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The benefit of aero efficiency is so real you can feel it. Next time you ride in a car, stick your hand out the window with your palm against the wind and feel the force pushing against you. Now, turn your hand streamline, parallel to the ground, and feel your hand turn into a sleek wing.

Efficiency gains from aerodynamic refinements are real for race cars, semi-trucks, airplanes, and—of course—bikes. Zipp pioneered wind tunnel research for the bicycle industry and, for years, has partnered with the Auto Research Center (ARC) Wind Tunnel. The ARC wind tunnel is just two miles from the Zipp factory and has decades of experience conducting wind tunnel tests on jet engines, race and passenger cars, RVs, and semi-trucks. The U.S. triathlete team also tested aero positions at ARC.

In this episode of the Zipp Speed Podcast, ARC Chief Operating Officer Henri Kowalczyk tells us all about wind tunnel research. Edited excerpts below:

The Zippcast · Inside Indy's ARC Wind Tunnel
On the ARC Tunnel

Most other tunnels were designed for airplanes, spaceships, and such. This one was specifically designed for race cars and automobiles, so the interaction with the road with the model is a big deal. We spin what is a big conveyor belt. That is simulating the car rolling over the road. The wheels are spinning, so you're getting the interaction of the wheels with the road, the flow under the car, all of that. We found it's important for any vehicles that move because there are always wind interactions, even with bicycles.

On the Value of Wind-Tunnel Research

The drag force you're generating changes as a square of the speed. If you double the speed, you've quadrupled the drag force. But to overcome that, drag changes with the cube of the speed. You don't need double the power; you need eight times the power. That's you pedaling on a bike!

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Anything you can do to eliminate drag is a big thing. Even at low speeds like 10 mph, 50 percent of the power needed to overcome aerodynamic drag.

–Henri Kowalczyk, COO of ARC Wind Tunnel

Scale-model semi-trucks in the wind tunnel
On Fuel Savings for Semi-Trucks

With trucks, it's all about fuel savings. The more expensive fuel gets, the better fuel efficiency is for the bottom line. They come in here and do testing and development. They can improve 10 or 15 percent with fuel economy. Just putting a "skirt" between the wheels (around the trailer) gets you some kind of improvement. You'd get 2 or 3 percent. Companies come in here and want to maximize that improvement; the angle and shapes, how it interacts with the landing gear, how it interacts with the rear wheels. Some (trucking companies) of them get 5 to 6 percent fuel-effeciency improvements.

We try to make the models as close to reality as possible. Some of our customers might be the designers of the trucks or trailers, so they have CAD. We take that CAD and size it down. With rapid prototyping, you can scale things down and print them out.

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On the Twisting Road to Speed Gains

We always people, if we knew the answer, we wouldn't be testing it. You know some obvious things to be true: a more streamlined shape will be better. Where you get the counterintuitive things are the interactions between things. That's really where a lot of the gains are like vortex generators. You're disturbing the air, so that would be bad. But sometimes you put vortex generators to control where that air is going and how it's sticking to shapes. For example, on airplane wings, they will put them on the leading edge to get better stall.