Building a 12-Speed Di2 Road Bike - components and wiring

Updated May 22, 2024 by BetterShifting Terry

My page on Building an 11-speed Di2 road bike is one of the more popular pages of the site, and that’s not surprising. Building 11-speed Di2 bikes involves all sort of junctions and there are plenty of wiring options.

While 12-speed Di2 simplifies things greatly, it isn’t always easy to know whether or not you’ve really bought all the parts you need, and wire lengths generally confuse people too.

I’ll cover all of that on this page, and then some. If there’s anything you feel is missing though, please leave a comment below or send me an email or WhatsApp message.

If you're building a 12-speed Time-Trial / Triathlon bike, be sure to also read that guide.

The process of building a bike

Building a bike involves more than just putting the parts together.

When thinking about the entire process of building a bike, I like to split this up into six steps:

  1. Planning
  2. Buy components
  3. Bench test
  4. Installing the components to the bike
  5. Setting up the bike (derailleurs)
  6. Test ride

This page will help you with the first three steps, planning, buying the components, and the bench test.

For more information on installing your Di2 components and setting them up, take a look at my Di2 Installation Guides

My tools and shopping lists

People regularly ask me what tools I use, so I added a list of my tools to my Amazon Affiliate Storefront, as well as lists containing 12-speed Di2 parts.

Note that while these lists contain all 12-Speed Di2 components, don’t just blindly order everything from one of these lists. Both rim and disc brake items are listed, and besides that: you’ll still have to determine the number of wires and their length.


Every time I’m building a Di2 bike I think about the components to use, and what length wires I need. While selecting the components may seem trivial, there are still plenty of things to keep in mind.

Will you be using rim brakes or disc brakes? Do you want to use satellite shifters? Do you want to run your shifters wired? Or perhaps save some money by swapping out Ultegra components for their 105 equivalent?

This is what the next section is all about.

General parts for a 12-Speed Di2 build

No matter your bike type, there are a couple of components you always need:

Most parts come in GRX, 105, Ultegra and Dura-Ace versions.

GRX is Shimano's gravel groupset, but the others are all 12-speed road Di2 groupsets - at three different price points and feature levels.

If you’re not sure which of these is right for you, please read my page on the differences between 12-speed Dura-Ace, Ultegra, and 105 Di2.

New crankset, or use your 11-speed crankset?

You’ll also need a crankset, of course. If you’re upgrading an 11-speed bike to 12-speed Di2, you may be considering using your 11-speed crankset with 12-speed Di2.

While this isn’t officially supported, I can confirm that it does work. The 12-speed chain and front derailleur work just fine with 11-speed cranksets.

There are a couple of things to be aware of though:

First, chain line. Shimano’s 12-speed road systems are designed around a 44.5mm chain line and 148mm Q-factor.

Older road systems have a 43.5mm chain line and 146mm Q-factor.

This means that the 12-speed front derailleur is optimised for chainrings that are 1mm further outboard than the ones on the 11-speed cranksets.

Usually this is not a problem, but you can run into problems setting up the front derailleur limits - you may not be able to adjust the front derailleur in far enough.

If you're planning to build a gravel bike, then I recommend using a gravel-specific crankset. Shimano's gravel chain lines are 2.5mm further outboard than its road counterparts.

WheelsMfg make crank spindle spacers you can use to push the crankset out

On my own bike I am using the 11-speed FC-R8000 crankset, since my 12-speed crankset came with a very bent chainring.

The 11-speed chain ring works well, but I did place a 1mm spindle spacer between the drive-side crankset and the bottom bracket in order to push my crankset out a bit.

Also, when using an 11-speed crankset there is only a small gap between the front derailleur and the crank arm. We’re talking 2-5mm here.

There is only a tiny gap between the front derailleur and 11-speed crank arm (left), while the gap is larger for the 12-speed crankset (right)

Not a problem if the limits are set up correctly, but get your front derailleur too far out and you could, literally, pedal it right off.

Finally, 11-speed Shimano cranksets do not work with 12-speed Shimano (outer) chainrings, even though the BCD is 110 in both cases. This is due to the design of the crankset - you physically cannot put a 12-speed Shimano outer ring on an 11-speed Shimano crankset.

Hydraulic Disc Brake specifics

If your frameset supports disc brakes, great! You can use the shift levers wirelessly, or choose to run them wired instead. Either way, the shift levers you’ll need are:

There are new hydraulic brake calipers too, and I recommend using them. Not only has the rotor-to-pad clearance improved, but they also feature new bleed ports which make bleeding your brakes a lot easier.

The new calipers are well worth the money if you ask me!

You can buy these parts either as a kit of two shift levers and brake calipers or buy everything separately.

The kits usually come pre-bled and with an easy connection kit (brake hoses). If you’re buying everything separately don’t forget to include a set of SM-BH90-JK-SSR brake hoses - one for the front brake, and one for the rear.

Are you installing the components yourself? You’ll also need a bleed kit, Shimano mineral oil, and a tool to cut the brake hoses. I have listed mine under the tools I use over on Amazon.

EW-SD300 wires - wired vs wireless

At the bare minimum, you’ll need two wires. One to connect the front derailleur to the battery, and one to connect the rear derailleur to the battery. In this setup, the shift levers operate wirelessly.

You can connect the wireless GRX, Ultegra and Dura-Ace shifters to the rest of the system using SD300 wires. Doing so increases the battery life (time between charges) by 50%, and eliminates wake-up shifts (wireless systems go to ‘sleep’ after 30 minutes of inactivity).

To do so, you’ll need three more EW-SD300 wires (total: five wires), and a four-port EW-JC304 junction.

The two shifters are connected to the EW-JC304 junction, and the third wire is used to connect the junction to the third port on the battery.

The length of those wires depends on where you place the JC304 junction. Some people put it in the stem or the handlebar, while others place it in the down tube.

Note that the 105 Di2 ST-R7170 shifters lack the required ports, and can not be run wired.

Rim Brake specifics

Prefer rim brakes? No problem! 12-speed Di2 works with rim brakes too, but not wirelessly - wired shifting only.

This means that you’ll have to use the Ultegra or Dura-Ace shift levers. There are no 105 Di2 or GRX rim brake shifters.

You can still build a budget-friendly 105 Di2 or GRX rim brake bike - but you’ll have to use one of these shifters:

These shifters are essentially the 11-speed models, with SD300 ports and updated firmware. Shimano has updated the calipers (Ultegra / Dura-Ace) too, but again, they’re just the older calipers with a new paint job.

Wiring up a 12-speed Di2 rim brake bike

When it comes to wiring up a 12-speed rim brake bike, you’ve got two options. You can either wire both shifters to each other, or use a junction box.

I have written an extensive guide on wiring up 12-speed di2 shifters, so here is a quick explanation of the two methods. For the full details, be sure to read the complete guide.

Method one: wire the shifters to each other

This first method involves running an EW-SD300 wire from one shifter to the other, and then running a single wire from either shift lever to the third port on the battery.

While the disc shifters only have one SD300 port, the 12-speed rim brake shifters have two SD300-type ports and one satellite shifter port.

You can use any of the two top ports on the shifters, in any way you like. In the image above the battery wire is connected to the middle port on the right shifter. Want to use the top port for that? Sure. Want to connect the battery to the left shifter? Go right ahead.

Note that those green coloured ports only work with the RS801 satellite shifters. You can stick regular SD300 wires in them, but nothing will happen.

This method works best when the handlebar supports internal routing. Routing the wire through the handlebar effectively hides the wire connecting the two shift levers, resulting in a clean front end of the bike.

Are you on disc brakes or perhaps you simply cannot run an EW-SD300 wire through your handlebar? Then take a look at the alternative method.

Method two: 4-port junction EW-JC304

The second method of connecting wired shift levers involves a four-port junction, the EW-JC304. This is also how you’d wire up GRX, Ultegra and Dura-Ace wireless shifters.

Each shifter is connected to the JC304 junction using a single EW-SD300 wire, and then a third wire runs from the JC304 junction to the battery.

It doesn’t really matter which port you use, as long as you do not use the bottom port. Those are meant to be used with the EW-RS801 satellite shifters only.

If you'd like to know more about connecting 12-speed shifters wired, I recommend you read the complete guide.

Wire lengths for 12-speed Di2

Getting the right length Di2 wire is relatively straightforward. You decide where your junctions (if any) and battery go and then measure or estimate the required wire lengths from those points.

When in doubt, choose a slightly longer wire than you think you need. All wires cost roughly the same, and having a wire that is slightly too long is not a problem - excess wire can be hidden in the frame.

A wire that is too short, on the other hand, is a big problem. You wouldn’t be able to connect your components, and would probably have to order extra wires.

Speaking of extra wires… if you plan to run your Di2 wireless, you may want to order a long wire (1000mm - 1600mm) and a 2-port JC302 or 4-port JC304 junction anyway. It definitely makes updating your wireless shift levers a lot easier.

Traditionally, Shimano has always recommended you measure the length from point to point (using a piece of string for example), then 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

This method still works well when determining wire lengths for wired shift levers, but the wires plugged into the battery require a slightly different approach.

Also, if you do not have a frameset yet, but want to order wires anyway, you can estimate the required length using the frame’s geometry charts. I’ll use my own Cinelli Veltrix Disc size M as an example.

Note that I will assume that your battery is in the seat tube. It is the most common location, and bikes that place the battery in the down tube are relatively rare. If your battery is in the down tube, however, keep that in mind when reading the text below.

The concepts still apply, but obviously, you won’t need to run a wire up the seat tube.

Rear derailleur to battery wire length

Connecting the rear derailleur to the battery is the same for both disc and rim brake bikes. It involves running a wire from the derailleur through the chain stay, past the bottom bracket, and up the seat tube.

You’ll want to be able to take out the entire seat post with the battery in it, and all wires still attached.

For this reason, I usually add an extra 100mm to the battery wires’ length.

Let’s look at the geometry info and determine the correct length for the wire between the rear derailleur and the battery.

The length of the chain stay (C) is 415mm. The distance between the bottom bracket and the top of the seat tube (S1) is 540mm. You’ll need to be able to take out the entire seat post with the battery and its wires still connected, so I like to add about 100mm - 150mm of wire.

In this case, I’d go with a 415mm + 540mm + 150mm = 1100mm wire.

You can hide the excess wire in the frame, and this way you can comfortably take out the seat post for maintenance.

Front derailleur to battery wire length

Determining the front derailleur to battery wire length is a similar process to that of the rear derailleur wire. Measure or look up the length you need, and then add a bit of excess wire so that you can easily take out the seat post.

I’ve highlighted the seat tube in the image above, but the real length on your bike is probably less than that. On my bike, the front derailleur wire entry opening is just a bit above the down tube, at the front of the seat tube - about 450mm.

Either way, I would use a wire of 450mm + 150mm = 600mm. Of course, 650mm would work too.

Shifter wire length

Shimano’s general rule of “measure, then go the next size up, then add 50mm” works well when determining the length of the wires between the shifters, and also for the wire between the shifter and junction.

Perhaps I’m stating the obvious here, but If you plan to use the four-port junction EW-JC304, decide where you’re going to place that before ordering any wires.

That leaves one wire - the one connecting the shifters to the battery. On most bikes this wire is routed through the down tube, so let’s go with that.

The down tube size isn’t listed, but it is about 650mm. Our seat tube has stayed the same, 540mm.

650mm + 540mm + 150mm = 1340mm.

That’s not all though, you’ll need to add the distance from shifter or junction to that as well.

You`ll want a bit of slack at the shift lever, as seen in this picture

Getting this size right without measuring is pretty tough, and I really recommend measuring the distance. It is not uncommon for this length to exceed 1600mm - the longest wire Shimano makes.

In that case, you'll need to combine two wires using an inline junction, the EW-JC302.

Again, I prefer having too much wire over having too little. The excess wire will be hidden away in the down tube and seat tube, and the performance is the same as that of a shorter wire.

Buy the components

Once you’ve decided what components and wire lengths you need, it’s time to buy the components.

You’ll find so-called ‘upgrade kits’ online, which are basically both derailleurs, shifters, battery, wires and cassette all bundled together.

These can be cheaper than buying all components separately… but there’s no guarantee. Quite often it’s worth shopping around and looking for discounts.

Also, the wires included with the upgrade kits work well with the most common bike sizes, but they can be a bit on the short or long end for extremely large or small bikes. You can usually determine the required wire lengths just by looking at the bike’s geometry chart, as explained above.

My Amazon Storefront

As I’ve mentioned at the top of this page, I have an affiliate storefront on Amazon. It’s basically a list of Di2 components and tools I put together myself. Any purchase you make through these links will earn the site a small commission, at no cost to you.

(note that while these lists contain all 12-Speed Di2 components, don’t just blindly order everything from one of these lists. They contain both rim brake components and disc brake ones)

Only use these links to buy Di2 components and tools if the price is right for you. If you’re in Europe and shops like Bike-Discount and Mantel work out better for you, by all means, buy your parts there.

There are also links to shop on most component pages, if you prefer that.

Bench test

With all the components in hand, it’s tempting to install them on the bike and get building right away. However, hold your horses!

Now is the time to connect the derailleurs to the battery (use a dummy plug in any unused port!), and perform a couple of basic jobs:

  1. Charge the battery
  2. Connect to the bike using the mobile app
  3. Pair the shifters
  4. Update the shifters and other firmware
  5. Make sure both derailleurs and shifters work

Updating the shifters can be especially troublesome if you’ve installed all components to the bike - they need to be wired to the system in order to be updated.

This is also the time you confirm every component and wire actually works. Better now than later, after you’ve installed everything on the bike.

Installing the components and building the bike

When it comes to actually building the bike, here are some pages that will definitely help:

  1. Install and set up a 12-speed rear derailleur
  2. Install and set up a 12-speed front derailleur
  3. Install the seat post battery
  4. Chain length and direction
  5. Install wired shift levers

And these guides on using the 12-speed Di2 software to set up and fine-tune the bike:

Order of installation

If you're wondering in what order you should install and connect Di2 components, then don't worry - it does not really matter. I generally install everything to the frame, pair the shift levers, and set up the derailleurs.

Shimano does mention a workflow in their dealer manual, and it is as follows:

  1. Install the shifters, satellite shifters, derailleurs, and battery to the frame
  2. Install the chain
  3. Set the rear derailleur limits and adjustment
  4. Set the front derailleur limits

Any other questions or suggestions? Please leave a comment below, or send me a message.

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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