It's good to see them designing specifically to reduce this issue, with cooling fins on the pads and rather pleasing bimetallic rotors. Shimano identified overheating as being of particular concern for discs on the road - longer, faster descents (and smaller rotors) being likely to result in rotors and pads heating up more than off-road. We used 160mm up front and 140mm at the back, simply because this was what suited our test bike. Shimano's road disc brake system has been designed for use with 140mm or 160mm rotors, with the idea being that users can choose the size to suit their weight and intended use.
We tested them with a 2014 11-speed Ultegra Di2 groupset, which will be reviewed separately - here we are focusing on the brakes themselves. Shimano has launched these brakes as non-series they don't strictly form part of the Dura Ace or Ultegra groups (although being Di2-specific, they need to be paired with one or the other). They are only compatible with Di2 electronic shifting - the hydraulic reservoir fits in the space where the mechanical gear-shifting gubbins would be - so the controls look a bit more, well, normal than SRAM's, too. With SRAM's fancy-pants Red hydraulic brakes still not available following their embarrassing recall, Shimano might well expect to sell quite a few of these, their first hydraulic disc brakes for road. There are a couple of kinks still to be ironed out and they’re not cheap, but they’re still a compelling upgrade. Shimano’s excitingly-named BR-R785 hydraulic road bike disc brakes are a genuine improvement in braking power and control.