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Back 2019-08-29

Zipp Engineer David Morse Talks Aero Testing

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Zipp Advanced Development Engineer David Morse in front of Zipp's history wall in Indianapolis.

It comes down to small details in the wind tunnel. Pigtails or ponytail? Which is faster? The answer may surprise you…

Measuring minutiae. Learn from what doesn’t work. Refine. Repeat. That’s how Zipp Advanced Development Engineer David Morse views aero testing, whether it’s in the wind tunnel, computer simulations, or on the road. In this edition of ZippCast, Morse discusses the role of aero testing in developing fast and stable wheels.

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EDITED EXCERPTS FROM OUR CONVERSATION WITH ZIPP’S DAVID MORSE

Crosswind stability crucial

“You definitely want that straight-line low drag, but that’s really not usable if your bike is so unstable in a crosswind that you have to come out of your aero position, or you’re not comfortable pushing hard because you’re spending all of your energy making sure your bike is going in a straight line. We try to take a holistic approach to the aero aspect of your equipment and make sure it’s a well-balanced set of features to make you, the whole system, go faster.”

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David Morse in the ARC wind tunnel in Indianapolis with pro triathlete Jordan Rapp.

Role of Wind Tunnel testing

“What the wind tunnel is best for is development, iteration, and learning. I would say 99 percent of the time we spend in the wind tunnel is discovering what doesn’t work. A lot of the runs we have we’re just seeing failures. If it’s slow, we need to figure out why it’s slow. Or if it did work, we need to figure out why it did work. You can use the wind tunnel to get really nuanced information and really good accuracy comparing very small details from one design to the next, and that’s really where I see the benefit of the wind tunnel coming in. You can dig down into minutia like internal vs. external nipples and actually find out what kind of difference that makes on the whole bike system.”

“There’s a lot of kind of buckshot design that happens, where we have a concept we iterate 10 different ways, and then see which one worked better… It starts off with a guess-and-check and then we try to get more sophisticated from there. An example of that would be our 454 and 858 profiles. We went through several dozen iterations of the amplitude of that tubercles, the number of tubercles, the shape of the tubercles, the shape of the cross-section and then on top of that we have a dimple pattern. There was a lot of back-to-back comparisons that happened in the wind tunnel to end up with the design we currently have on our 454 and 858. Once we think we have something, we’ll go back and dig deeper with CFD to understand why that result came to be, and then we take that and try to do some predictive engineering to refine the shape and optimize.”

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Pro triathlete Sarah Haskins digging into aero details, right down to her hair.

Pigtails or ponytail? Which is faster?

“We had pro triathlete Sarah Haskins wear a ponytail with her helmet on and then pigtails. We found, surprisingly, that her pigtails were faster than her ponytail. That kind of brings up the point that there’s a lot of interaction going on aerodynamically on a bike. You have a person moving, you have your rotating wheels, your crankarm, a lot of stuff going on that all really play into each other. The front wheel changes how the wind comes across the crank, which changes the rear wheel and so forth. It makes it really difficult to predict, and sometimes you just have to put it all together and measure it.”

What about tire pressure?

“Understanding that a road bike doesn’t mean one set tire pressure: You have to dial in your pressure according to tire size, road surface conditions, how heavy you are. All those things should be taken into account to get the proper pressure. What you’re going gain from that is optimal rolling resistance. You’re going to gain much better feel in the corners…. If you lower your pressure and you’re railing a corner, you really feel comfortable leaning the bike over and taking that corner as hard as you want to…. Lower the pressure down and go tubeless and really see first-hand what that feels like.”

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So much of the value of going to the wind tunnel is about learning from ideas that don't work. The wind tunnel is a development tool.

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CFD (Computational Fluid Dynamics) post-processing is a vital tool in reminding designs and understanding aero efficiency down to the smallest details.

Basic aero tips for all cyclists

“Staying focused that aerodynamics matters all the time will help you tuck a tighter, keep your elbows in, keep your head a little lower… Stay on the details. It’s difficult to perceive the difference it makes, but it’s going to put you in a little better position. In a triathlon, you’ll come in 30 seconds, maybe a minute faster. It’s going to make that small little bit of difference, but it all adds up if you pay attention.”

MORE ON DAVID MORSE AND AERO ANALYSIS

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