Building Shimano Di2

Building Shimano Di2

Even though it’s been more than 20 years since I worked full-time as a mechanic, I still try to do as much of my mechanical work on bikes as possible. I do this for a few reasons. The first is simple; I like to work on bikes and my time in the garage is pretty special to me. Then there’s the fact that in working on bikes I learn more about how they operate and the quality with which they are built.

That said, bikes have evolved enormously since I last attended a tech seminar with Shimano. Pardon the obviousness of the following statement, please: Di2 is a group of wholly different operation than all its cabled forebears. I’ve learned how to adjust the shifting from a trim standpoint, but I honestly had no idea how you’d build a bike with Di2, so when the opportunity came up to review a Felt AR FRD, I figured that was the perfect opportunity to learn how to install Di2 from Big Blue’s well-trained staff.


My contact at Shimano, Dustin Brady, arranged for tech Nick Murdick to walk me through the process. Nick teaches tech seminars and provides support for a broad range of Shimano-sponsored events. He reminded me of any number of guys I worked with over the years—calm, focused, resourceful and organized. Guys like him have always been in short supply.

Following installation of the headset, fork, stem, bar and levers, Nick began running the wires that would lead from Junction Box A, mounted under the stem, to Junction Box B, inside the frame.


This is what Junction Box B looks like with one of the four wires inserted. Pretty simple.

The basic premise of Di2 is that the signals from the levers travel to Junction Box A, mounted beneath the stem. You want that there so that you can make adjustments to the derailleur trim easily. I’ve managed to make the sorts of adjustments on the fly that I would otherwise have to pull over for with a mechanical derailleur. So two leads enter the junction box, and one lead (or E Tube as Shimano calls them) leaves and enters the frame.

That lead goes to Junction Box B. Three other wires plug into that junction box; there’s one for the battery, to power the entire system, then there’s one for the front derailleur and finally one for the rear derailleur. That’s the system, in a nutshell.


The trick, of course, is getting the wires routed though the frame. Shimano uses the Park Tool IR-1 Internal Routing Cable Kit for this operation. This kit is essentially three plastic-coated (of course they are blue) cables with magnets affixed to the ends, plus a guide magnet. This operation strikes me as equal parts ingenious and forehead-slapping duh. That said, once more than one wire is running through the down tube, it’s pretty easy to grab the wrong wire with the guide magnet. The process starts with connecting two magnets with cables in the down tube, then pulling the magnets back out the top tube and substituting a wire for the upper magnet and then drawing it down through the frame. Rinse and repeat.


The other thing a frame without an excess of resin inside offers as a satisfying click that resonates through the frame when the magnet makes contact with the wire. Also, as I watched Nick do this, it occurred to me that this operation would be a great deal harder with many frames that have spider webs of resin clogging their tubes. And what of steel builder? How the hell do they manage?


If the opening at the bottom bracket looks large, that’s because Junction Box B has to pass through it with two wires running alongside it. The one blue lead emerging from the down tube is for the rear brake cable. The AR, like a growing number of frames, uses a direct mount rear brake.


Here, Nick feeds a wire into the frame. Even once several wires were run, it wasn’t hard to draw wires back and forth. IMG_9504

Here is a closeup of the contact for a wire connected to a magnet. In addition to the magnet, there’s a rubber sleeve that fits over the connector to make the attachment more secure.IMG_9505

Early internally routed Di2 bikes had an unusual issue in that they developed an odd buzzing. Shimano devised these little clips, not unlike zip ties to keep the wires from vibrating around in the frame and turning it into a, uh, giant thing that vibrates.

The seat tube run was so simple the wired magnet was almost unnecessary. IMG_9514

Junction Box B with all the leads attached. One thing I failed to shoot was the cradle that Felt designed to integrate with the seatpost and hold the battery in place. The cradle keeps the battery from rattling around all noisy-like in the frame. IMG_9517

Shimano’s direct-mount brake comes with a plastic guide to aid installation. I’d had no idea that it wasn’t as straightforward as a traditional single-bolt brake. Once the two bolts are installed but not fully tightened, the plastic guide can be pulled upward for removal. It’s very nearly a three-hand operation. IMG_9519

When you’ve built as many bikes as Nick has, you’ve got more torque values memorized than there are highways in California.  IMG_9522

Once Nick got to derailleur setup, he was nearly on autopilot. While the front derailleur took a few minutes, the rear derailleur went quicker than a banana peel on concrete. And while I’ve yet to adjust the mechanical front derailleur perfectly (I have gotten close), thanks to the auto trim on the Di2 front derailleur makes it nearly easy to adjust perfectly. We’re not talking automatic transmission easy, but certainly easier than a five-speed stick. IMG_9633

The lever body features three connectors. One runs from the lever to Junction Box A. The other two can be used for auxiliary shifters. I’ve ridden Di2 plenty, but until this bike, I’d never had the opportunity to use either of the auxiliary shifters. IMG_9634

This is the sprint shifter; because it’s mounted on a simple plastic clip it’s easy to position exactly where you want it. I went for a spot just below where my thumb wraps around the bar. All I need is roll my thumb down slightly to brush the button to execute a shift. The button only executes upshifts, though as I understand it, you could theoretically program it to make downshifts—not that I can fathom why you’d do that.IMG_9680

This is the other auxiliary shifter, for climbing. Because of the two buttons this one can obviously shift both up and down. The smaller, slightly lower button downshifts, while the other button upshifts. It’s a system that makes implicit sense to a guy like me—a guy like me meaning that I’m more likely to downshift than upshift these days. 

I spent more than four hours with Nick going through the assembly. I’m sure that if he hadn’t been demonstrating for me, he could have had the entire bike finished in that time. On my own, I suspect I’d have spent upward of six hours trying to do this and I can’t be certain I’d have ever figured out how to run the wires without the aid of the Park magnets. I’m reminded of the Foghorn Leghorn cartoon where he goes to lift the lid on the box, stops and says, “No, I might still be in there.”

Like that.

The question on my mind is if this is the first electronic shifting system on the market—and it’s this good—I wonder what these systems offer two generations down the road.

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  1. adam

    I can’t wait for the review of this bike. It’s on my shortlist, I think Felt is underrated in the engineering department.

  2. Sam

    Timely article and one I’ll probably come back to again and again when I’m ready to proceed with my maiden build. One question which I wonder if you could help with, some frameset come in electronic or mech specific frame. What are the differences in term of building electronic vs mech and how is this manifested on the frame?

  3. Craig P

    Great article! I’ve been thinking that it would be fun ( and less expensive ) to build my next bike myself. Thanks for the inspiration !!

  4. Mark

    The next generation will be wireless, as per SRAM’s latest offering, so all this jiggery-pokery will be redundant. Good thing too. This was an easy install, some are time-sucking nightmares, with right angle bends everywhere.

  5. AusTex

    So here’s the thing about all the electronic systems I found out the hard way…

    They are strong, very strong and they have no shear pins so if you get a chain wrap, or drop a chain and try to shift out of it, like you can with mechanical systems you’re liable to snap the linkage for the front derailleur or the cage itself and there goes $400 into the gutter. If you’re away from your LBS do you really want someone experimenting on your Di2?

    In my mind the Di2 offers what the Campy mechanical system has had for a decade, the ability to upshift and downshift more than one gear at a time. For me, my next bike (because its hard to have too many) will be Campy mechanical.

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