RobbieTunes

I'm a C&V guy, with only a couple carbons, but my girlfriend rides modern Di2 Ultegra, so I thought I'd ask here.
She is running 50/34 front and long-cage RD with an 11-34 cassette. She'd like to go 36T or more at the cassette.

I know, I know...chain wrap is an issue, and there are other ways to skin this cat, but this question is purely about the RD.
I'm building her an '88 Merckx that should fit the bill with balanced use of crankset/cassette, but an RD/cassette swap on the Di2 is tempting, if it works.

She is looking at a Di2 Shimano GRX RD-RX815 model, but is open to the RD-RX817 model, if they can give her a wider range.
The literature indicates the RX815 can handle a 34T max/low with a road cassette, but a 40T max/low with an MTB cassette.
Why would there be a difference between handling a road cassette and an MTB cassette? Does the literature assume 1x with the MTB?

Is the RX815 just a clutched and Shadow version with the same capabilities (34T) as her long-cage RD?... or..
Can she go with an RX815 or RX817 and possibly pop in a wider rear cassette?

I know about chain wrap, I know about Wolftooth's Roadlink...so I'm only asking about the RD
(Her backup Ultegra 6700 is running a 50/34 and 11-34 cassette, on a short-cage RD with a Roadlink. It works)

Thanks.

Cyclist0108

I'm running 2014 11-speed Di2 Ultegra with a 46/30T GRX crankset and on one wheelset a Shimano 11-34T and on the other a SRAM 11-36T cassette. Works fine. All I had to do was adjust the B-screw. I've been doing this for years with none of the issues you are worried about, and no need for the Wolftooth. The more modern Di2 rear derailleur is actually spec-ed for 11-34T. Mine is spec-ed for 32T max. Shimano is very conservative.

For the mtn cassette, I think Shimano expects you to be running a 1X.

Before the GRX crankset was released, I was using first an Ultegra 50/34T crankset and then a White Industries 46/30T crankset. (The GRX one works better). I did have to adjust the limit screws on the front derailleur to accommodate the +3mm chainline.

So for an extra $150 for the GRX crankset, I have stupidly low gearing (which I use).

I think for GRX Di2, if you change the rear derailleur, you would also be forced to change the front to GRX Di2. But I found there was no reason to do it. The Ultegra Di2 derailleurs work fine with my setup.

Iride01 

Where are you finding the spec for a max low of 40T if MTB cassette? If it's not Shimano docs but instead the configuration of a bike being sold, then the manufacturer may have decided that combo works for that bike.

Not sure why this isn't more appropriate for bicycle mechanics sub-forum.

RobbieTunes

Thanks.

RobbieTunes

Saw this on the internet; it must be true. (The RD-RX817 references 42T for MTB)

"For rides on smooth surfaces, the derailleur features and ON/OFF switch for the chain tensioner. The RD-RX815 rear derailleur is compatible with mtb cassettes with a 42T low sprocket max. FEATURE. Adjustable chain stabilizer prevents chain slapping and drops."
Reference: bike.shimano.com/en-us/product/component/grx-di2/rd-rx815.html "

Tried that link: not found.

I wanted to hear from users. I know great mechanics, former pro's, and nothing is broken or needs repair.
Mechanics tend to quote manufacturer documents. I have those and don't need to re-read them here.

That being said, my surety is my surety, you're "not sure why" is totally up to you.
Not sure why I have to explain it to you. Only you can answer that, not that I'm asking.

Iride01 

Several places I looked on Shimano USA's site didn't mention anything about a difference for MTB cassettes. But I didn't look hard.

If you are building the bike and definitely going to use that Di2 DR, then just try and see. I think in a road cassette from Shimano 34T is the max you'll find.
You mentioned that 50/34 and 11-34 is currently working for her on another bike, so it'll be at least that. You might consider a crank with smaller chainwheels though if you need lower gearing.

Also be careful in mixing GRX and standard road components. GRX is made for a wider chain line for bikes with a wider rear spacing. Rear DR should be fine, I'd think...... but thinking sometimes creates troubles.

RobbieTunes

It does, but ignorance s bliss....I’ve been happy a few times by accident.

HTupolev

I haven't seen that. But, if the RX815 behaves similar to its mechanical version (the RX810), it should handle a wide 2x crank and an 11-40 cassette pretty well. I actually know a guy who's running an RX810 with a 48-32 crankset and an 11-50 (!!!) cassette, with surprisingly reasonable results. The b-screws on these derailleurs provide an absolutely enormous adjustment range, and although this does increase the chain gap in the small cogs, it seems like you need to push things quite far beyond the specs before stuff starts to get tangibly problematic.

If it "assumes 1x", you'd think it would point to the RX817. The RX817 isn't a wider-range derailleur than the RX815, it just lists a larger max cog because it's intended for 1x applications. The RX815 actually has a longer cage.

Bryan C. 

I'm using the RX815 rd with an 11-42 cassette and the 48/31 chainset. Works great, shifts perfectly. Plenty of b-screw left if needed. Some people will say it isn't the "correct" way but it does work.

I was running an 817 in a 2x configuration. It has an SS length cage that was workable but obviously too short. It also locked out the smallest 4 cogs while using the small chainring. Wouldn't allow the sync shift function to operate either. While it was done as an experiment, thinking the 817 would handle the larger cassette better, I wouldn't recommend it for a long term solution.

Elvo

I think 11-40 should work fine and maybe 11-42, maybe 11-46 with the road link. But you will have chain wrap issues and probably won't be able to use small/small and probably shouldn't be in big/big either.

Personally I have the RX800 with 11-36T rear and it works great. Why not switch to a 46/30 crankset?

RobbieTunes

It’s not for me, and while I can build a bike, I cannot change long-held beliefs in others, so I have to stick to “the plan”

Cyclist0108

Switch to the 46/30T crankset and don't tell her. Or the 48/31T. Unless she is really obsessive, she won't count the teeth.

Psimet2001 

The difference in part numbers from Shimano in the grx line denote 1x and 2x setups for the RD. The 1x RD (RD-RX817) is set up to have a larger chain wrap so that it has that large rear cog capacity. The RX815 is a 2x setup. Anything in GRX rd that ends in a 5 or 0 is 2x and 7 (or 2)? is 1x. The higher numbers are Di2.

a 1x RD is set up with pulleys that are designed to help retain the chain in a 1x scenario. Shifting on a 2X setup would kinda suck. They also have a clutch but everything (I believe) in the grx lineup has a clutch. I am running 2x in grx right now with a clutch.

When introducing a 2x setup to that 1x RD you will run into a reduced cassette capacity on the big cog end. Not as much because of the geometry of the RD and it's clearance to the large cog but because the chain wrap needed. The chain length just won't work out to allow use in all gear appropriately.

Keep in mind though that the crank chainring sizes in GRX are 48/31. So...if you paired the 1x RD with say a 46/36 you'd most likely gain some on the large cog side but I'd say you'd be pushing it to run the 40t.

I mean how low do you really need to go. I ride the 2x GRX at stock 49/31 x 11-34 and the lowest gear is completely ridiculously low. Like too low. Go di2 1x GRX and use their crank.

Canker

No such thing as too low. Just means you haven't hit a steep enough hill yet .
<----- Another 46/30 11-32 rider here. It is my gravel/road bike that spends 98.34% of the time as a road bike. At the time GRX wasn't around so if I were doing it today I'd go 48/31 crank which seems to be the perfect compromise. 11-32 to keep my cassette tightish and the 31 chainring for those crazy steep gravel hills.

RobbieTunes

I'll try this in stages, I think. Budget-wise, for sure.
The Madone is going to 50/32 on the front (FSA ring), and 11-36 rear. That's 2T front, 2T rear. Should be enough.
The backup Amira is staying 50/34 and will go 11-36 rear. If the FSA 32T ring works on the Madone, I'll get one for the Amira, too.

Since the '88 Mercks is Campagnolo, and I'm not rich, the 11-32 rear is the max.
Also, the um, "need" for polished components don't help. I have a 50/34 polished Centaur crankset.....
BUT, given the advice here and elsewhere, I've been shopping for 46/30 cranksets.
I like the Praxis (out of stock) if I decide on carbon. Pretty much can't go Shimano....
I like the Sugino OX ($400 to start), with it's 110BCD and 74BCD on the same plane of the spider.
I like the White Industries new variable BCD, but actually buying one seems to be a search in itself.
Time will tell.

We'll probably start with the full polished 11-sp Centaur, and go from there as cranksets become available.

mrbubbles

Running 50-34 with 11-40, there's definitely enough room for 11-42. There's no mod on my bike, just plain all Ultegra 8070 series with medium cage. You can definitely go 46-30 or 50-34 with 11-40 or 11-42, just adjust the b-screw all the way in.