Are discs a drag? Wind tunnel testing disc brake road bikes
Are disc brakes ready for pro racing?
It seems a simple question, on its face. Discs are simply better brakes; they stop better, under a wider range of conditions, than any rim brake. But racing isn’t all about stopping. Racing is about speed, the pursuit of even the most marginal gains to give a leg up over the competition. In this dog-eat-dog world, do discs stand a chance?
Ask pro riders, pro mechanics, team directors and staff whether they believe disc brakes should be allowed in professional road racing and the opinions will be as diverse as those asked. Both sides believe bikes will be safer, easier to use, and faster with their chosen stoppers. Both are right, and wrong.
We set out with a focus on speed. Safety and ease of use are somewhat subjective, with a heap of arguments on both sides, but speed is quantifiable. Speed, in most instances in road racing, is synonymous with aerodynamics.
The question, then, truly is simple. Can disc brakes keep up with rim brakes in the wind tunnel?
Repeatability and accuracy are the twin goals of any wind tunnel testing. To that end, we compared two mostly identical frames, a pair of Specialized S-Works Tarmacs, one built for disc brakes and one for rim brakes. Other than the brake mechanisms, the shape of both frames is identical.
Two sets of Rail52 wheels from November were built, using the same rims; one set was built with disc hubs while the other used standard hubs.
Both bikes were built identically, with Shimano Dura-Ace Di2 drivetrains and Specialized cranksets. The brake levers were different, of course, as the disc bike used R785 hydraulic levers and the other was mounted with standard Dura-Ace Di2 levers.
A full yaw sweep, or wind angle sweep, from -20 degrees to 20 degrees, was key as disc brakes are only mounted on the left side of the frame.
Testing was performed at the A2 Wind Tunnel in North Carolina, our preferred test facility. In all, 70 runs were performed.
The rim brake version of the Specialized S-Works Tarmac is faster than the disc version across all wind angles, with the disparity increasing as the wind moves from left to right.
Discs are the most detrimental in strong crosswinds, specifically right-side crosswinds. At -20˚ yaw, which is an uncommon but still feasible wind angle, the difference between the two Tarmacs is an astonishing 8 watts.
Across more common wind angles, the gap between disc and rim-brake bikes is generally between 1 and 3 watts — not much, but not an insignificant amount either.
Though Shimano’s R785 brakes are excellent, they still use a caliper that was originally designed for mountain bike use. We’re testing a holdover product, designed to allow Shimano to put a toe in the disc game while the UCI deliberates over the brakes’ legality in road racing.
Therefore, we are confident that the small gap present at low wind angles could be closed over time through improved integration and engineering. The larger aerodynamic losses at high wind angles are likely caused by the disc rotor and therefor are unlikely to be decreased significantly.
It is worth noting, too, that Specialized itself recently performed a similar test and saw a smaller difference between the disc and non-disc Tarmac. That test was performed with a rider, and ours was not; adding a rider adds another variable, but it also changes the way the wind hits the back end of the bike. Neither protocol is a perfect one.
Rims that do not require a brake track could be shaped differently, improving aerodynamics. The brake calipers will likely shrink over time as well. Shimano’s new Flatmount caliper mounting standard brings all the hydraulic lines inside the frame and mounts the caliper flush with the frame and fork. Frames are already being released using this new standard and we expect to see a compatible high-end disc offering from Shimano, perhaps at the Dura-Ace level, within the next 18 months.
We tested brakes with standard Dura-Ace shift levers and hydraulic-ready R785 levers, the latter of which has a pronounced, bulbous head that holds the hydraulic reservoir. These larger levers certainly had some negative impact on the disc bike’s results. If Shimano and other brands succeed in decreasing the size of their levers to match normal mechanical levers, this disparity would disappear.
It is also important to note that the Tarmac Disc is part of a very small group of race-ready disc frames. The vast majority of disc road bikes available today fall outside the race mindset — they take bigger tires, are built with slacker geometry, and are generally more adventure-oriented. For these types of bikes, a few watts lost is largely irrelevant. The fat tires and upright positions require far more power to overcome.
The UCI is considering the use of disc brakes in pro racing within the next few years. For professionals, even single-digit wattage losses are a big deal; teams spend thousands gaining far less. That means that the results shown here make it unlikely that any pro rider would willingly ride a bike with disc brakes unless, of course, everyone moved to discs at the same time. This would level the playing field once again.
Editor’s note: Due to a miscommunication with the A2 wind tunnel, this story published in Velo Magazine with a misleading graphic. The negative wind angles represent the drive side, while the positive wind angles represent the non-drive side. We regret the error.