It would be a tight fit getting the JC41 through the bar end area. You'd probably have to check it with the parts in hand.

So a few potential options to mention:

-You don't indicate which Shimano component series you're planning to use. The latest single button shifter offerings have female e-tube ports while the previous gen two-button shifters have e-tube pigtails with male ends. So you're fine including y-cables for the single button models while you'd need the JC200 to couple every two button shifter connection.

-I would suggest taking a look at the Shimano XTR Di2 SM-BTC1 external mount battery holder for internal batteries. Included is a 6-Port Junction-B that is cylinder shaped. I would guess it would fit in the Vuka Bull. You'd need enough wire slack to get everything plugged in externally out one end of the bar before pulling it back into the bar. I've used this Junction-B to wire up a Vuka Stealth with all four shifters connected inside the base bar.

-You didn't mention what you were using for extensions/hardware. Assuming you aren't planning on using Saltstick dispensers or similar in the back of your extensions I would also suggest considering the Shimano Di2 EW-RS910 Junction A mounted in the back of one extension. You would just run the extension e-tube wire to junction and one out the extension to basebar. This depends somewhat on extension type and i.d. if you need wire to come out back, but I've done this with both carbon (tight) and aluminum (easy) extensions. This setup eliminates routing the junction A to under your seat and finding a way to mount it, along with dealing with in when taking out seatpost for travel, etc.

-If your extension length is adequate and you use the 6-Port Junction B you may also have the opportunity to hide your battery in one extension with the e-tube wire fed to the last unused Junction B port (4x shifter, 1 to downtube, 1 free)

That’s a really cool bit of kit, the BTC1!

It’s 6870 series stuff with dual buttons, using Vuka B1 aluminium Evo extensions on the carbon Vukabull base bar. I’m running the RS910 on my R8070 road bike using a steerer tube battery holder coupled with a Pro Vibe stem and aero bar for a really neat installation, and it’s scratched the itch to sort out the aesthetics of my tri bike. I have been looking at the RS910 as I understand it will fit in the alu extensions, but wasn’t convinced I could get a wire out neatly.

I’m going to strip everything off and see how much cable I’ve got on the shifters. I never knew this piece existed - thank you for that SummitAK!

Ok. So the new e-tube y-cables aren't a great option as every link would require a JC200. These shifters should have very long pigtails with male e-tube ends. The installs I'm talking about above used 9070 brifters and the SW-R671 extension shifters. I'm guessing the DA and Ultegra brifters offer the same length pigtails and you probably have the same extension shifters which also have long pigtails too. I've got extras on the bench and will try to remember to measure them along with the diameter of the BTC1 junction B.

For aluminum Zipp extensions there is room to both insert the RS910 and run a wire past the junction too. I grind a little half moon at the bottom of the extension and wrap the e-tube wire around the end of the extension (from inside to outside, sort of back on itself) and heat shrink it to the outside of extension then run it down back of extension riser to base bar. The RS910 can then sit flush against the end of the extension. You can do the same on the other side and then use a bar end plug. The RS910 will also fit into most carbon extensions with some of the sharp edges ground off the junction and cleaning up the extension i.d. with a burr or sanding drum.

Ok. Some bad news. The BTC1 Junction B is about 22mm O.D. From memory I thought it was closer to the diameter of the Shimano internal battery, but the BTC case is quite a bit larger diameter and the 6-Port Junction is sized to insert inside one end.

The good news is I measured the JC41 Junction B at 30mm x 14mm x 7 mm. It easily slides into a Zipp EVO aluminum extension which has an I.D. of about 20mm. Bummer is the need for multiple 4 port Junction B's and an extra wire. I checked some Bonty carbon extensions and the I.D. was about 18mm. I don't have Zipp carbon extensions or cutoffs on the bench, but the I.D. on these were a bit less than the Bonty.

ST-9071 Shifter/Brake e-tube pigtail measures 480mm from where it leaves shifter to end of male e-tube plug

ST-R761 Extension Shifter e-tube pigtail measures 620mm from where it leaves shifter to end of male e-tube plug

Hope this helps!

You, Sir, are a legend.

Thanks for taking the time to measure all of that - that’s really appreciated. I’ll put together a plan of attack!

Thanks again, SummitAK. This has been a huge help!

JC41 fitted into each bull horn of base base bar with the shift/brake and extension shifter plugged into each side’s JC41. Was trying to join each JC41 with a Y-cable (JC130), which would then go to the frame, but the Zipp Vuka Bull carbon base bar isn’t hollow all the way through. No matter what I try I can’t get the Di2 cable through the stem clamp area. I’m using the Park Tools IR-1.2 internal cabling tool kit.

Thinking of ditching the base bar now for something that will be hollow. This has been one of those itches I really regret scratching... I recently swapped the Vuka Clip for he B1. Should have just gone all in and got an integrated base bar/extensions setup!

@natezipp

Able to confirm the Vuka Bull carbon base bar isn’t hollow through the central clamp area?