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I'm specifically talking about rear shifters....

Can I mix and match any of Shimano's DI2 shifters with rear derailleurs and internal geared hubs?

Shimano makes several DI2 shifters

  • integrated shifters for drop bars (e.g. ST-R9170, and ST-R785)
  • thumb shifters for flat bars (e.g. SW-M8050 and SW-M9050)
  • standalone buttons (e.g. SW-R600 and SW-S705)

They also make several DI2 rear derailleurs and internal geared hubs

  • rear derailleurs (e.g. RD-R9150 and RD-M9050-SGS)
  • internal geared hubs (e.g. SG-S7051-8 and SG-S7051-11)

Are all of these combinations completely compatible?

For example, can I run ST-R9170 (Dura Ace 11spd "road" integrated shifter) with SG-S7051-8 (Alfine 8sp internal geared hub)?

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Looks like Shimano provides a handy guide: https://bike.shimano.com/en-EU/e-tube/project/compatibillity.html

source

Posted here since Shimano is already deleted it:

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    Can you summarise the information here? if the link doesn't work in future, this won't be very informative on its own
    – Swifty
    Nov 16 '20 at 8:07
  • Yes the link is not that easy to understand. My understanding is that 10 speed road Di2 is not compatible with 11 speed road Di2, but to what extent this manifests itself I don't know. I didn't really get much sense from that chart of this.
    – thelawnet
    Nov 16 '20 at 9:15
  • FFS. Shimano already removed the pdf. Luckily I still had a local copy. Just added links to pages on Imgur.
    – tir38
    Nov 29 '20 at 3:46
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You can use the 785 shifters with Alfine 11 di2 for definite. You should be fine with the other combinations. I was told by the shimano tech rep in 2015 that any di2 shifters work with any rear end shifting. the shifting switches are pretty dumb and just say up or down to the rest of the system.

FYI Shifting is disabled in a derailleur system where the rear derailleur is 11sp and the front is 10sp (though this combination did work prior to a certain firmware upgrade).

XT rear mech is commoniy used with road systems to get a 1x11 on drop bars too.

As noted by Terry in the comments, the sprint switches SW-R610 are an extra part that need to plug into existing road shifters.

The only thing that will not work is SW-R610 - those sprint shifters ONLY plug into the dedicated sprint shifter port on ST-6870 / ST-9070 / ST-R8050 / ST-R9150. The main "rules" are: do not mix 11-speed and 10-speed derailleurs and do not mix road/grx and mtb derailleurs (all derailleurs must be from the same category road/gravel, urban or MTB but shifter controls can be from either). Also, 7970 is not compatible with any current derailleurs as it doesn't have the e-tube bus connector.

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    This^^ The only thing that will not work is SW-R610 - those sprint shifters ONLY plug into the dedicated sprint shifter port on ST-6870 / ST-9070 / ST-R8050 / ST-R9150. The main "rules" are: do not mix 11-speed and 10-speed derailleurs and do not mix road/grx and mtb derailleurs (all derailleurs must be from the same category). Also, 7970 is not compatible with anything (different connector) :) Nov 16 '20 at 14:12

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