You’re talking about the piece that mounts in the place of a standard cable guide, right?

I think if you get some 3M automotive trim mounting tape, that would hold it. Alternatively you could just drill a hole and I set a rivnut, if it’s an aluminum frame.

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MS: Exercise Science
Your speed matters a lot, sometimes you need to be very fast, where sometimes you need to breakdown your speed.

It's a carbon frame. I was thinking, would this work...get a single 5-port junction box, 2 ports for the STI, 1 goes to the battery, 1 to the FD and 1 to the RD? I know it's a lot of long fugly wires, but would that work? Or is a b-junction necessary?

A single 5-port would work if all you're plugging in is 5 components. This would actually make the install I'm doing now tidier.

I thought I could slip the narrow 4 port junction into the bottom bracket once the cable plate was removed, but that unfortunately isn't the case. So my intent was 3-port A jn (b/c I already had it on hand) plus the 4-port (that I also already had on hand). 2-days ago I ordered a new 4-port to mount exteriorly under the BB, but another solution would have been to use a new 5-port A jn.

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To breathe, to feel, to know I'm alive.

Here is what I would do to minimize the mass of wires...

Use the bar-end Junction A: EW-RS910
Use the Junction B splitter cable to connect both brakes to the Junction A: EW-JC130-SM

At this point, you have one cable coming from the cockpit to the frame. I recommend the Wireless Unit EW-WU111 as both a junction between the cockpit and frame and for its cool connectivity features.

Then, you have a single wire running from the Junction B.

I would figure out a way to glue a 4-port Junction B (SM-JC41) to the battery mount. Then, you have the one wire coming down from the cockpit, the one short wire looping back to the battery, and two wires routing to the FD and RD. That should look very clean and be relatively easy to do all external.

Are you saying your frame has no holes whatsoever for the under BB cable guide? Does the Front Derailleur cable guide run through the chainstay or goes around externally to the seat tube?

Curious as I converted my externally routed bike to Di2 but it had two (thru) holes for the under BB cable guide and for the FD cable (that routed behind the BB/seat tube). Frame is a Cannondale Supersix Evo actually. Not my bike but here is a picture of the cable guide:



I used the external JC41 and bolted it where the cable guide goes. But since it is a thru-hole I instead bought a longer bolt and secured it with a matching nut along with a rubber washer to protect the carbon from contact with the nut edges. See pics on my actual Di2 Setup:





I ran 1 wire from the Junction A along the downtube and to the Junction B (with call "JB" for short). Then ran 1 wire from JB to RD along the chainstay and used the Di2 tape in black (since my frame is black it hides it pretty well). Ran 2 wires from JB through the opening where the FD cable goes through opening behind the BB (if you look at above pic on the left side you can see the Di2 wires where I ran it through that opening) which connected the FD and the external battery (DN-100 mount). Junction A was just a 3 port and had both shifters hooked up to it directly.

Here is a pic of the setup after I initially finished (I have since installed one of those stem spacer Junction A mounts but haven't taken a pic of that setup yet):



Technically a junction B is not necessary if you have enough ports available on the Junction A to get everything connected. You can do that for now and get it to work but make sure you measure out how many long cables you need to run everything. Depending on the frame size you might not find a single cable long enough to make it from the the Junction A mounted on/near the stem to the RD.

Correct, no holes under BB at all. It is a 90s Kestrel 200sci. There were very thin cable guides built in structurally.

I may either look into this:

Use the bar-end Junction A: EW-RS910
Use the Junction B splitter cable to connect both brakes to the Junction A: EW-JC130-SM

Or just live with a tape job. I "think" the 1600mm wire should just make it from A to the RD, using the 5-port junction box method. I don't want to spend too much on this effort if possible.

Here's the other angle: why not do a SRAM eTap installation. Functionally, I like it a lot better than Di2 on a road bike. And installation is crazy easy.

I did a full Di2 upgrade to my TT bike and eTap upgrade to my road bike on the same weekend. One of the reasons I chose eTap for the road bike was that the frame was not setup for Di2. The other was that I preferred the road bike shifting UI of eTap versus Di2. Now that I have had it for a while, I will probably never consider anything other than eTap for a rode bike-- it is that much better. (Conversely, I will probably always prefer Di2 on a TT bike.)

I'd probably myself then just double sided tape (and also zip tie for extra security) a 4 port Junction B Internal (JC41) on the chainstay instead and hide it behind the chainrings. Run 1 long cable from the Junction A on the downtube to the Juction B. Then run the wires to the FD, RD and Battery. Single wires are much cheaper than those Y cables when you add them up on how many connections. Still would stick with a 3 port Junction A though but instead go with this one https://bike.shimano.com/...50-di2/SC-MT800.html

I actually use this one on my Tri Bike to see battery level and also gear (plus this one has a built in bluetooth unit too and saves money over getting an inline one). Use a stem face plate convertor to mount it actually. This exact convertor is what I bought: https://www.ebay.com/...?hash=item41f5018199

But since you are putting on a road bike you could as easily mount it how it was originally intended though on the bars.

This is a great idea, thank you. I didn't even think about using the 4-port internal. Hopefully I can work out the wire length so I won't have too much excess zip-tied to the chainstay, etc.

Good luck on your build. While I do agree with exxxviii that eTap is a better option for most bikes that are not setup for electronic with external routing I assumed you already had sunk costs into Di2 already and were looking for solutions (not alternatives). I personally didn't want to go eTap simply because I wasn't willing to buy it new and didn't want to risk stuff breaking buying secondhand since it was still (at that time) having some teething issues with the new product). Di2 first gen 11 speed was dropping in price on the used market with the 2nd gen coming out it was economical enough for me to go used and if parts break replacements can be had for relatively cheap.