CdA, your air drag
Above 25 km/h most of your power goes into pushing air aside. CdA says how much: from two photos in a minute, or on the road with the Chung method.
Free. Your photos stay on your device.
Photos are quick and need no power meter. A road test is the most accurate, and it also calibrates the photos.
Ready
One photo from the front and one from the side, your height and weight. Your browser finds you and your bike, and you see your position as a 3D model with the air flowing around it.
Coming soon
Ride a loop a few times with a power meter and upload the FIT file. The Chung method works your CdA out from the ride itself, for several positions in one session.
CdA is the drag coefficient times your frontal area, in square metres. Air drag grows with the cube of speed, so at 40 km/h a small change in CdA is many watts.
≈ 0.40 m²
Hands on the tops, upright
≈ 0.32 m²
The usual road position
≈ 0.29 m²
Lower and narrower
≈ 0.23 m²
Time trial position
From 0.32 to 0.29 m² saves about 26 W at 40 km/h. Your own numbers depend on your body, your bike and your clothes.
CdA is two numbers multiplied: how big you are against the wind, and how smoothly the air passes you.
Your browser outlines you and your bike. The front wheel is the ruler: its size is known from the rim and the tire. You sit further from the camera than the wheel, and the side photo tells how much, so that is taken into account.
Your torso angle sets the drag coefficient: lower and flatter lets the air pass more smoothly. This part is an estimate from wind-tunnel and road tests of riders.
You get CdA with its likely range, the watts it costs at 30 and 40 km/h, and your position as a 3D model with the air flowing around it.
Finding you and your bike in the photos happens in your browser. Our server receives only measurements, such as how many pixels you take up, and never the photos.
Save the CdA to a setup in your bikit account, and ETA uses it for your ride times instead of guessing from your position.
The frontal area comes out within a few percent with good photos. The drag coefficient is the uncertain part, so the result is within about 10 to 15%. Comparing two of your own positions is more exact than that.