What Is CdA? and Why Is It Important as a Cyclist to Measure It?
By Marc Graveline, a.k.a “Data Cruncher”, Co-founder, Notio Technologies
The energy that a cyclist produces goes towards overcoming various forces such as wind resistance, gravity and different forms of friction.
The amount of energy used to overcome wind resistance can be a significant percentage of the cyclist’s overall effort. For instance, in order to go 36 km/h on a flat course, a 68 kg cyclist (plus his equipment) must produce 200 watts of power. Of these 200 watts, 170 watts will serve to overcome air resistance and the remaining 30, rolling resistance (tire to road friction).
However, not all cyclists have the same efficiency for overcoming aerodynamic drag. In order to quantify this efficiency, we use the term 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. The above cyclist going 36 km/h with 200 watts on a flat course has a CdA of .28. By improving that CdA by 10 % and dropping it to .252 on that same 200 watts, the cyclist will go 37.5 km/h. With a world class CdA of .205, he will go 40 km/h on that same 200 watts.
As mentioned, 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 (square centimeter) 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 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 his arms by bringing arm pads closer together, lowering his shoulders by dropping the handlebars or stretching his or her posture. However, these changes in 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.
As a result, the cyclist will typically find many more watts of aerodynamic savings that he will be able to use over an entire season of training to improve.