Big Island Innovation

  • By Aaron Hersh
  • Published Oct 4, 2012
  • Updated Oct 31, 2014 at 4:39 PM UTC
Photo: Nils Nilsen

3. True wheels

Until recently, aero wheels have been judged only by the wind resistance they create. If a wheel spits out low drag numbers from a wind tunnel test, it must be fast, right? While these numbers truly are important, they aren’t the only factor. Stability, especially in the violently unpredictable crosswinds of Hawaii, also impacts how quickly an athlete arrives at T2.

When Zipp began testing prototype rim shapes that would eventually become its current Firecrest wheels, engineer Michael Hall and technical director Josh Poertner came across a phenomenon that explains the shaky feeling of riding deep wheels in the wind. Crossing air doesn’t stream smoothly over the wheels; it builds up and releases in bursts. When a pocket of air separates from the rim—Poertner calls this “vortex shedding”—it forces the wheel to twitch slightly, creating unstable handling.

To solve this problem, Zipp blunted the inner edge of its deep wheels instead of using a typical pointed shape. This new design prevents air from clustering on the rim and releases it in very small, frequent bursts that don’t influence handling as dramatically as wheels from the previous decade. Enve, Hed, Rolf Prima, Bontrager, Shimano and others have adopted similar designs, and the difference on the road is astounding.

Most of the contenders now ride aero wheels created to subdue crosswinds, but there are a few exceptions. Athletes riding rims that predate this technological evolution will be at a substantial disadvantage while trying to control their bikes through strong gusts.

RELATED: The Next Evolution Of Aero Wheels

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FILED UNDER: Bike / Gear & Tech / Ironman / Pro Bikes

Aaron Hersh

Aaron Hersh

Aaron Hersh is the Senior Tech Editor of Triathlete magazine. To submit a question, write Aaron at

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