As before, the electronic group shares mechanical Dura-Ace's crankset, chain, cassette and brakes. Elsewhere, there have been big changes though. The old wiring has been ditched in favour of the slimmer, modular, dual-core E-Tube configuration used on Ultegra Di2, and now Alfine Di2 too –  apparently “the aim is to have a standard wiring system through all of Shimano’s Di2 groupsets”. 

Dura-Ace Di2 9070 is looking good, even in prototype form

This means we now have the option to mix and match the groups, so expect some value-added options in complete bike specs next year. It also means it’ll no longer be necessary to heat-shrink the connections as with current Dura-Ace, as Di2 9070 will use the same waterproof connections as Ultegra.

The next big change is a new optional internal battery, available to fit either in the seatpost, seat tube or down tube. To get around any charging issues, a new junction box port will allow the battery to be charged in situ. As the internal battery doesn't require the same level of weatherproofing as the external version (though it's still fully sealed), it weighs 60 percent less.

The Dura-Ace Di2 9070 battery is designed to be housed inside the frame or seatpost

Shifter ergonomics have been improved, with an 8mm longer button and a new cable path on the brake lever, and the derailleurs are said to provide slicker shifting. They've also been significantly reduced in size and, more importantly, weight – as a pair they come in at 90g less than the current models. 

The goal Shimano set out for the new Dura-Ace Di2, weight wise, was a 160g reduction overall, giving it parity with the mechanical group. The whole system weighed just 1,995g when we saw it in prototype form earlier this year – just 30g heavier than the mechanical version – and Shimano say the production transmission should still weigh sub-2,000g.

New for Dura-Ace 9070 is programmable multi-shift. This means that if you hold the button down, the rear derailleur will keep changing until it gets to the end of the cassette, reaches a pre-set (by you) number of shifts or you let go. This wrests back the major advantage of Campagnolo's EPS.

The Dura-Ace Di2 9070 derailleurs are said to be more compact, lighter and provide slicker shifting

"Tunability" is a buzzword Shimano kept returning to at the launch. The new Di2 can be customised to suit your needs, with the PC interface (sold separately) allowing you to configure everything from shifter operation – want to switch over the left and right shifting? Well now you can – to the speed at which multiple shifts are made. 

Myriad secondary shifter options to tune your bike to your needs are also available, including sprinter triggers, time trial shifters, tri shifters and a new top mounted Roubaix shifter. No additional wiring is needed to add these, and there's no setup procedure either – the new junction box below the handlebar allows you to plug-and-play. 

The under-stem junction box is the key to Di2 9070's versatility. It allows the addition of numerous 'plug & play' accessories

There are two versions of the box – SM-EW90-A, with three ports, and SM-EW90-F, with five ports – and combined with the three ports on the back of each STI lever, this offers a huge range of setup options.

The five-port box is also designed to work with Shimano's latest Flight Deck computer (SC-R770) via an ANT+ wireless connection. When used with mechanical Dura-Ace, the touch-screen computer shows what gear you're in and can be paired with ANT+ devices to show your speed, cadence and heart rate. It also offers remote operation via a bar end switch. Connect the Flight Deck to Di2 9070, however, and it comes into its own. You can use it to adjust your gears, show the battery level, perform simple system diagnostics and even programme the multi-shift function. 

Shimano's latest Flight Deck computer can be used with both mechanical and electronic versions of Dura-Ace

Also in development is a time trial/triathlon-specific version of the new transmission called Dura-Ace Di2 9071. We'll bring you more on the new electronic groupset as soon as we get it.