# AeroTune app experiences?

Any experiences for cda measurements by using aerotune garmin app? https://www.aerotune.com/de/app/

Sounds nearly too good to be true. In a podcast I heard that they claim on a day with good conditions the test can be fairly accurate, I think I remember it was like within 2% or so. Can’t find any data that validates the statements though nor any experiences.

I’m figuring this uses the so-called “Chung method” where you calculate differences in drag based on power outputs for repeated out-and-back time trial. But this does the math for you which is nice, even though you could just as easily do it yourself in Excel/Google Sheets. I’ve not tried it but I like it - as I understand it your power output doesn’t need to be precisely the same for each effort, it just needs to be relatively close. It does require either a completely windless/traffic free day, or at least a fairly constant wind speed and direction. Otherwise that will contaminate the estimates.

If you like math, here’s a blog post where a rider tests the accuracy by fixing different sized styrofoam spheres to his bike, and then he calculates their size based on the changes in predicted drag coefficients for each run. The upshot is it does a pretty good job.

Without a Garmin you can use Best Bike Split’s “Race Analysis” tool https://www.bestbikesplit.com/features: “Race Analytics tool estimates aerodynamic drag (CdA)…”

Exactly right. That’s what I saw in a video about it (it was german though as far as I remember). Conditions need to be fairly stable, wattage might not play a huge role.

But I was thinking about trying this at some point. Maybe first validating that it works by comparing upright to tt position and then going from there.

Yeah I know. But it’s too dependent on conditions I’d guess to be any accurate.

If you have access to a velodrome, that would be ideal. But any sort of quiet course that you can do multiple loops on (say, an industrial park crit course) can work especially if you collect a lot of data for each run.

See this slide deck by Chung himself for some discussion. As long as wind speed and direction are not changing too drastically, the other variables are things you can control. Slide 55 has some suggestions for where you can run the test: http://anonymous.coward.free.fr/wattage/cda/indirect-cda.pdf

I’d forgotten this, but Golden Cheetah has a calculator you can use to analyze the data. It’s totally free software, works in Mac, PC and Linux.