CdA calculator for cycling

What CdA is

CdA is your drag coefficient, Cd, multiplied by your frontal area, A, in square metres. Together they set the aerodynamic drag force, ½ · ρ · CdA · v², where ρ is the air density and v your speed through the air. Because the power needed to overcome drag grows with the cube of speed, aerodynamic drag is the dominant resistance at racing speed on flat ground [3]. Halving your CdA does far more for your flat speed than any realistic gain in power.

How the estimate works

The calculator first estimates your projected frontal area from height and mass, with a regression reported by Faria and colleagues [2] from Bassett and colleagues [1] for the aero-racing position:

A = 0.0293 · H^0.725 · M^0.425 + 0.0604   (H in metres, M in kg)

It then multiplies that by a factor for your riding position. The factors were chosen so that a 178 cm, 75 kg rider lands on the CdA values typically measured in each position. They are a calibration rather than a result from a study, and the regression itself was reported for the aero position even though the calculator uses it as the starting point for the hoods.

For a 178 cm, 75 kg rider in still air at sea level, with the planner's defaults for the bike and tyres.

Worked example

Take a rider who is 178 cm tall and weighs 75 kg. The regression gives a frontal area of 0.339 m². On the hoods the factor is 1, so the estimated CdA is 0.339 m², and 250 W on flat ground holds about 35.8 km/h. On aero bars the factor drops to 0.73 and the CdA to 0.248 m², and the same 250 W now holds 39.4 km/h: a gain of 10 percent from position alone.

Limitations

This is a population regression, not a measurement. Two riders of the same height and weight can differ widely in CdA because of their shoulders, flexibility, helmet, clothing and bike, none of which the estimate sees. If you know your real CdA from a field test or a wind tunnel, use it instead. Field-test methods are reviewed by Debraux and colleagues [3], and the race planner accepts a measured value in its advanced settings.

Want the effect on a whole course rather than one number? The planner turns your CdA into a target wattage for every climb, flat and descent of a GPX route. To see how much of your speed comes from weight instead, try the watts per kg calculator, and the full model is on the methodology page.

References

1. Bassett DR Jr, Kyle CR, Passfield L, Broker JP, Burke ER. Comparing cycling world hour records, 1967–1996: modeling with empirical data. Medicine & Science in Sports & Exercise. 1999;31(11):1665–1676.
2. Faria EW, Parker DL, Faria IE. The science of cycling: factors affecting performance – Part 2. Sports Medicine. 2005;35(4):313–337.
3. Debraux P, Grappe F, Manolova AV, Bertucci W. Aerodynamic drag in cycling: methods of assessment. Sports Biomechanics. 2011;10(3):197–218. doi:10.1080/14763141.2011.592209