What is CdA?

What is CdA?

CdA is an abbreviation for the coefficient of aerodynamic drag. It’s a dimensionless number (no units), which is the result of a body’s drag size, shape and surface texture.

CdA depends on the size, shape and surface texture of the object. The most important component of the size is the frontal area, i.e., the area in contact with the wind from the front. But the shape of the object is also very important. A 1 cm² cube will have a higher CdA than a sphere with an area of 1 cm², which in turn will have a higher CdA than a 1cm² tear drop with the same frontal area. So, while the frontal area is important, it does not represent the full picture of the aerodynamic drag.

Ways to reduce the frontal area include lowering the cyclist’s head, narrowing their arms by bringing arm pads closer together, lowering their shoulders by dropping the handlebars or stretching their posture. However, these changes in the frontal area sometimes affect the overall shape of the object, which can have unexpected consequences on the CdA. While there are general trends, such as lower and narrower is better, it’s not always the case. Some positions, while theoretically faster may impact the cyclist’s ability to generate power over long periods of time.

The CdA measure enables the cyclist to find the optimal position to improve his aerodynamic efficiency while preserving his ability to generate power. It also allows her or him to make equipment choices such as helmet and clothing. The cyclist can thus experiment to find the most efficient setup for him.

Time trialists can aim to have a CdA below 0.2, however this is by no means the average which is closer to 0.22-0.23. A road bike racer can be as low as 0.24 but the average is closer to 0.27. A regular road bike and standard helmet in the summer is likely to be 0.3-0.32 however in the winter this can be much higher.

Cd = The coefficient of drag (wind).  A = How much frontal area a rider projects.

A very, very good CdA for a time triallist would be around 0.16, a fairly average CdA for a road cyclist would be around 0.4. Typically CdA is measured by team in the wind tunnel prior to the season starting, but there are tools like the Aeropod that can help you measure it if you’re an average Joe.