Learn · Equipment
What is CdA, and how to estimate yours
The short answer: CdA, your drag area, is how big a hole you punch in the air. It is your frontal area times a drag coefficient, in square metres, and it sets how much power the air costs you at any speed. Riding position changes it most. A typical road rider on the drops is around a quarter of a square metre.
What the number means
Air resistance grows with the drag area and with the square of your speed. Two riders at the same speed and the same air density feel drag in proportion to their CdA: 10% less drag area is 10% less air resistance. That is why it matters so much on a fondo, where you spend hours at speeds where the air is the biggest force you push against.
CdA covers rider and bike together. In one wind-tunnel study the bike made up a little over a quarter of it in the upright position, and the body the rest 1.
What your position does
The same rider, on the same bike, measured in a large wind tunnel 1:
| Position | CdA, rider and bike |
|---|---|
| Upright | 0.270 m² |
| Drops, arms straight | 0.243 m² |
| Aero bars | 0.211 m² |
Going from upright to the drops cut his drag area by 10%; aero bars cut it by 22%. Whether you may ride aero bars depends on your event’s rules; the drops are available to everyone.
Riding in a group helps more than any position. A rider sitting right behind another had 27% less drag upright, 23% less on the drops and 14% less on aero bars, and the rider in front gained 1–3% as well 1.
Bigger riders, bigger holes, but not in proportion
A heavier rider presents more frontal area, but not twice as much at twice the weight. In 21 male racers on drop handlebars, the frontal area of rider and bike grew with body mass to the power 0.594 2. So a rider twice as heavy has about 1.5 times the frontal area, and per kilogram the heavier rider has the smaller hole.
How FondoLab estimates yours
The rider profile starts from the wind-tunnel values for the position you pick, measured on a 72 kg rider, and scales them to your weight with that 0.594 exponent. For the hoods, which the study did not measure, it uses a value halfway between upright and the drops: 0.256 m². A 60 kg rider on the hoods gets about 0.230 m², a 90 kg rider about 0.292 m².
Know the limits. The wind-tunnel rider wore an aero helmet and a racing suit, and his legs did not move 1, so a rider in a club jersey will usually sit a little higher. Scaling by weight assumes the drag coefficient does not change with size, which is our assumption, not a finding. The estimate is a sensible starting point, not a measurement: if you have measured your CdA, enter that instead, and the profile uses it.
Every number behind the estimate, with its source, is on the method page.
References
- Blocken B, Defraeye T, Koninckx E, Carmeliet J, Hespel P (2013). CFD simulations of the aerodynamic drag of two drafting cyclists. Computers & Fluids 71:435–445. doi:10.1016/j.compfluid.2012.11.012
- Heil DP (2002). Body mass scaling of frontal area in competitive cyclists not using aero-handlebars. European Journal of Applied Physiology 87(6):520–528. doi:10.1007/s00421-002-0662-9