As disc brakes have gotten more popular on road bikes, the industry is slowly answering questions about their relevance for skinny-tire riders in terms of reliability, safety, power and, for performance-minded riders, aerodynamics. Specialized, currently the only bike company with its own wind tunnel, attempted to answer that last question in the newest video in its “Wind Tunnel” series. The company ran two identically equipped 2015 Tarmacs through its wind tunnel: one with conventional rim brakes and one with Shimano’s new R785 hydraulic brakes. The results may surprise you. Head-on, there was no quantifiable difference in drag. Only when the bike was angled 10 degrees to simulate a sidewind did Specialized see a split. In this scenario, disc brakes are slightly slower. How much? About eight seconds over a 26-mile course, the company estimates, based on an analysis that takes into account a wider variety of speeds than the standard 30mph wind-tunnel fan speed. One interesting quirk: Since discs are only on the non-drive side of the bike, the aero deficit is most significant when the sidewind comes from the rider’s left. Chris Yu, Specialized’s aerodynamics engineer, said they found that when testing yaw on the drive side, the spinning wheels effectively shielded the brakes and minimized the difference. So in the real world, where yaw angle (the direction of a sidewind) shifts dynamically depending on weather and terrain, the aero gap from rim to disc brakes may be even less. And, Yu reminds us in the video, the speed boost from being able to brake later, ride with more confidence, and better modulate speed could make up for the rest of the time difference.