thadoggfather

Hey guys anybody put a larger cassette on a GRX 815 Di2 than 11-34? according to shimano largest supported is exactly that, but then I read this a post on GRX (dammit can't link it yet) it says you might be able to go bigger depending on your bike.
I have a canyon grail CF SL8

Bob the Mech

Well the next cassette size up in Shimano terms is in the MTB 11 speed territory (11-40). Type this in to Google: 11 speed mtb cassette on road bike...make yourself a coffee (or any preferred beverage) and select the videos option. A picture (well moving pictures) paints a thousand words. Plenty of folks out there have tried this and they will show you how Another option is the SRAM PG-1130 11-36T 11 speed cassette. A road link may be required and ofcourse reprogram the Di2 with a new non-stardard cassette profile. Or change the crankset to the Praxis Works Alba X M30 Direct Mount Compact Crankset (48/32)...https://road.cc/content/review/21912...-4832-chainset ....and keep the 11-34, the more expensive option as this isn't the standard 24mm hollowtech spindle so adaptors are required or a new bottom bracket. So a couple of options.

mstateglfr 

11-36 is a really common cassette size and ive seen it used on various di2 setups. Admittedly I dont know the details of what di2 components were each time.
Shimano's stated limitations are conservative and GS RDs that show as 32 or 34t max are routinely used for 36t cassettes.

Jazzguitar

The SRAM 11-36 cassette works perfectly for me. I've tried my Shimano 11-40 and it works but the B screw was maxed out and I wasn't comfortable running it like that. 11-40 should work fine if you use a Goatlink or something similar.

Bob the Mech

It's not beyond the realms of posibility that an 11-36 will work right out of the box...with no additonal chain length...on the medium (GS) rear mech. I tend to sit on the side of caution when chain sizing at least when running non-spec cassettes. Gives it a fighting chance of success as apposed to catastrophic failure

Darth Lefty 

This is a touring bike trick, and touring bikes usually have mountain bike derailleurs. MTB derailleurs have some extra capacity for a trick like this because their tooth and wrap ratings take into account the possibility of rear suspension changing the geometry. GRX probably doesn't have this fudge factor built in, or at least I wouldn't count on it as much.

Bryan C. 

I run an 11-42 xt cassette with a GRX 815 RD with 48/31 chainrings without the need for a roadlink. Zero shifting issues in any gears. Proper chain length and b-screw adjustments took some trial and error but overall I wouldn't hesitate to do it again. I originally had an 817 RD installed but the 815 handles the bigger cassette in a 2x configuration much better. The only thing I would do differently is to get the lighter XTR 11-40 cassette, the xt cassette being a few grams heavier isn't a deal breaker for me though.

Jazzguitar

I wish mine was the same, it sounds like it all will come down to the derailleur hanger geometry.

Cpn_Dunsel

On what bike?


Same set up will not work as well on every bike, even when the group sets are identical. Wheel base, chain line, geometry and even frame size are major factors that matter and what works on one specific frame may not work on one that is a different size.

Bryan C. 

2019 Specialized Diverge carbon frame

I wasn't the first one to try a big MTB cassette on a road RD. I agree that some frames may not be compatible with this set up. But based on the posts that I read when researching this subject the people who actually tried it had pretty good luck.

Lots of naysayers and people who are slaves to max capacity specs will say it won't work at all, and won't work long term. My suggestion is to try it on your bike and see what happens. It took some time to find the right balance between chain length and b-screw so this mod probably isn't for those with limited experience setting up derailleurs from scratch. But for those of us who like to tinker with their bikes it's good fun to try something out of the ordinary.

Cpn_Dunsel

I am definitely not a naysayer nor a hater and agree 100% especially with the part about those of us that like to tinker. Success in these endeavors does tend to require a degree of perspicacity to which most may not subscribe.


(edited to add)
The right mix of specific bike and specific person is the crux here, not the gear, I think. People told me not to put drop bars on an Epic Hardtail and ( ! FLEX AHEAD !) I just had my personal best finish in the Unbound 200 on it. Yet, I would not be telling others to do likewise.

Jazzguitar

I'm certainly not a naysayer, and I built my bike, and many others, up from scratch so I think I know a thing or two about tinkering. The fact, for me, is that on my particular frame the only way to make it work is with a goat link, no amount of fiddling with chain length or b-tension will change the fact that the upper pulley wheel will not clear 42T with a 815 derailleur in stock form. Even with a 40t the b-tension screw is barely making contact with the bracket axel and I wouldn't feel comfortable riding with it like that on a trail. I've also used a M8050 rear mech which easily cleared 46T, but that was running 1x and doesn't help the OP.

OP, try it out, it might just work out of the gate, if not there are options such as a Goatlink, etc...