Connect 12-speed Di2 shifters by wire

While 12-speed Di2 shift levers support wireless shifting if you’re on disc brakes, there’s still plenty of you riding rim-brake bikes. Those bikes don’t support wireless shifting, and the only way to connect them to the rest of the system is by wire.

And even if your bike does support wireless shifting, you may opt to connect the shift levers via wire to take advantage of the 50% longer battery life (time between charges), or to eliminate the wake-up click 12-speed wireless users experience every time they go for a ride.

Whatever your use-case is, let’s take a look at how to wire up 12-speed shift levers.

At the time of writing, these shift levers can be wired:

• Ultegra ST-R8150 (Rim brakes)
• Ultegra ST-R8170 (Disc brakes)
• Dura-Ace ST-R9250 (Rim brakes)
• Dura-Ace ST-R9270 (Disc brakes)
• GRX ST-RX825 (Gravel, Disc brakes)

The 105 ST-R7170 shift levers cannot be wired to the rest of the bike, they lack the required ports. There is only one SD300 port on a 105 shifter, and it can be used for updates and diagnostics only.

Note that while the compatibility charts claim you cannot set up ST-RX825 wired, you can in fact do that, no problem.

If you’re using one of the disc brake shift levers then there’s only one way to wire up your shift levers, but rim brake riders have two methods to choose from when it comes to connecting shift levers.

All about those ports - SD300 only!

The 12-speed rim brake shifters have two SD300 ports, and one satellite shifter port - the bottom one.

Twelve-speed disc brake shifters have one SD300 type port, and one satellite shifter port. Again, the satellite shifter port is the bottom port.

And yep, as the text above implies, the bottom port on both rim brake and disc brake shifters is for satellite shifters only.

Regular EW-SD300 wires can be plugged into that bottom port, but doing so won’t have the desired result.

Using the bottom port to wire shifters to the battery will NOT work.

Only satellite shifters work in the bottom port.

So, with that out of the way, how do you wire up shift levers?

Method 1: for both disc brake and rim brake shifters

This method involves connecting both shifters to a junction (EW-JC304) using one EW-SD300 wire each, and then running a single wire from that junction to the battery.

The parts in this setup are:

• One EW-JC304 junction | Amazon.com | eBay
• One 350mm EW-SD300 wire from the left shifter to that junction | Amazon.com | eBay
• One 350mm EW-SD300 wire to connect the right shifter to the junction | Amazon.com | eBay
• One 1400-1600mm EW-SD300 wire from the JC304 junction to the battery | Amazon.com | eBay

There is a good chance you'll also need:

• EW-JC302 in-line junction | Amazon.com | eBay

using the affiliate links earns the site a small commission, at no cost to you

A note on wire length

Note that I put 350mm and 1400-1600mm EW-SD300 wires in the list above. This will work in most cases, but I really recommend measuring what length wire you need.

This is how that works:

• Decide where your shifters will go
• Determine where the JC304 junction will be
• Run a piece of string from the location of your shifters to the JC304
• Measure the piece of string
• Add 75-100mm to that to get the length of wire you need

For example, the distance between my left shifter and the stem, where my EW-JC304 junction will be, is roughly 250mm. I could probably get away with using a 300mm wire, but it’d be close.

I’m using a 350mm wire instead, hiding the excess wire in the handlebar.

The golden rule when it comes to wire length is that a wire that is slightly too long is better than one that is too short.

Use wires that are too short and you run the risk of putting extra stress on the wires and sometimes even break them. Wires that are too long can simply be hidden in the frame.

Get a long battery wire

Especially important for the wire between the battery and junction is that this needs to be longer than just the distance between JC304 junction and battery.

Ideally this wire extends about 50mm - 100mm beyond the top of the seat tube, since you need to be able to take out the entire seat post with the battery still connected. If it doesn't reach that far, you'll have to extend the wire using an EW-JC302 inline junction and another EW-SD300 wire.

Connect the shifters to the JC304 junction

First, install both shifters to your handlebars. Whether you install the brake hoses/cables now or afterwards is up to you. It doesn’t really matter.

Next, make sure the EW-JC304 is in the right place. In this case, I’m placing mine in the stem.

I have already connected two 350mm SD300 wires to the junction.

Connect the other ends of the SD300 wires to the shifters, and be sure to use the top port.

If you're having a hard time getting the wire or cables through the handlebar, using an internal routing tool can help. I have added a couple of them to my Amazon StoreFront.

Finally, plug the remaining 1400-1600 wire into the junction run the other end through the frame, to the top of the seat tube (assuming your battery is in the seat post).

The wire should comfortably reach beyond that point, but depending on your frame size, it may not. See the section at the bottom of this page for more information on extending wires

At this point you’ll have three wires coming out of the JC304 junction.

This is what my stem looks like with the wires connected. I’m using bubble wrap to prevent the JC304 junction from rattling around in the stem.

That's it - this is all method one involves, really.

These components on Amazon

I have added the components used in Method 1 to a separate page on my Amazon Storefront. Using this affiliate link helps support the site, at no cost to you.

Method 1: connect shifters using junction

If you're building up a bike from scratch, you would probably install the seatpost battery, the derailleurs, and the chain next.

Method 2: rim brake shift levers only

This method avoids using a junction at the front of the bike, which is great if your handlebar/stem don't let you hide the junction out of sight.

Instead of connecting both shifters to a junction and then running a wire to the battery, you connect the shift levers to each other, and then run a single wire from either shift lever to the battery.

This method only works for rim brake shift levers, because the disc brake shifters have only one SD300 type port.

Also, I recommend using this method only if your handlebars support internal routing. If they don’t, using this method will look a bit messy.

You need:

• One 550mm EW-SD300 wire from the left shifter to the right shifter | Amazon.com | eBay
• One 1400-1600mm EW-SD300 wire from the shifter to the battery | Amazon.com | eBay

using the affiliate links earns the site a small commission, at no cost to you

Potentially, you’ll also need:

• EW-JC302 in-line junction | Amazon.com | eBay
• An additional EW-SD300 wire | Amazon.com | eBay

So why would you need a junction and SD300 wire? Well, depending on the frame size, your handlebars and your stem, a single 1600mm EW-SD300 wire may not be long enough to connect a shift lever to the battery.

The solution is to extend that wire using an in-line junction and another SD300 wire - more on this further down the page.

Wire length

The wire lengths I mention on this page are just suggestions, no more than that.

To get your wire length right, I recommend measuring the distance between two connection points.

This is how that works for the wire between the two shifters:

• Determine where your shifters will go
• Run a piece of string between the two shift levers, following the path the SD300 wire would take
• Measure the piece of string
• Go the next size up, and then add 50mm to get the recommended wire length

For example, if the distance between two shifters is 430mm, the next size up SD300 wire is 450mm, and I recommend you use a 450 + 50mm = 500mm wire. If the desired wire length is not available, always choose the next size up.

Having a wire that is slightly too long is no problem - you can hide that excess wire underneath the handlebar tape… but a wire that is too short is a real problem.

For the wire between shifter and battery, getting the right length wire can be a challenge.

• Decide which shifter you’re going to connect to the battery
• Run a piece of string from the shift lever to the top of the seat tube, following the path the SD300 wire would take
• Measure the piece of string
• Go the next size up, and then add 200mm to get the recommended wire length

Why 200mm? Mainly because you need to be able to take out the entire seat post with the battery in it, and the wire connected.

When holding the piece of string along the frame (step 2), take into account that you need to be able to rotate the handlebar. Don’t wrap this wire around your head tube too tightly - you don't want it to break or snap under repeated stress.

The maximum length EW-SD300 wire that you can buy is 1600mm, and if that isn’t long enough, you’ll have to extend that using the EW-JC302 in-line junction and another EW-SD300 wire.

There is more information on extending wires further down this page.

Connect the shifters to each other

Start by installing both shifters to the handlebar, and make sure they’re roughly in the right place.

Take your ~550mm wire, and connect that to both shifters. Remember to route the wire through the handlebar.

Decide which shifter will be connected to the battery. I prefer using the one that also operates the rear brake. That way you can shrink wrap or simply zip-tie the Di2 wire to the brake outer cable and it’ll look as good as it can.

These components on Amazon

I have also added the components used in Method 2 to a page on my Amazon Storefront. Using this affiliate link helps support the site, at no cost to you.

Method 2: Wire up shifters without junction

Extend wires using the EW-JC302 junction

Assuming your battery is in the seat post, you'll want the wires connected to the battery to extend about 50-100mm beyond the top of the seat tube.

Why? Because you need to be able to take out the entire seat post, with the battery wires still connected.

If even the longest (1600mm) SD300 wire doesn't reach that far, you'll have to extend it using an EW-JC302 junction.

For example, on my medium size 54 frame, a 1200mm wire only just made it to the top of the seat tube.

I didn't have any longer wires at the time, so I extended that wire using an EW-JC302 inline junction and another EW-SD300 wire of the right length.

Use dummy plugs in unused ports

One last thing - always use dummy plugs in unused shifter ports.

While the system will work just fine without dummy plugs in unused ports, these plugs do play a crucial role in keeping water out of the shifter electronics.

Di2 is pretty water-resistant, but not completely waterproof. Getting water in the shifter circuitry could cause ghost shifts, battery drain, and all kinds of other hard to track down issues.