Field Tested: Shimano Ultegra Di2

First off, this is NOT Ui2. There is no such thing and never will be. A lot of folks think the “D” is for Dura-Ace, the trim level where the electronically-controlled shifting first appeared. It actually stands for “Shimano Digital Integration Intelligence” – Di2, get it?
In the six years since it’s introduction, the concept of a battery and servo operated shifting system has proven itself capable in nearly any conditions, from Grand Tour victories to continent-spanning tours—in the winter mud of cyclocross to the drenching rain of spring in Belgium. It has even come to mountain bikes with the introduction of the latest Shimano XTR group. (Read about that in our sister magazine, Dirt Rag.)

The group we have in to test is the second generation Ultegra 6870 Di2 group, essentially a trickled-down version of the current Dura-Ace version. It features the second generation of the E-tube connector cables and firmware that make it super simple to set up and adjust, and is cross-compatible with most of the Dura-Ace 9070 system and even some of the previous generation 6770 Ultegra group. Just as with a mechanical groupset, you can mix and match shifters, derailleurs, cranksets, etc., with the lone exception of the sprint shifters that are only compatible with Dura-Ace 9000.
The plug and play nature of the E-tube connectors means that setup time is actually easier in many ways than a mechanical drivetrain. The biggest concern is how to route the cables. If you have full-length internal cable housing, you’re all set, the E-tube will slide right through. If you have internal routing with a bare cable moving inside the frame, you have to get creative.
This Specialized Tricross Disc was an excellent candidate, as it has a wide opening under the bottom bracket to route cables, and required only a tiny hole drilled in the bracket where the mechanical cable enters. Because this piece is welded into the downtube I had no concern with drilling an extra hole in it, and the bike can still be returned to mechanical shifting if desired and the hole will even be covered.

Installation

Click through this gallery for some insight on how we installed the system.
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Shimano offers a few different options for connecting things and powering them, with internal and external versions of the junction box and the battery. In this case I used the external junction box and the internal battery, which slots perfectly in the Shimano PRO seatpost, though can be adapted to fit nearly any round seatpost.

If you have to route the cables externally, you have some challenges. I used a piece of Shimano’s own stick-on cover and some cable ties to hide it behind the chainstay. Standing next to the bike there are almost no visible cables, so I’m very pleased with the installation.

Shifting

The rear derailleur moves the chain across 11 cassette cogs, and is available in a short-cage and a mid-cage, pictured here, which is compatible with up to a 32 tooth cassette for the steepest terrain.

The crankset is shared with Shimano mechanical groups and features the new four-bolt arms that are used for both compact and standard chainring sizes. In fact there is a new 52/36 combination that should appeal to nearly everyone. Pictured here is a more cyclocross-friendly 46/36.

The reason this practice wasn’t adopted with five-bolt chainrings is that a 110 bcd chainring with 53 teeth was usually far too flexy to shift well. Shimano’s new hollow and 3D chainrings solve this problem and shift unbelievably well. In fact, it’s hard to believe the Di2 system could shift any better than the mechanical groups already do, but it does. Not only is a switch between chainrings available at the touch of a button, it operates as quickly as a rear shift, and can even shift under load.

Another cool advantage of Di2 is the ability to mount shifters virtually anywhere—after all, it’s just a button. These climbing shifters are great for riding on the tops and shifting the rear derailleur. Time trial bikes make good use of this feature by mounting shifters in all sorts of places. You can even reprogram the buttons on the main shifters to alter the shift functions if you’d like.

When you click them, the action is similar to that of clicking a mouse button. It moves a little and there is some tactile feedback that it worked. The shifts happen instantly thereafter, no timing or soft pedaling required. You can jump on and just abuse it and it will shift fine. You can hold the button for multiple shifts, but it seems faster to just click it a bunch of times as fast as you can. Because it’s so easy and shifts so well, I found myself shifting more often than I probably would on a mechanical setup. Think > Click > Shift. It’s that simple. The wide range and smaller steps on the 11-speed 11-32 cassette also means you can make tiny adjustments to your cadence without interruption.
So far the Di2 system has been flawless and nearly without fault. The only minor problem I has was a shifter that wasn’t working until I realized the E-tube wire had become unplugged under the shifter hood. Yes you have to keep it charged, but it’s not like your phone. We’re talking once every couple months, depending on how often you use it. If it does get low, you’re not screwed. On one ride the battery did get low enough where the front derailleur stops working, a signal that you should probably head home—which I did, still shifting away with full control of the rear derailleur.

I will say the move to electronic shifting is incredibly easy too. Once in a blue moon I hit the wrong button, but for the most part you don’t need to reprogram your brain with new shift systems or control locations. Everything works exactly as it always has, just faster and crisper. There is a slight loss in tactile feedback of pushing that big brake lever inwards to bang out a downshift, but I can’t say I really miss it.
One drawback is that in extremely bumpy terrain (such as cyclocross or in spring classics) it might be harder to shift, and you see some pros running the mechanical drivetrain at races like Paris-Roubaix. The other downside is that it can be difficult to shift with bulky gloves on, as you lose the tactile click feel and the buttons are inherently smaller than moving the entire lever blade.

Braking

Included in our test setup is a pair of Shimano’s CX75 mechanical disc brakes. They are compatible only with short-pull levers (like road shifters) and are much smaller and lighter than the less-expensive R505 calipers. Like most mechanical disc brakes they operate the pad on one side only while the other side is fixed, though still adjustable for pad wear.

Now, I just have to say, in my experience Shimano brakes are the best on the market. Road, mountain, disc, whatever. That’s why I was a bit disappointed in the CX75s. My initial setup resulted in a super-long lever throw, which does help with modulation, but certainly didn’t instill confidence. It also required quite a bit of muscle to get some stopping power, more than a traditional rim brake. Because there are no barrel adjusters built into the design, I cheated the caliper’s lever arm up the cable to get a shorter throw, which got the lever pull where I wanted it, but still didn’t find the braking power I was looking for.

Speaking of, I’ve also been riding another bike with Shimano’s R785 Di2 levers for hydraulic brakes, and they are sublime. All the braking power you could dream of, with silky-smooth operation, at the tug of a single finger. Probably one of the most game-changing products I’ve sampled. But you’ll have to wait for that bike review for more…

Conclusion

After spending a few months with Di2 I’m really struggling to compare it to cable actuated derailleurs. It’s just so, so different that it doesn’t seem fair. Is it “better”? Well it shifts perfectly every time, the cables never stretch, it never goes out of adjustment and the battery issue is basically a non-factor. Would I install it on my everyday lock-up bike? Of course not. But on a nice road or cyclocross bike, it has some clear advantages over its cable-operated cousin. I, for one, welcome our robot shifting overlords.

Pricing

Most likely anyone’s first introduction to Di2 would be when purchasing a complete bike, but if you want to build a set à la carte, here is the MSRP pricing from Shimano:

• ST-6870 shifters: $379.99
• RD-6870-GS rear derailleur: $279.99 ($10 less for SS cage)
• FD-6870 front derailleur: $269.99
• Junction box A (under stem): $129.99
• Junction box B (inside frame): $34.99
• Wire (prices vary depending on length): $29.99 – $34.99
• Internal battery: $179.99
• Climbing switch: $129.99