### How To Calculate Aerodynamic Drag Using a Power Meter

By Gary Tingley

Using details in the screenshot above, and the calculator provided by Analytic Cycling, you can estimate the power required to complete a time trial for any specific time. In the example screen shot above, I used a 20k time trial at sea level on a calm day. An alternative method for estimating aerodynamic drag can be found by measuring anthropometric data.

The calculators and links required are located at:

For this example, rolling resistance for the course is in the numbers in the screen shot. For purposes of the estimate, use the defaults in the image for pedal range, wind, drag, air pressure, Crr, and rise.

Also plug in the speed in meters per second, then change the frontal area up or down until the power matches the average from the race, with the caveat of the time lost to inertia at the start (add 5 seconds to the time).

An example for a hypothetical "aero-tuned" cyclist competing at the Fiesta Island Time Trial in San Diego, California, which is a point to point 3-lap 20k course:

.44 frontal area [.440m^2 x 0.5 = .220m^2 of CdA]
83kg rider and bike Slope – 0.0015, crr 0.004, 12.69 m/s – 332w – 26:15 for 20k
+ 5 secs first lap = 26:20

.44 frontal area [.440m^2 x 0.5 = .220m^2 of CdA]
83kg rider and bike Slope – 0.0015, crr 0.004, 12.82 m/s – 341w – 26:00 for 20k
+ 5 secs first lap = 26:05

Once you know your CdA, you can calculate your Functional Threshold Power (FTP) as it relates to your aerodynamic drag (CdA) in terms of W/m^2. With this number you can then compare your aerodynamic drag/power ratio to other riders.