How We Calculate Aerodynamic Drag at RaceYourTrack

The Chung Method, developed in the early 2000s by Robert Chung, makes it possible to determine the aerodynamic drag coefficient (CdA) from real-world power data — without the need for a wind tunnel or lab testing. A detailed description can be found in his article Estimating CdA from Power Data (PDF), licensed under Creative Commons Attribution (CC BY 3.0).

Chung’s approach was both elegant and practical: ride several laps on a consistent circuit, record power and speed, and find the parameters that bring the energy balance into equilibrium.

Try it in the Bike Calculator: Choose a CdA and see how fast you would go under different conditions. Open the bike calculator

The Idea Behind the Chung Method

Chung’s key insight was simple: if you model the full energy balance of a ride, you can fit the unknown parameters (especially CdA) so that the physics matches what you actually measured.

In the original setup, riders repeat laps on a consistent circuit and use the fact that effects like wind tend to balance out over a complete lap. What remains is a robust, repeatable signal that can be used to estimate aerodynamic drag from real data.

Why this works so well in practice:

  1. Lap-based wind averaging – Headwinds and tailwinds can partially cancel out over a full lap.
  2. Robust to offsets/drift – Small sensor drift (power or elevation) matters less when you fit for overall consistency rather than single moments.
  3. Field-friendly – No wind tunnel or lab equipment required; only ride data (power + speed, and ideally consistent conditions).

That combination made aerodynamic testing accessible: practical, repeatable, and grounded in real-world riding.

Next step: Once you have a CdA estimate, plug it into our calculator to see how much speed it costs or saves. Simulate speed for a chosen CdA

Our Implementation for Real-World Routes

In practice, few riders train or race on perfectly flat, closed circuits. That’s why RaceYourTrack uses an extended implementation of the Chung Method that directly accounts for real elevation profiles.

We compare the simulated altitude with the actual recorded data and optimize the parameters that minimize the deviation. This allows for accurate estimation of aerodynamic drag (CdA) even on real GPX tracks.

Whenever power data is available — from a rider’s own power meter — our system automatically performs the calculations in the background. CdA and rolling resistance parameters are determined without manual input and seamlessly integrated into the physical simulation model.

Explore the impact: Use the Bike Calculator to compare speed vs. CdA in seconds. Open the bike calculator

Key Features of Our Implementation

  • Wind is taken into account directly through selected weather conditions.
  • Fully compatible with real GPX data – including elevation changes.
  • Numerically stable, since optimization is based on altitude rather than instantaneous power.
  • Physically consistent through complete energy balance.
  • Automated parameter optimization integrated directly into our simulation.
  • Automatic computation whenever power data is available (power meter).

For Those Who Think in Formulas

The following equations summarize the physical principles behind our implementation of the extended Chung Method:

$$\Delta E_{\text{pot}} = m g \Delta h = 0$$

$$P_{\text{mech}} = P_{\text{roll}} + P_{\text{aero}} + P_{\text{acc}}$$

For routes with elevation changes:

$$P_{\text{mech}} = m g \dot{h} + C_r m g v + \tfrac{1}{2} \rho c_w A v^3 + m v \dot{v}$$

$$\dot{h}_i = \frac{P_i - C_r m g v_i - \tfrac{1}{2} \rho c_w A v_i^3 - m v_i \dot{v}_i}{m g}$$

$$h_{\text{sim}}(t) = \int \dot{h}_i \,dt + h_0$$

The optimal parameter pair $(c_wA, C_r)$ is found by minimizing the deviation between simulated and measured altitude:

$$\text{RMSE}(c_wA) = \sqrt{\tfrac{1}{N}\sum_i (h_{\text{sim},i} - h_{\text{real},i})^2}$$

Want a feel for the magnitude? Pick a CdA and compare speeds under your conditions. Try the bike calculator

Conclusion

  • The Chung Method provides a robust, wind-independent way to estimate CdA from real ride data.
  • RaceYourTrack applies this method in an extended form to real-world courses with climbs and descents.
  • Comparing simulated and measured altitude enables a precise, transparent analysis of aerodynamic drag.
  • Once power data is available, the calculation runs fully automatically — no manual setup required.

From a clever concept to a powerful analytical tool — bringing precision aerodynamics directly to the road.

Continue with a what-if: Use the Bike Calculator to see how different CdA values translate into speed. Open the bike calculator

Ready to test the Chung Method on your own rides?

If you want to run the Chung Method on your own data and estimate CdA from real power files, create an account and get started.

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Source

This method is based on the work of Robert Chung: Estimating CdA from Power Data (PDF), licensed under Creative Commons Attribution (CC BY 3.0).

Photocredit: Pexels/ Paolo Bici