Closer look: Shimano Dura-Ace Di2 transmission

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In general, the Di2 levers have a much more finished look and feel about them relative to 7900. We still want more feedback from the buttons and a stiffer paddle but aside from that, it's nearly perfect.(Image credit: James Huang)

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Di2's most obvious benefit is its remarkably smooth and precise shifting action, even as compared to the most ideally adjusted and tuned mechanical setup from Shimano or any other component manufacturer - yes, even Campagnolo. It hits its target each time, every time, and absolutely, positively nails each gear without fail in virtually any condition. If there's such a thing as a tireless 'shift robot', this is it.

Shift action is especially astounding up front, particularly when used in combination with Shimano's ultra-stiff Dura Ace chainrings. The Di2 front derailleur faithfully moves the chain onto the big ring - or down to the inner cog - even when mashing the pedals in a low-cadence grind with an uncanny silkiness we've experienced with no other system. Think about how well one-tooth shifts work out back, translate that up front and you've pretty much got the idea.

Even better, however, is that Di2's performance is unflappably repeatable and doesn't degrade over time. Commands issued by the levers are always received and translated exactly as intended at the other end with no 'telephone effect' that can plague cable-actuated systems as a result of cable wear, housing compression, or system contamination. Barring things such as a bent hanger, what you get on day one is seemingly what you get for every day afterwards.

For example, Di2's rear derailleur pulleys actually overshoot the centerline of each cog slightly to help nudge the chain into place - but only momentarily. After a very brief pause, it then centers itself for quieter running. In theory, you could do this with a mechanical system, too, and in fact most already do for downshifts anyway. But Di2 does it in both directions, which just isn't possible with conventional ratchet internals.

On the other hand, the front derailleur applies just the right amount of pressure so that the chain moves only at its prescribed gate instead of possibly overriding the designated timing by just muscling it over. Again, you could do this with a mechanical system, too, but it'd required you to precisely apply the appropriate amount of force at the lever and then hold it there until the shift is completed - not exactly ideal in the heat of competition. With Di2, you just hit the button and it does the thinking for you, reliably hitting the next available gate.

It's the same story in the other direction as well. Instead of a strong return spring abruptly jerking the chain down at an inopportune position, Di2 smoothly coaxes the chain down to the inner ring at the prescribed release point. Admittedly, Campagnolo's Ergopower system is capable of this as well unlike mechanical systems from SRAM or Shimano but again, with Di2 it's only a matter of hitting a button - no extra thought required.

A word on shift speed, too: modern drivetrains rely more than ever on their highly engineered systems of gates, ramps and pins to help move the chain from gear to gear and Di2 is no exception, especially given that it shares its crankset and cassette components with Dura-Ace 7900. As a result, 'shift speed' is strictly speaking more a matter of how quickly the chain encounters one of those gates when a shift is initiated than how fast the derailleur moves, making Di2 no quicker than anything else.

But from a user standpoint, Di2 is still effectively faster since once your job is completely done once you hit the button - at least for single shifts. There's no holding the lever for that split second while the shift is completed or ever-so-slightly nudging the chain over with a hair more force. With Di2, it's tap, shift, tap, shift - and that's it. It's a subtle distinction and one that riders who haven't tried Di2 for themselves will invariably debate but it's real and noticeable.

With no mechanical guts to occupy a bunch of room in the lever bodies, Shimano engineers had fewer restrictions in crafting their shape. As a result, the Di2 levers are slimmer in width and smaller in girth than 7900 and have more gently rounded corners that feel more natural in your hands. In fact, they're a much closer approximation of the previous Dura-Ace 7800 generation but thankfully without the sharp dip in the top surface.

Shift button placement closely approximates the mechanical analogue for a reasonably easy-learning curve for current Shimano users, at least when on the hoods. Here, the buttons' fore-aft offset is a familiar layout and their differentiated height and texture are easy enough to tell apart. When in the drops however, Shimano users accustomed to reaching up for the inboard paddle and to the very tip of the main lever to shift in the opposite direction will be caught out a bit.

Instead of the familiar top-bottom layout, you again have the same fore-aft button offset - the main lever is stationary and you can push on its tip as much as you'd like but it's not going anywhere. In this situation the buttons' height and texture differences aren't as easy to discern and your fingers have smaller targets to hit from this angle. We got used to it after a few rides but still would have preferred a more natural transition.

In either case, we still want more tactile feedback from the buttons to indicate when a shift has been properly initiated. The exceptionally short button throw is barely a couple of millimeters but the return springs are rather stiff and it's still possible to move the button without actually hitting the contact points in the switch - the clicks are just too light and inaudible. In contrast, the time trial shifters have far more feedback.

No discussion about Di2 would be complete without addressing the battery life and we're pleased to report that unless you're an exceptionally neglectful rider in terms of maintenance, it's simply not an issue. On average, Shimano says a full charge will last about 1,600km (1,000 miles) but that's heavily dependent on shift frequency - not to mention the number of front versus rear shifts - and other factors such as temperature.

There's no sugarcoating the issue here; Shimano's Dura-Ace Di2 components are horrifically expensive. The rear derailleur alone retails for US$900 - nearly the cost of an entire SRAM Rival group - the front derailleur is only slightly cheaper at US$750, and a pair of shifters fetches US$930. And that's not even including the required wiring harnesses (US$450), the battery (US$100), or its proprietary charger (US$120). Add it up and it's a choice between a fancy Di2 transmission or a decent used car.

Shimano naturally intends Di2 to be filled out with the remaining Dura-Ace 7900 bits and admittedly it seems to work best that way given the brake calipers' modified cable pull ratio and the crankset's ultra-stiff outer chainring. If you're so inclined, other cranksets with standard spacing will work, too, as will other brake calipers though the feel at the lever - and the stopping performance - won't be quite as good.

Shimano Dura-Ace Di2 is not perfect. There are some ergonomic quirks, it's still not the best option for multiple shifts and it's incredibly expensive. But the level of refinement is also unlike anything else out there and we would offer up the argument that truly innovative developments usually don't come about by playing it safe and maintaining the status quo. It's not a game changer just yet but it just might be once the technology invariably trickles down and prices drop.

"It absolutely makes sense for other disciplines. We've already got guys who have been racing with it in cyclo-cross and have done well with it," said Shimano US press officer Devin Walton. "It makes a lot of sense for mountain bikes but there are a lot more obstacles on a trail. These parts are a lot more expensive to replace and repair and that's definitely a concern plus the level of potential contamination steps it up a notch. It makes hypothetical sense but to make the jump into real world execution is a whole other issue."