Ask a Mechanic: Di2 Wire Installation

Kyle writes, “I just got a new Di2 group for my road bike and was wondering what length e-tube wires I will need to complete it. Any advice on finding the correct lengths?”

Making sure your e-tube wires are the correct length will ensure that you have a clean set-up without a lot of excess wire to try to hide when you are done with your build. Wire lengths are more critical for external set ups, but still shouldn’t be overlooked by those doing an internal set up. The main difference is that internal routing allows excess wire to be hidden inside the frame if you measure wrong. With that said, even if you are doing an external set up it’s better to have the wire be a bit to long rather than a bit short.

Before starting, you’ll want to make sure you know where your junction boxes and battery will be mounted. Once that’s established, you can start measuring. The process for each wire is the same; you’re simply measuring the distance from point A to point B. A flexible tape measure is helpful, since it will bend to follow the frame where the wire will travel.

If you don’t have your Di2 components mounted to the frame yet, just use the existing barrel adjuster on the rear derailleur as a starting point for that component, along with the anchor bolt on the front derailleur. Most people who are doing an internal set up will position Junction Box B around the bottom bracket shell at some point, usually just in front of the bottom bracket on the down tube.

When you measure, it’s important to measure along the frame where the wire will travel, not just a direct line. Measure from point to point for the wire you need, remembering to follow the lines of the frame, and add 100mm to the wire length. You’ll appreciate the flexibility the extra wire length provides when it comes time to actually connect and run the wires through the frame. If you have an external Junction Box B, I would only add 50mm to the measured length, because you won’t have the luxury of hiding any excess wire inside the frame. The same applies to the length of wire from the shifter to Junction Box A. Now simply repeat the process for all the components that you will have on the bike, and you’re all set.

One thing to remember when measuring for an internal seat post battery is that you need enough slack in the wire to allow you to remove the post. For that wire I will usually measure from the top of the seat tube down to the junction box, and add the same 100mm. This allows for flexibility when disconnecting and connecting the battery as well as decrease the chance of the wire falling down into the seat tube when it is disconnected. Once you have all the correct lengths, simply run the wires, make your connections, and you’re ready to hit the road.