Rule 28 · Performance Lab
Aero Calculator.
Test two positions on the same flat road, enter what your power meter and speedometer recorded, and we'll back-calculate your CdA for each — then translate the gap into the time and power it's actually worth.
01Your tests
Two runs, same road, same effort as close as you can manage. The numbers come from your head unit — nothing to estimate here.
Test A
Test B
Sea-level air density is 1.225. Road tyres on tarmac run 0.004–0.006 Crr. Leave drivetrain at 100% unless you're comparing a power meter at the crank against one at the hub.
02The comparison
Your coefficient of drag area for each position — this is the number aero kit and position changes actually move.
| Position | Power | Speed | CdA |
|---|
03What the gap is worth
Same two numbers, translated into a race result.
A simplified Chung method (virtual elevation, steady-state form): CdA is back-calculated from the power needed to overcome rolling resistance and aerodynamic drag at a measured speed. Assumes flat ground, no wind, and steady effort through each run — real-world variance in either will shift the numbers. Treat this as a strong estimate for comparing two positions on the same day, not a wind-tunnel-grade absolute.