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Demystifying Di2: Cut the Guesswork Out of Harnesses and ETubes

It’s not an exaggeration to say that electronic shifting has revolutionized cycling, and as we all know, Shimano has been at the forefront of this electronic revolution. Accordingly, its Di2 groupsets have set a lofty standard, but if there’s any remaining drawback to upgrading, it’s this: Compared to their mechanical counterparts, Di2 groupsets are tricky to understand, let alone to just buy what you actually need. Seriously, though, I research and write about this stuff, and I’m continually left with a residual WTF expression every time that Di2 comes up in conversation. However, after numerous discussions with our expert mechanics and Shimano, I’ve attained Di2 enlightenment, and I’m able to share with you just what it is that you’ll need to complete your build, regardless of build variables.

In this piece, I’m going to demystify contemporary Di2 setups by:

1) Identifying the various parts of Shimano’s Dura-Ace 9070 group set

2) Showing you how to select the correct wire lengths for both internal and external configurations

3) Explaining all of the compatibility options that you’ll have to consider when outfitting your bike with either a 6770 or 9070 groupset.

Note: While Dura-Ace 7970 is still available, it uses different wiring and junctions. Therefore, we won’t be discussing it here.

The Components


You have quite a few options here, all of which depend on if you’re configuring a road or triathlon build and how many satellites shifters that you’re looking to integrate with it. For triathlon base bars, you’ll want the ST-9071 TT Brake/Shift Levers. These attach at the base bar, leaving the extensions clear for either SW-9071 or SW-R671 shifters. For a road build, you’ll need the ST-9070 shifters. Keep in mind, though, that both of these configurations will benefit from the inclusion of Shimano’s satellite shifters, whether it’s a Climb or Sprint remote shifter. However, this requires the use of a five-port junction box, which we’ll discuss later.

Front Derailleur

This is a simple choice — you’ll need the Dura-Ace FD-9070 front derailleur for either a triathlon or road build. This derailleur attaches directly to frames with braze-ons, but clamp adapters are available separately.

Rear Derailleur

Again, you really only have one option here, the Dura-Ace RD-9070 rear derailleur.


Shimano makes two lithium-ion batteries for 9070 — an external battery called SM-BTR1 and an internal battery called SM-BTR2. The external battery (SM-BTR1) is typically fitted near the bottom bracket using a mount attached to the nearest water bottle braze-on. In rare cases, such as with the BMC TMR01, this battery may be fit inside the frame. Some frames also position the battery under the chainstay.


The internal battery (SM-BTR2) is placed inside the seatpost. It may only be used with compatible seatposts, or for those with a corresponding adapter, like the Pinarello Dogma 60.1, Dogma 2, and Dogma 65.1. Additionally, it requires a frame with designated holes for internal wire routing.

Wire Junction Boxes

There are two wire junction boxes in any Di2 set-up. These are where the wires leading to the various components come together. The cockpit, or “A” junction, which hangs below the stem, is part of the Front Wire Harness, so we’ll discuss it in the wiring section.


The lower, or “B” junction, comes in two forms — an external (SM-JC40) and an internal version (SM-JC41). The external version is fastened underneath the bottom bracket. The internal version is generally tucked inside either the seat tube or down tube through the bottom bracket shell.

E-Tube Wiring

Your configuration variables are vast, so this tends to be the most confusing part of the process. We’ll break it down by section, while providing some rules to follow along the way.


The Five Wire Sections, From Front to Back

Front Wire Harness

The Front Wire Harness connects each shifter to the cockpit, “A” junction box, which hangs underneath the stem. The standard harness only comes in one length.

There are two different “A” junction boxes available from Shimano. The standard “A” junction has three ports — one for the down tube wire, and two for the wires leading to the shifter/brake levers.


An alternate junction box has five ports — this is to accommodate extra remote shifters in a standard set-up, or shifters mounted at the bar ends and on the brake levers in a time trial/triathlon set-up with aero bars. Both junction boxes have a charge port built in for charging an internal battery without removal.

Down Tube Wire

The Down tube Wire connects the cockpit, “A” junction, and thus, the Front Wire Harness, to the lower “B” junction. It can be run inside the down tube of the frame (compatible frames only) or outside it. To select the appropriate length, do the following:

1) Measure from the bottom of the bottom bracket shell to the back of the bottom of the head tube to find the length in millimeters. Call this measurement “B.” Then measure from that same point on the bottom of the head tube to the bottom of the handlebar next to the stem. Call this measurement “A.” Add A & B together, and then subtract 30 to get “X.” Note that the size needs to be in millimeters.

2) Round your “X” measurement up to the closest-sized wire. The Short Down Tube Wire is 700mm, the Medium Wire is 750mm, and the Long Wire is 900mm.

Note: Err on the longer side for internal set-ups, since any extra wire can be tucked into the frame. This same rule of thumb should be applied to the other wire sections, too.


Front Derailleur Wire

The Front Derailleur Wire runs from the lower “B” junction to the front derailleur. If the junction is located anywhere near the bottom bracket, whether inside the frame or outside of it, the short, 350mm length will usually suffice.

Battery Wire

The Battery Wire connects the battery to the lower “B” junction. When you’re using the standard, external battery, these parts can normally be placed very close to one another — the short, 350mm length will usually suffice. You’ll need a longer battery wire for the internal battery that’s mounted inside the seatpost.

Rear Derailleur Wire

The Rear Derailleur Wire runs from the lower “B” junction to the Rear Derailleur. To determine length, a good rule of thumb to follow is to add 200mm to the length of your chainstays and round up to the closest-sized wire. To reiterate, err on the longer side for internal set-ups, since any extra wire can be tucked into the frame.


In my opinion, the simple, fast operation and dependability of Shimano Di2 is well worth both the financial investment and the slightly more complicated assembly. Of course, its complexity is an admitted deterrent. Remember, though, that our bike experts are always willing to lend a hand. Feel free to give us a call or send an e-mail if you have any questions.