Chilepines

I have the same 785 shifters on my tandem, but I routed the wires inside the bar and used the bar-end A-junction. Those shifters only have one port, but that shouldn't really matter. With the external junction A you need 6 wires - one from each shifter to the junction, one from A to B, then one to each derailleur and to the battery.
I posted about my di2 hack on the forum below. Not something everyone would want to do but it actually worked out well.
https://www.bikeforums.net/bicycle-m...-di2-hack.html
I think you will need the B-junction in the seat tube, and it will still need enough length below it to be able to remove the seatpost without pulling all the wires apart.

Polaris OBark

I did read that hack, which is in fact what gave me the idea, but I think the B-junction box is the way to go.

BTW my frame builder tried very hard to make it work with the 30mm White Industries spindle, so I have to take his word for it that, at least for that bike, it couldn't be done. In retrospect, with your "hack" I think it could have worked.

Polaris OBark

Almost there...



I fixed the handlebar rotation after the photo -- it is horizontal now with respect to hood ramps.

Polaris OBark

I just realized I never posted a final post-build photo (not that these things ever really get finished -- eg: I still need to make the steerer tube final cut, and I still need to get that ******g wire inside the chain [corrected] stay):

Chilepines

What wire are you putting in a seat stay?

Polaris OBark

Sorry, chain stay.

The one that goes to the rear derailleur. It is currently hidden behind the lizard skins chain stay protector. I can get it in, but I can't get it out through the hole next to the rear derailleur. I tried for hours, and used those little cable caps you sent (thank you, again!) in conjunction with a (conventional) shifter cable to pull it through. The main problem is I need to get it exactly perpendicular to the 5.5 mm hole, and I've tried everything I can think of (your wire trick, shop vac and dental floss, wire pick, magnet, etc), and failed. It's perfectly fine now, behind the chain stay protector, but it really bothers me that it is the one thing that I wasn't able to get to work as designed. (I noticed they re-designed this for the current generation of the frame.)

Badger6

Could you install the wire by inserting it from the RD and pull it toward the BB? I had to do that on an older frame.

Polaris OBark

Thanks. I tried.

The problem is symmetric.

I need to email the maker and ask if there is a trick. Also, I have heard there is a second type of Di2 wire that is narrower. I don't know if they are cross-compatible, but that might be a way.

Functionally, this isn't really a problem because I have the wire running under the chainstay protector. It just kind of bugs me.

Koyote

I have a custom built frame designed for Di2 (11sp) with all of the ports for the wires...And the port for the RD wire is a problem: the little Shimano grommet, which sits in the port and serves as sort of a guide and holder for the wire, will always pop out -- then the wire is just dangling slightly. I've tried new grommets, I've tried really jamming it in there, etc. Finally gave up and wrapped a bit of electrical tape around it. "It just kind of bugs me," or at least it did at first. But after a while, ehh, whatever. Hopefully your issue will end up being the same.

Badger6

The EW-AD305 will allow using the newer generation wiring with older gen systems…it appears that you’d need three total wires to make it work: two older gen (SD300 wires on the ends) with two of these adapters to run the smaller SD50 current gen wiring.

Polaris OBark

Yeah, that isn't going to help, then, unless the EW-SD300 could plug in directly into the rear derailleur (GRX).