Shimano) is that if you use a GRX Crankset, you'll need a GRX FD (and vice-versa). Shifters and RD are not affected by this.

 

[Grog McCog]

FWIW, I am running Di2 Ultegra (2014) with a 46/30T GRX crankset and 11-34T cassette with no issues. It might be possible to do something similar with your existing Di2 components.