How to measure your CdA without a wind tunnel: the Chung method, step by step
Methodology
±0.005 m²
Typical precision of field CdA estimation with the Chung method.
With no wind tunnel: a power and GPS file over a suitable stretch is enough.
In the previous article we saw why measuring CdA is the most cost-effective performance lever. Now for the how. And the good news repeats: you do not need a wind tunnel, just a well-executed ride and a bit of method.
The idea: virtual elevation
The method was popularised by Robert Chung and it is elegant. If you know your power, your speed, your mass and the resistances (rolling and air), physics lets you compute at every instant how much energy is left over or missing… and therefore how much you should have climbed or descended. That reconstructs a ‘virtual’ elevation profile.
There is only one CdA value for which the virtual elevation matches the real elevation of the route. That is your CdA.
In practice, the algorithm tries different CdA values and keeps the one that makes the two curves coincide. The quality of the fit is measured with an R²: the closer to 1, the more reliable the estimate.
The field protocol
For the number to be good, the ride has to isolate CdA from noise. Recommendations:
- A closed, repeated stretch: out-and-back on the same piece of road, or several laps of a circuit. Having the start and finish at the same altitude helps a lot.
- Little or no wind: wind is the great enemy of the measurement. Early morning is usually calmer. If there is wind, out-and-back partly cancels it.
- Race speed: ride at the speed and in the position you want to measure (drops, aerobars…). CdA depends on position, so measure the one you will actually use.
- Clean power and altitude: calibrated power meter and, if you can, a barometric altimeter. Avoid harsh braking and do not coast for long stretches.
- Know your Crr and your mass: rolling resistance and total weight (rider + bike + kit) go into the equation. A good Crr value for your tyre improves the result.
How to read the result
You will get a CdA and its R². Read it like this:
- High R² (> 0.9): solid measurement. The two curves match; you can trust the CdA.
- Medium R²: probably wind, traffic or a noisy altitude trace. Repeat in better conditions before making decisions.
- Compare, do not absolutise: the method’s real power is in A/B testing. Measure your current position, change something (drop the torso, narrow the elbows, another helmet) and measure again. The CdA difference is more robust than the absolute value.
From measurement to plan
A measured CdA is not a number to file away: it is the ingredient that makes your race plan realistic. In AthletePro Super Record you upload the ride file (including .fit and .fit.gz), get the CdA with its R² and a chart overlaying the virtual and real elevation so you can see at a glance whether the fit is good. From there, that CdA feeds the watt-by-watt split, the times and the strategy.
Measuring your CdA once per position and updating it when you change something important is, by far, the cheapest equipment upgrade that exists.