Build a 12-speed Di2 Time Trial / Triathlon bike

Updated December 18, 2023 by BetterShifting Terry

Time trial bikes are not all that different from regular road bikes, right? They consist of a frame, a set of wheels, one or more derailleurs, and all the other things you’d expect on a bike.

So why would building a TT/Triathlon bike be any different from building a 12-speed road bike?

If you're familiar with TT/Triathlon bikes, feel free to skip the introduction and go right to the relevant section:

It’s all about the bars

There is one part where they differ though - and if you’re reading this I’m sure you know this… the handlebar area. Time trial bikes generally do not just have regular drop handlebars. The cockpit area of most Time Trial bikes will consist of either a one-piece TT bar or a base bar with aero bars attached to it.

Also, most Time Trial bikes will have four shifting switches installed on the bars. This is what makes building a Di2 TT bike different from building a regular bike.

Rider on TT Cervelo

12x vs 11x Di2

This page describes the process for building a 12-speed Di2 TT/Tri bike. The process is very similar for 11-speed Di2 bikes though.

If you’re building an 11-speed Di2 TT/Tri bike then you can read the rest of the page just fine, but instead of the BT-DN300 battery, you’ll need BT-DN110. Also, you’d need R8050 / R9150 series components, not the 12-speed R8100 / R9200 series.

The 11-speed Di2 systems use the EW-SD50 wires, while the newer 12-speed systems use EW-SD300 wires - and EW-AD305 adapters to connect older components.

1x or 2x?

Depending on the type of riding you do and the environment, you may wish to build a 1x Time Trial bike - one without a front derailleur and with a massive front chainring.

Don’t worry, this isn’t something I just made up… the Specialized S-Works Shiv TT is a (SRAM eTAP) 1x Time Trial bike: Shiv TT / Tri bike

You can build Di2 1x TT bikes too, of course. The concept is not just limited to SRAM.

However, the majority of the Time Trial bikes out there are still 2x bikes.

There are pros and cons to each of these:

  • 1x bikes will give you a harder time on the climbs
  • 1x bikes generally have larger gaps or jumps between the gears
  • 1x bikes do not have a front derailleur. this saves weight and improves aerodynamics
  • 1x bikes are a bit cheaper to build

There is no way for me to decide which of these is best for you - you’ll have to decide that yourself. However, there’s this great Triathlon site - Especially good are the forums, where people will offer advice on both your fit, and bike tech.

If you go 2x12 - Synchronized Shift only

On a TT/Tri bike, using full synchronized shift makes a lot of sense. Basically, this means that you only need to shift the rear derailleur, and the Di2 system will shift the front of the bike for you.

In fact, on a 12-speed Di2 bike you don’t have a choice - you have to use (full) synchronized shift. All 12-speed compatible TT/Tri shifters have one button each, so they simply do not have enough buttons to operate the front derailleur.

TT / Tri shifters

What parts make up a 12-speed Di2 TT/Tri build?

Let’s start with the easy parts - the ones the same for every Di2 system. You’ll need a battery, and rear derailleur, a front derailleur (for 2x), and wires to connect the derailleur(s) to the battery.

Whether you pick 105 Di2, Ultegra R8100 or DURA-ACE R9200 doesn’t really matter, except for your wallet.

Sure, 105 and Ultegra weigh more than Dura-Ace - but are you really going to notice the extra weight on a TT/Tri bike?

Building a bike without shifters and switches is a bit pointless, so don’t forget about those:

While regular 12-speed Di2 bikes are all about wireless shifting, TT/Tri bikes are not. You see, the shifting signals are transmitted by the shift levers.

These are not installed on TT/Tri bikes, so you’ll have to connect your shift buttons and switches to the battery by EW-SD300 wire. Also, you need one or two SM-JC41 B Junctions at the front of the bike, depending on the amount of shift units you have - as well as other small parts:

With that out of the way, let’s take a look at how you’d wire up the bike. How you’d connect it all. Don’t worry about wire lengths for now, I’ll come back to that later.

I’ll use images from Shimano’s Dealer manual where applicable - read more about looking up Shimano’s documentation here.

Compatibility warning: firmware update REQUIRED

While the 11-speed shift units I mentioned above are 12-speed compatible, in most cases you'll have to update their firmware first. The 2021 firmware is required.

If any 11-speed components with outdated firmware are connected, the entire system will shut down until the offending part is disconnected.

Since the bike doesn't function when outdated components are connected, you can not use a 12-speed bike to update 11-speed TT/tri shifters. You'll either have to connect them to an 11-speed bike to install the updates, or use the SM-PCE1/SM-PCE02 tool.

In case you do not have access to either of those, your best bet is to ask a friendly local bike shop to install the firmware updates. It should take them about 10-20 minutes.

Connect the derailleurs and battery

On 11-speed Di2 bikes, you would install a Junction B (SM-JC41) around the bottom bracket area, and connect the front derailleur, battery, and rear derailleur to it. Not so for 12-speed Di2.

The new BT-DN300 battery has three ports, and you connect both derailleurs directly to the battery - no junctions required.

bike schematic rear

Let’s take a look at the wire length for these two derailleurs. Normally you’d measure the distance (by running a piece of string along the frame for example), then go the next size up and then add 50mm to get the recommended length.

However, having the derailleurs connected to the battery complicates this a bit. You’ll want enough slack in the wire so that you can take out the entire seat post, with both wires still attached to the battery.

I recommend doing this:

Measure the required length of wire from the derailleur to the top of the seat tube. Then add 150mm. On my Cinelli road bike this means my rear derailleur to battery wire is 1100mm.

This should give you about 50-100mm of excess wire. You can hide the excess wire in the seat tube, and it’ll come in handy when taking out your seat post - when cleaning the bike or doing maintenance for example.

Shift buttons, switches, and levers

Before I show you how to connect the battery to the front of the bike, let’s first discuss your options for the “cockpit” area.

Generally speaking, a TT/Tri Di2 bike uses a set of SW-R9160 remote triathlon shifters for the aero bars, and one of the TT/Tri brake levers. The one you use depends on the type of brake on your bike:

All of these have just one button per lever, and the difference between the Ultegra and DURA-ACE rim brake levers is purely the weight and the looks.

With your switches and levers selected, how are you going to connect all of those?

Well, there are a couple of methods, and they depend on how you want to, or can run wires through your base bar.

This is what Shimano suggests in their dealer manual:

base bar setup dealer manual

The two shift buttons on the aero extensions are connected to a four-port junction, the SM-JC41. A Y-splitter (EW-JC130) is used to connect both brake levers to the junction. Finally, the fourth port of the JC41 junction is used to connect all of this to the rest (rear) of the bike.

Alternatively, you could use two SM-JC41 junctions, and not use a Y-splitter at all. You end up with one unused port, so you can connect extra shifters if you want to.

base bar setup alternative

In the end, it doesn’t really matter - as long as the components are connected to each other, they will work. If you want more open or unused ports in the first setup, simply use two EW-JC130 Y-splitters instead of one.

Di2 is pretty flexible, and you can get creative. Use one SM-JC41, two, or ten. It’s all up to you. There is no limit to the number of SM-JC41s or EW-JC130 Y-splitters you can use in a Di2 system.

Not sure about your wiring? Feel free to leave a comment below or send me a message.

Putting it all together

With the wiring for the front and the rear of the bike set, it’s time to figure out how to connect both ends of the bike.

bike schematic - front and rear separate

You may have already spotted this, but there is a small problem - the front and the rear of the bike use a different wiring standard. 12-speed Di2 bikes use the EW-SD300 wires, while the TT/Tri shift units all use the SD50 wiring type.

Fortunately, Shimano makes an adapter that will let you connect the two. The EW-AD305 adapter is a small, inline unit, that simply lets you connect these two wires.

AD305 graphic

You run an EW-SD300 wire from the battery into the down tube, connect the EW-AD305 to that, and then run an EW-SD50 wire to the SM-JC41 at the front.

This is what that looks like:

bike schematic - all connected

The purple box in the down tube is the EW-AD305. It connects the front of the bike (yellow EW-SD50 wire) to the battery using the green EW-SD300 wire.

Note that the location of that EW-AD305 is up to you. I’ve put it in the down tube in the image above, but it can also go closer to the handlebar, near the bottom bracket area, or even in the seat tube.

Where you put the EW-AD305 affects the lengths of the EW-SD300 and EW-SD50 wires connected to it.

Speaking of wire lengths - in this case, the Shimano guideline applies:

Measure the distance between the two components by running a piece of string along the frame. Then go the next size up, and then add 50mm to get the recommended wire length.

For example, if the measured distance is 330mm, the next size up is 350mm, and Shimano would recommend a 400mm wire.

This should give you enough slack in the wire, and you can generally hide any excess wire in the frame.

Set up the bike

That’s it! You should now be able to order all the components and extras you need.

Next up is actually building the bike, and putting it together. The installation guides section should help with that. It’ll show you how to install the derailleurs, connect the wires, set up derailleur limits, and more.

Important: Enable Synchronized Shift

After installing and setting up the bike, be sure to enable Synchronized Shift. This will trigger automatic front shifts as you shift the rear derailleur up and down the cassette.

To enable synchronized shift, double-press the button on the rear derailleur and the bike will cycle through the modes.

Read my complete guide to synchronized shift to learn more about setting up synchronized shifting.

For your convenience, here is a list of every component mentioned on this page. Each component page also has a link to Amazon/eBay/Wiggle, in case you feel like shopping there.

Anything you purchase through those links will earn the site a small amount of money, but only use them if it makes sense - don’t spend $100 more on components just to support the site.

Small parts

12-speed derailleurs / battery

TT/Tri shifters

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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