Homemade Wind Tunnel

Measure your own aero drag using a power meter and a (fairly) simple test.

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Measure your own aero drag using a power meter and a (fairly) simple test.

If you have a power meter on your bike, you can replicate a wind tunnel with a little patience and a free program you can download from the Internet. Aerodynamic field testing—measuring your drag while riding on the road—can be even more realistic than wind tunnel testing as it takes the human mechanics of riding into account. The “Chung Method,” named after its inventor, Robert Chung, is the simplest and most effective way to conduct an at-home aerodynamic drag (CdA) test.

It works like this: Find a quiet stretch of road with a dip in it and ride down one side of the hill and up the other side in the position you want to test, turn around at the top without hitting your brakes and repeat a few times. That’s it. The hill allows you to slow down without touching your brakes or changing your position.

Record data on your original position and then make a single fit or equipment change and record data again. Altering more than one thing for each test will obscure the real cause of any aero changes you measure. Once you have the before-and-after data, the rest is crunching numbers.

Free Golden Cheetah shareware (Goldencheetah.org) has a tool that does all the math for you. Download your test session results into Golden Cheetah and open the data from your baseline position. Go to “Aerolab Chung Analysis” view (currently located in the “+” pull-down in the upper right) and enter the following variables:

Combined bike and rider weight (“Total Mass”): Dressed for riding, step on your bathroom scale with your bike.

Air density at the time of the test (“Rho”): Find temperature and pressure data during the time of the test session from Wunderground.com and plug those two numbers into Golden Cheetah’s “Air Density Calculator” (located under Tools).

Coefficient of rolling resistance (Crr): If you are not certain of your tire’s Crr, enter a value of 0.004 if you tested with race tires or 0.005 for training tires.

Once these variables are entered, you are ready to calculate drag. You will see that the lowest dip points on the blue line in the data gradually descend in elevation as you go from the first “half-pipe” run on the left to the ones on the right. The descending blue line labeled “V-Elevation” represents the rider’s aero drag. Adjust the CdA slider bar beneath the graph until the lowest dips of the blue line align horizontally. It is helpful to note that clicking your mouse on any point of the blue line will show you the elevation at that point, making it easier to compare all the dips.

When aligned, you will see that the CdA number will have changed. This “aligned” CdA number is the aerodynamic drag of the tested position. Analyze the data from your second test the same way. Lower CdA numbers mean less drag and thus better aerodynamics.

Once you’ve nailed the Chung Method testing process, you can repeat the steps for your fit and gear selection to unlock your own aerodynamic puzzle.

Click here for the complete explanation of why the test works from Chung himself.

Ian Buchanan and Dean Phillips are fit and technology specialists at Fit Werx bike shops in Vermont and Massachusetts, respectively.

Note: Aerodynamic field testing should not be used to determine major positioning changes. Before diving into drag testing, start with a well-fit, biomechanically solid riding position and only test changes that do not affect the sustainability or efficiency of your position.

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