Before you whip out your credit card to buy the latest aero wheels in an attempt to take the lead on a local Strava segment, consider this: Those aero-savings numbers that accompany the bikes and equipment you buy are determined for elite racers, not you and me.
Aerodynamic benefits are real, but the questions we all need to ask before making a purchase are “For whom?” and “To what extent?” Brands often present a product’s aero savings in an “X watts saved at 40 to 50km/h (25 to 31mph) format.” That’s really fast.
To understand why brands use those speeds and not numbers more relevant to the average riders among us, I asked some of the leading industry experts in aerodynamics research to explain. Perhaps the most obvious reason brands test at those speeds is because aero equipment is primarily designed to aid the pro racers who can ride that fast.
More From Bicycling
“[Racing] was and is a main driving force behind aerodynamics research and development,” says Cannondale engineer Damon Rinard. Cervélo’s engineering manager Scott Roy concurs: “Thirty miles per hour was the typical speed achieved by professional athletes, in a time-trial configuration. From the very beginning, our aero development was focused on making top riders faster.”
This isn’t to say slower riders don’t benefit from aero equipment—after all, if you’re moving, you’re fighting air resistance—it just means that brands are more interested in measuring a product’s aerodynamic drag at pro speeds.
Another reason for testing that fast is because the data collected is more accurate at higher test speeds. However, the tunnels commonly used by bike brands were designed for cars and airplanes, vehicles that obviously travel much faster than a bicycle.
According to Silca president Joshua Poertner, an aerodynamics expert and former technical director at Zipp, of all the wind tunnels in the United States used to test cycling equipment, the San Diego Low Speed Wind Tunnel (LSWT) and Indianapolis’s Auto Research Center (ARC) are the most accurate and precise. If a test is run at 50km/h in these tunnels, the uncertainties, says Poertner, are “+/-0.4 percent speed and +/-3 percent drag.” But if a test is run at 20km/h, the uncertainties jump to “+/-1 percent in speed and +/-20 percent in drag.”
Those speed-related changes in accuracy affect testing at every wind tunnel, not just the ones in San Diego and Indy. When using these tunnels designed for other industries, the 40 to 50km/h test speed is slow enough that it’s in the realm of possibility for cyclists (even if only on downhills or when riding into a strong headwind), but fast enough to reduce those uncertainties to an acceptable level.
So why not just build a tunnel designed for cycling speeds—which, using data provided by Strava and Garmin Connect, likely averages about 25km/h (15.5mph)? Specialized did. It tests cycling equipment down to 12.5mph in its Win Tunnel (yes, Win). However, some experts aren’t convinced that Specialized’s tunnel is as accurate as the LSWT and ARC. To build one that is would cost a “shocking” amount of money, says Poertner.
Now, it is possible to estimate slower-speed drag savings from the available wind-tunnel data, but it requires some math—math that “only holds true if the airflow at 30mph follows the same pattern as it does at 20mph,” says Gerard Vrooman, Cervélo’s cofounder and, more recently, designer of 3T’s Strada and Exploro aero bikes. “If all of a sudden the flow starts tripping or sticking the shape better or worse, or completely different effects start happening, then it doesn’t work at all.”
Enve’s vice president of production and consumer experience, Jack Pantone, provides an example: “Some wheels are, quite literally, designed for 30mph performance. We found that dimples only made a difference at 48km/h. At 32km/h [20mph], dimples appeared to have no [aerodynamic] benefit.”
Though Poertner agrees with Vrooman that, in some cases, airflow can be different at lower speeds than at higher ones, based on his own experience, he’s found that the math works “exactly” when scaling data collected at 48km/h to 32km/h. Anything below that, he admits, he is uncertain of the data’s accuracy.
But assuming the math works, power and drag are nonlinear—it takes eight times the power to go twice the speed. That also means the potential power savings fall rapidly as the speed decreases. A significant 15-watt savings at 50km/h translates to a measly 1-watt savings at 20km/h, which is hardly substantial enough to build a marketing campaign around. That said, when looking at savings from the perspective of time, slower riders save more of it because they’re on course longer. Example: The same aero benefit that saves a rider traveling 30mph one minute every 30 miles saves the rider traveling 15mph two minutes every 30 miles.
[Want to fly up hills? Climb! gives you the workouts and mental strategies to conquer your nearest peak.]
When I set out to write this look at aerodynamics, my goal was to provide some clarity about why the data is presented at such high speeds, but also to suggest that brands provide aero-savings information at an average rider’s speed. I learned that there are good reasons for the former and difficulties in gathering the latter. Still, we deserve to know how much—or if—an aero product benefits us. After all, we buy this stuff and fund its development.
I’m not saying we should ditch the 40 to 50km/h claims; I’m just saying a lot of us would benefit from a second set of numbers. Or, better yet, a chart that shows savings—watts, time, or both—at a variety of speeds so we can determine if a product truly benefits us and, if so, how much.
Being armed with the information as it relates to the average rider could be the difference between completing the transaction on that set of aero wheels or deciding to spend your money on lighter ones.
A gear editor for his entire career, Matt’s journey to becoming a leading cycling tech journalist started in 1995, and he’s been at it ever since; likely riding more cycling equipment than anyone on the planet along the way. Previous to his time with Bicycling, Matt worked in bike shops as a service manager, mechanic, and sales person. Based in Durango, Colorado, he enjoys riding and testing any and all kinds of bikes, so you’re just as likely to see him on a road bike dressed in Lycra at a Tuesday night worlds ride as you are to find him dressed in a full face helmet and pads riding a bike park on an enduro bike. He doesn’t race often, but he’s game for anything; having entered road races, criteriums, trials competitions, dual slalom, downhill races, enduros, stage races, short track, time trials, and gran fondos. Next up on his to-do list: a multi day bikepacking trip, and an e-bike race.
