Building a bike – cable routing, lengths and how to connect it all
If you’re building a new Di2-equipped bike there are a couple of things you’ll need to decide. We’ve already published a post on what components are available and how to select them. In this post we look at figuring out how to route the Di2 wires through your frame so that you can put it all together.
As the previous post states, you’ll need the following components:
- Front Derailleur (optional)
- Rear Derailleur
- Shifters / brake levers (maximum of 6)
- Internal or external junction box A (or System Informations Display – MTB)
- Internal or external junction box B (optional)
- About 4-6 Electric wires
- Wireless Unit (optional)
Not sure what to buy? Have a look at the components page for an overview of all available components. I've added a shopping list to the bottom of this page, just in case you want to go shopping right now. Alternatively, use the component pages to find the correct Amazon/eBay/Wiggle pages.
Di2 Cable routing options
There are a couple of ways to set up your Di2 system. These include fully internal, fully external and mixed setups. Regardless of the option you choose, the process of selecting your EW-SD50 electric wires is the same:
- Decide which components will go where on your frame
- Figure out how to route the electric wires (inside or outside the frame)
- Give each wire a label (e.g. “Junction A to Junction B”, “Junction B to battery”, “Junction B to rear derailleur”, etc).
- For each wire, measure the length of the frame sections it has to pass through and write down the required length
Shimano recommends you measure the length from point to point, choose the next size up and add 50mm to make sure the wire is not too short. For example:
If the measured length is 325mm, the next size up is 350 and the correct wire length would be 350 + 50 = 400mm
I won’t discuss all the different ways to set up your Di2 bike – there are simply too many possibilities. Instead I’ll show you the full internal and fully external options – you should be able to figure it out for yourself after that (don’t hesitate to contact me for help though).
Internal battery and junction A
This is by far the cleanest and most used setup on modern frames. It hides as much of the Di2 wiring as possible, making your bike look as clean as it can, and it improves aerodynamics slightly over a more external setup.
The list of components in this build is:
- A) Battery BT-DN110
- B) Bar-end junction EW-RS910
- C) Junction B SM-JC41
- D) EW-JC200 inline junction or Wireless Unit EW-WU111 (either)
- E) EW-JC130 Y-connector cable
- F, G, H, I) EW-SD50 electric wires
The EW-JC130 Y-connector is available in different sizes. Look at its hardware page for a list of sizes. The one you pick depends on your handlebar size, how you route the wire and where your EW-RS910 Junction box is located. Have a look at the EW-RS910 page for more information on the available routing options.
Note that the Y-connector is optional. I haven't used it on my main bike, but it's up to you. The pros and cons of the Y-splitter are explained in more detail in the EW-RS910 installation guide. Basically, using the Y-splitter gives you more flexibility and E-tube ports, but generally makes the cockpit area a bit messy.
To get a better picture of the sizes you can take a piece of string and use that to see how the different lengths measure up to your handlebars. You connect the Y-connector wire to the rest of the system using either the inline 2-port junction or wireless unit.
Besides the above mentioned EW-JC130 and EW-JC200, there are four EW-SD50 electric wires installed. One wire runs from the stem (part D or E if you don’t have a wireless unit) to the Junction B (SM-JC41). This wire is labeled I. There are three wires that connect Junction B and the other components. Wire F runs to the battery, H to the rear derailleur and the last wire(G) connects the front derailleur. Note that on some frames there is no space to put Junction B in the downtube and in these cases it is usually placed somewhere between the bottom bracket and the battery.
Wire lengths depend on the size of your frame. You’ll have to measure the frame sections the wires run through to determine the correct length. Can’t decide between two lengths? Then choose the longer one – you’ll be able to hide excess wire inside your frame.
It’s also a good idea to select a slightly longer wire for the seatpost battery. At some point you will want to be able to take out the entire seatpost to do maintenance, a system reset or cleaning and it’s nice being able to do this without having to stretch the Di2 wire and possibly damaging the connector. You can determine the seatpost wire length as follows:
Measure from the bottom bracket shell to the top of the seat tube. Add that measurement to the length of the seat post and then add another 250mm to get the correct length
External below-the-stem junction A
While the internal bar-end junction is the cleanest option, not every frame supports having the junction box there. Some handlebars don’t allow you to run electric wires from one handlebar drop to the other. In cases like these you’d use the traditional below-the-stem Junction A SM-EW90.
Since we’re not using the Y-connector cable in this build we’ll have to replace that with two EW-SD50 electric wires. Use one of them to connect the left shifter to Junction A and use the other to connect the right shifter. Measure the length you need carefully – the electric wire will run along the handlebar all the way from the e-Tube port on the shifter to the Junction box. Once again; when in doubt, choose the sightly longer wire. It’s good to have some slack so you don’t pull the connector loose easily.
Note the external battery and junction b below the bottom bracket. (both colored red) Even though we recommend routing as much of the wiring internally as you can, sometimes it is simply impossible to do. Not all frames support internal Di2 wiring, even if they do allow internal mechanical cables. While Di2 electric wires are smaller (2mm) than mechanical cables and cable housing, the Di2 connectors are slightly larger (5mm diameter) than the holes/cable stops in most frames. If this is the case for your frame and you’re determined to set it up with Di2 then you’ve got two options:
- make the cable entries larger using a drill (not recommended unless you know what you’re doing and definitely do not try to do this on carbon bikes)
- route your Di2 wires externally
If you’re going the external route, keep in mind that you need to attach your electric wires to your bike somehow. There are a couple of options. You could use double-sided tape, tie-wraps, cable guides that screw into existing holes or buy the Shimano Di2 Wire Cover.
The list of components in this build is:
- A+B) Battery BT-DN110 or the external battery SM-BTR1 and mount
- b) Front Derailleur
- c) Rear Derailleur
- C) Below-the stem Junction A SM-EW90
- E) External Junction B SM-JC40
- 6x) EW-SD50 Electric wire
This setup is very similar to the fully internal build, except for the battery location. If there is no way for your frame to accommodate an internal battery you’ll need to attach that somewhere to your frame. There are some creative people out there that have put the external battery below the saddle or bottom bracket, but it is usually placed near the bottle cages. You can use either the external SM-BTR1, or get the internal BT-DN110 and the SM-BTC1 Bottle cage battery mount.
No junction B
Just because you can build a Di2 system without a junction B doesn’t mean you should
Yep, it is possible to set up a Di2 system with just a junction A. This means running cables from your Junction A to every component you want to connect. I won’t go into the details as much as I did above, but one way to do this is:
- Connect your shifters using the Y-connector cable EW-JC130 and EW-JC200
- Run a long wire to a 5-port junction A. You’d probably attach this junction box to your saddle or somewhere easily accessible (for charging and configuration)
- From junction A run an electric wire to each component you want to connect.
Another option would be:
- Connect the shifters to one (use the Y-connector) or two (EW-SD50 wires) ports on the 5-port junction A
- Use the remaining ports on Junction A to run electric wires to the battery, front derailleur and rear derailleur
- Using the second option means you’d run wires from the below-the-stem junction to each component. That’s four or five electric wires coming out of the Junction box – not a pretty sight.
So now that you have decided what components to buy and what length and how many electric wires you need it’s time to go shopping.
You could get the Front/Rear derailleurs off eBay, Facebook marketplace, or any other good second-hand source. I recommend buying a new battery and EW-SD50 Electric wires. This means you get a brand new battery at full capacity and wires with new connectors. Worn connectors are rare, but can be a real pain when they cause components to disconnect at seemingly random times.
Junction boxes can easily be bought on both eBay and Amazon – I recommend getting them where they are cheapest. They don’t really wear so there’s no real disadvantage to buying them second-hand.
Using the links below to buy anything will earn the site a small commission at no additional cost to you.
- SM-EW90 stem Junction A: Amazon | eBay | Wiggle
- EW-RS910 bar-end Junction A: Amazon | eBay | Wiggle
- SM-JC40 external Junction B: Amazon | eBay | Wiggle
- SM-JC41 internal Junction B: Amazon | eBay | Wiggle
- EW-SD50 Electric Wire: Amazon | eBay | Wiggle
- BT-DN110 battery: Amazon | eBay | Wiggle
- SM-BCR2 battery charger: Amazon | eBay | Wiggle
If you’re planning on putting the battery in your seatpost you might also want to look for a seatpost battery mount on Amazon, Chain Reaction Cycles or eBay to securely mount your battery in the seatpost.