GRX Di2 questions

[Nick38]

So I have a Specialized Diverge Carbon, it came with SRAM Apex, I hate it with a passion but I'm putting up with it for the time being.
I'm looking to fit GRX Di2 x1, either 817 or 815 RD I'm assuming the difference is the cage length?

My main query is the shifters, those of you that have them what's the shift quality like compared to Ultegra and Dura Ace? Also how do the shifters feel compared to road shifters? From photos etc they look a strange shape, I can't find any locally to try.

I've got a couple of Tarmac SL7s and an Allez Sprint all with 11 speed Di2, just love it, can't have this SRAM carry on

[TerryDi2C]

The RD-RX815 is Shimano's 2x GRX Di2 RD. Its max is 34T and its total capacity is 38T.

The RX817 on the other hand has a max of 42T, but it is 1x only. Its total capacity (31T) isn't sufficient to run it 2x really.
(that said... it does shift in 2x, it isn't blocked in firmware)

Required RD total capacity >= (Large front ring - small front ring) + (Largest cassette sprocket - smallest cassette sprocket)

[IrrelevantD]

As I recall, the 815 is based on Ultegra and is very similar to the UltegraRX and has the same clutch, it just has a little more capacity. I have the UltegraRX and I love it. The system I have is basically just an upgrade from the original 6870 Ultegra Di2 system I've had since 2016. The shift performance is about the same, just as quick, just as percise, it just has more capacity for a bigger cassette.

817 is based on the XT from the MTB side. It has the same clutch as the XT, a little more robust and easier to adjust, just a little less capacity. The 817 will work in a 2x system unless they've removed that compatibility in a firmware update, but you don't have as much total capacity, so this may be an issue unless you're running close ring sizes such as 36/46 CX rings. If you are going 1x, you could even use an XT Di2 RD as it will work with road levers. You just can't mix a MTB RD with a Road FD.

As for the shifters, you don't have to use GRX. Any Di2 shifter will work. If you would like you can use R8050 Ultegra, or the older non-series R785. I'm personally using the R785 as they are a little narrower and feel more like the older 6870 cable pull levers.

[warthog101]

The RD-RX815 is Shimano's 2x GRX Di2 RD. Its max is 34T and its total capacity is 38T.

The RX817 on the other hand has a max of 42T, but it is 1x only. Its total capacity (31T) isn't sufficient to run it 2x really.
(that said... it does shift in 2x, it isn't blocked in firmware)

Required RD total capacity >= (Large front ring - small front ring) + (Largest cassette sprocket - smallest cassette sprocket)

I have cable GRX 810.
It is also supposedly limited to 34t as the largest rear sprocket.
It will run larger though, I am using a Sram 11-36 cassette.
https://velonut.com/blog/shimano-grx-2x ... t-cassette
Is the di2 able to run larger than the specified capacity also?

[warthog101]

Yes it does

https://youtu.be/wNeYYM9_aeM

[Nick38]

Thanks for the replies so far guys, are the Di2 cables the same as other Di2 systems? I know the new 12 speed systems are different. Same charger etc?

[TerryDi2C]

All GRX Di2 cables/junctions are the same as 11-speed road

[Nick38]

All GRX Di2 cables/junctions are the same as 11-speed road

Magic, Cheers TerryDi2C

[bet1216]

I have both GRX di2 and Dura-Ace di2 (both 11 spd and disc). The GRX shifter shapes are very nice IMO. They are slightly different shaped and utilize a different aux button configuration (GRX on the side - DA on the top - I like the button on the side better). They have different rubber covers also. I actually prefer the GRX ergonimics (the shape seems to make them feel smaller). It is my understanding that the new 12spd DA copies some of the ergonomics of the GRX. Shift quality wise in the real world they are the same, button feel, speed, feedback, etc (I think the DA derailleur is supposed be faster but not something I notice). Both work flawlessly. I don't recall of the shifter buttons are different sizes or not but neither has impacted me. Just my $.02. All the 11spd Di2 are cross compatible. FYI, if you compare to mechanical disc brake to di2, the shifter sizes of both are significantly different, with the Di2 being considerably smaller.

[Nick38]

I have both GRX di2 and Dura-Ace di2 (both 11 spd and disc). The GRX shifter shapes are very nice IMO. They are slightly different shaped and utilize a different aux button configuration (GRX on the side - DA on the top - I like the button on the side better). They have different rubber covers also. I actually prefer the GRX ergonimics (the shape seems to make them feel smaller). It is my understanding that the new 12spd DA copies some of the ergonomics of the GRX. Shift quality wise in the real world they are the same, button feel, speed, feedback, etc (I think the DA derailleur is supposed be faster but not something I notice). Both work flawlessly. I don't recall of the shifter buttons are different sizes or not but neither has impacted me. Just my $.02. All the 11spd Di2 are cross compatible. FYI, if you compare to mechanical disc brake to di2, the shifter sizes of both are significantly different, with the Di2 being considerably smaller.

Thanks bet1216

[de lars cuevas]

Grx Brake feeling should be better too, thanks to Servowave.

[Nick38]

Thanks again guys. So if wiring for 1x am I right in thinking I just need to take one cable from shifter to rear mech via the charging port and junction box? No need for a cable from other shifter as that’s just used for braking?

[FIJIGabe]

The beauty of Di2 is you can change what the buttons do and you don't need to worry what is wired into what, because the system will figure it out. You can probably wire the shifter to a bar-end Junction A, then wire it directly to a junction B in the BB that's wired to the battery and RD.

[la85]

I've got Shimano GRX Di2 RX815 RD and Shimano Ultegra Di2 R8050 FD.

What's the largest cassette and front chain rings I can use?

[urostasa]

Front Praxis 32/48 teeth, rear 11 regular cassette 11/42. GRX DI2 815 gearbox.