Light or Aero?

WHAT TO CHOOSE TO GET FASTER

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 for 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’ 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. “[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: “50 kilometres 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 aeroplanes,

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 (where much of the cycling world’s research and development in this arena is done) 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 per cent speed and +/-3 per cent drag.” But if a test is run at 20km/h, the uncertainties jump to “+/-1 per cent in speed and +/-20 per cent 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, probably averages about 25km/h? Specialized did. It tests cycling equipment down to 20km/h 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 slowerspeed drag savings from the available wind-tunnel data, but it requires some maths – maths that “only holds true if the airflow at 50km/h follows the same pattern as it does at 30km/h,” says Gerard Vroomen, Cervélo’s co-founder, 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 high speed. We found that dimples only made a difference at 48km/h. At 32km/h, dimples appeared to have no [aerodynamic] benefit.”

Though Poertner agrees with Vroomen 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 maths works “exactly” when scaling data collected at 48km/h to 32km/h. For anything below that, he admits he’s uncertain of the data’s accuracy.

But assuming the maths works, power and drag are non-linear – 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 saving at 50km/h translates to a measly 1-watt saving 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 the course for longer. Example: the same aero benefit that saves a rider travelling 50km/h one minute every 50km saves the rider traveling 25km/h two minutes every 50km.

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 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 choosing that set of aero wheels or deciding to spend your money on lighter ones.