With so much braking now being done by the torque reversal upon the rear axle of the ERS, several teams have reduced the size of their brake discs - mainly for weight and aerodynamic benefit. In most cases, diameter and thickness has been reduced by 3mm than the maximum allowable 28mm. Six-pot calipers have been replaced by four-pots at Mercedes, Red Bull, McLaren and Sauber. (And some engineers have even suggested that it would be possible to run no rear brake discs at all if the regulations allowed it, with all the braking done by the ERS!). In the event of a failure of the MGU-K unit a back-up hydraulic system will intervene. But the downsizing of the carbon fiber discs provides the system with a limited efficiency, as was witnessed in Sakhir last Sunday, but also in Canada last year, a track which is particularly demanding on brakes and where Lewis Hamilton suffered a brake disc failure when the MGU-K unit ceased to function following an electrical break down.