Pro road racers who use Berner’s uniquely oversized rear derailleur cages have sworn time and again that they reduce drivetrain friction thanks to their humongous 15-tooth pulleys. Now, Jason Smith of Boulder-based lab Friction Facts has confirmed that claim with his own independent testing. Berner-equipped racers such as Fabian Cancellara, Edvald Boasson Hagen, Philippe Gilbert and others will be relieved to know that the advantage is genuinely there – but it’s also pretty small at just 1.76 watts compared to a standard Shimano Dura-Ace setup. According to Smith, the Berner cage derives its advantages from three key principles: pulley size, cage tension and bearing friction. Smith isolated the effects of pulley size by machining 10, 11, 13 and 15-tooth pulleys to the same bearing bore and then transferring the same two cartridges between tests. Lower chain tension (i.e., the chain tension between the lower rear derailleur pulley and the bottom of the chainring), cadence, and chainring and cog sizes were kept constant throughout testing. The difference from best (15-tooth) to worst (10-tooth) was very small but still measurable at just 0.49 watts – similar to what others have c...