Shimano Dura Ace 7971 Di2 10 Speed TT STI Lever Set

Review from Cycling News

Shifting performance that's second to none



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.



Tap the rear upshift button and you're rewarded with that telltale 'ka-chunk' of a perfectly executed change. Go a step in the other direction and the transition is so seamless as to be virtually imperceptible save for the feeling in your legs of an easier ratio. In fact, it's almost too smooth.



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.



Is it possible to confuse the system? Yes, but only if you're really trying to. The only way we could get Di2 to miss a step was by doing things we'd never consider doing anyway and even then it sorted itself out impressively quickly.



Weight-wise, there's only a modest penalty to be paid. In comparison to the 7900 mechanical analogue, Di2 only adds about 50g by our measurements - right in line with Shimano's claims.



Why it works the way it does



The key, of course, is the small stepper motor in the front and rear derailleurs that precisely controls their movements but it's not necessarily a matter of how fast they move - in fact, the front derailleur actually moves slower on average than its mechanical equivalent. Instead, Di2's advantage lies in the fact that it does things automatically that would be impractical on mechanical systems.



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

In conclusion...

Whereas we've been somewhat underwhelmed by Dura-Ace 7900, Di2 has blown us away.

Rating: 5/5