Installing the handlebar junction and battery in aero extensions

Updated April 11, 2023 by BetterShifting Terry

Ever since I built that Cinelli a few years ago, my 6870-series Di2 has been on Zwift duty - installed on my dedicated indoor bike. It worked great, but I didn't use the front derailleur all that much. Mainly because I just did a lot of ERG mode workouts.

At the same time, my commuter bike was restricted to 1x, and there were times when I wanted more gears.

When I spotted Profile Design's Aeroport adapter I could no longer resist - it was time to move my old Di2 gear to the commuter bike! Not only would I gain a front derailleur and therefore more gears, it would also let me experiment with the 7972 series satellite shifters my local bike shop gave me.

SW-7972 box

Aeroport? What's that?!

The Aeroport is a Profile Design product that will let you secure the EW-RS910 junction as well as the BT-DN110 Internal Battery in your aero bars.

Why would you not just put the EW-RS910 in one of the drops? Well, time-trial bikes generally don't have drops. Sure, you could use the older SM-EW90-B below-the-stem junction instead, but that is hardly aerodynamic, and it looks a bit messy.

And yeah… my commuter isn't really a time-trial bike. It does have a regular road handlebar with drops. I thought it'd be a fun experiment to put the battery in the aero bars, and that's what I did!

SW-7972 and Ultegra 6870 are not compatible

If you've ever played with older Di2 components, you may know that the SW-7972 10-speed DURA-ACE shifters and 11-speed Ultegra 6870 are not compatible.

The 7970 series has a different plug and will only work with other 7970 series components. It does not work with anything other than 7970 Di2.

EW-SD50 wire on the left, 7970 Di2 wire on the right

However, these SW-7972 satellite shifters are very much like SW-R610 sprint shifter and the 12-speed SW-RS801 satellite shifters.

They are dumb shifters, without any firmware. Plug them into the dedicated satellite shifter port, tell E-Tube Project/Cyclist that you're using satellite shifters and that's it, they'll work.

Any ‘dumb' button will work in the satellite shifter ports, as long as the plug fits.

Obviously, the 7972 plug doesn't fit my ST-6870 shift lever satellite shifter port, but there is a way around that. You see, regular SD50 plugs do fit into that port…

To make the SW-7972 shifter work I simply took one of my spare SD50 wires, cut it in half, and then spliced the SW-7972 wire into that.

When done correctly, you end up with two SW-7972 shifters with SD50 connectors.

Let's walk through the process step by step. This is what I started with, the SW-7972 sprint shifter.

It was given to me by a local bike shop, It was of no use to them anymore, and they figured I could use it somehow.

So yeah… I cut it up.

Even though the 7970 series uses a 5-wire system, each of the sprint shifter wires has just two tiny wires inside.

See that "KG" sticker on the shifter? That means this component was made July 2012.

Next, I took an EW-SD50 wire, and cut that in half.

Inside were - no surprise here - two small wires.

The SD50 wire is shown on the left, the SW-7972 shifter and wire on the right.

Stripping the wires was the hardest part of the process. They may look pretty beefy in the photo above, but keep in mind that the diameter of an SD50 wire is just 2mm. Each of the small wires inside is less than 1mm in diameter.

It took me a couple of tries to get it right, but eventually, I stripped the wires.

All that was left now was to splice the wires - connect the two together. There are several methods of doing so. I went with the soldering method, but the others would probably work well enough too.

I repeated the process for the other sprint shifter and other half of that SD50 wire, and made sure to test the end result before I actually installed it on my bike.

Installing the battery and junction

With the sprint shifters working just fine, it was time to unpack the Aeroport and see how that would fit. I decided to put the EW-RS910 junction in the right aero extension, and the BT-DN110 Internal Battery in the left.

Why not the other way around? Ultimately this is up to the user, but the EW-RS910 junction is usually installed in the right handlebar drop. It just feels natural to me to have it on that side, so that's where I put it.

The Aeroport comes in a little box, and inside you'll find two pieces of plastic/rubber - the battery/junction holders.

You essentially slide these over the battery and the junction box. That looks something like this:

Note that even though I've used the older SM-BTR2 battery in the image above, that is not the best battery for this bike. With only one button at the end of each aero extension, that means I won't be able to shift the front derailleur.

This BTR2 battery does not support synchronized shift, but the newer BT-DN110 does.

I may still swap out the battery at some point, but for now, just being able to shift the rear will suffice. It is my commuter, after all. I'm not going to ride time trials using this bike.

Wiring up the battery, junction, and shifters

With the Aeroport figured out, I decided it was time to install everything on the bike and connect the shifters, battery, and EW-RS910 Junction.

This should be a piece of cake. After all, the satellite shifters plug into the regular shift levers, and you can then choose from a number of methods to connect everything to the EW-RS910 Junction.

However, since I'm installing both the battery and and the main junction in the aero bars, things got a little complicated… and I decided to draw some wiring schematics first.

Each ST-6870 shifter has two SD50 type ports, and one satellite shifter port. The EW-RS910 junction has two ports as well, and the battery has just one port.

You can definitely get this to work, but it takes careful planning. For example, you could

  • Connect the left shifter to the battery
  • Connect the left shifter to the EW-RS910 Junction
  • Connect the left shifter satellite shifter to its dedicated port
  • Connect the right shifter to the EW-RS910 Junction
  • Connect the right shifter satellite shifter to its dedicated port
  • Connect the right shifter to the rest of the system using a long EW-SD50 wire

Or any other combination of the above. As long as all components are connected, they will work.

In my specific case though, I had only a limited set of EW-SD50 wires to work with - the ones in my big "Di2 stuff" box. They didn't match my situation perfectly, so came up with an alternative wiring setup.

Besides, I didn't really like having three wires come out of each shifter, and I wanted to have a spare E-Tube port… just in case.

So I added a second SM-JC41 internal junction to the system.

That looks like this:

This seems alright, but I'm not done yet.

I haven't connected the satellite sprint shifters yet, so let's do that now. I placed the satellite shifters at the end of the aero bars, letting me shift the rear derailleur at all times.

This means I have to run a Di2 wire from each aero extension to the closest shift lever, adding yet another wire to the handlebar:

This looks OK, right? Tell me it's not just me going crazy here.

Most of the wires should be completely hidden from view. They'll be in the aero bars, or underneath the handlebar tape.

With the wiring all thought out, let's actually install the components. A diagram is nice, but it won't make the derailleurs shift.

Installing the components on the bike

Before installing the battery and all the wires, I checked whether or not I would actually be able to route the Di2 wires the way I intended. My main worry was the battery - it's 15cm in length, so would I actually be able to access the aero bar's wiring port?

Well…. It turns out I can!

Only just, but yep - looks like we're all good. Let's go!

With a wiring setup this extensive, where do you start? I decided to start where everything comes together - that SM-JC41 junction.

I used the PRO Internal Routing Tool to run all wires through the aero bars, and connected these to the JC41 junction. This meant connecting the battery, as well as running a Di2 wire to the left shift lever.

Here is an image of the battery right before I connected it and inserted it into the aero bar.

On the right side of the stem, you can see the SM-JC41 junction sticking out of the aero bar, with the battery wire and left shifter wire already plugged in.

There was a bit of room for the JC41 junction to bounce around, and that's not surprising. It's only 30.5mm long, 14.5mm wide, and 8mm high. I wrapped it in a bit of tape to stop it from moving around. No one likes noisy bikes.

Next, I connected the EW-RS910 Junction A to the B-junction and the right shifter, and installed the Profile Aeroport holder. I shoved it into the aero bar, and that was that. All done!

Well, not really.

While this looks okay at first glance, those two wires crossing over from left to right look problematic. This is a commuter bike, and it'll probably see some above. I definitely don't want these wires to get caught in someone else's handlebar, and damaged in the process.

Besides, having the two wires just hanging there looks pretty bad.

Looking at my wiring schematic, these wires turning out this messy isn't a surprise at all. In fact, I saw this coming and thought of a solution before I even got started.

Profile Design Aerobridge

Not only do Profile Design make an Aeroport to securely hold the Di2 battery and main junction, they also created the "Aerobridge".

What's this? Well, it's basically another piece of plastic that you secure to both aero bars using two clamps and bolts, bridging the aero between them.

The Aeroport is a "multi-purpose computer mount that can be positioned on any round section of the aerobar". This is exactly what I'll do - and at the same time, it's a neat bit of camouflage for those wires.

Before I show you the finished bike, here's one last work-in-progress image.

The Aeroport is in place, but there's still a lot of exposed wire in the shot. At this point I decided to wrap the handlebars and aero bars, hiding all of these wires from view.

(and don't worry, I also installed the brake cables)

This is what it looks like from above. Pretty neat, right?

And finally, here's the view from below. You can see some exposed wires, but unless you look underneath the handlebar, they're all hidden from view.

What's next?

That's it - the bike is all done. I've ridden it to work a couple of times, and it rides just like any Di2 bike should - perfect and immediate shifting.

So what's next for this bike?

Well… I'd really like to upgrade that battery to the BT-DN110 someday. That'll let me enable synchronized shift so that I no longer need to take my hands off the aero bars to shift the front derailleur.

This being a commuter bike though, I'm not in any hurry. If I run into a discounted BT-DN110 battery I'll probably buy that, but it can definitely wait.

How about you? Have you ever hacked your Di2 components to complete your special project? I'd love to hear about that, so be sure to send me a message, or leave a comment below.

BetterShifting Terry

About the Author - BetterShifting Terry

I enjoy playing with bike tech - both bike building and wheel building, bike maintenance and of course, Di2. Besides writing content and working on the technical side of BetterShifting, I also work as a Software Developer in The Netherlands. Read more on the About this site page.

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