More speed, same watts: why so much talk about CdA?

The Coefficient of Aerodynamic Drag (CdA) is the number most often used to measure aerodynamic efficiency. Why? While training with heart rate and power numbers are generally understood, less is known about CdA. That’s changing, however, as teams and triathletes start to dig into the science.  

CdA varies less with air flow velocity than other outputs; its reliability makes it a good measurement. Drag Force (N), for example, rises with speed; drag will sit at 10N at 30km/h but shoot up to 50N when speed reaches 70km/h. Same with Drag Power (W); at 40km/h, drag power is approximately 200 watts but spikes to 1000 when speed reaches 70km/h. By comparison, CdA at 20km/h can measure 0.22; at 70km/h it can decease but just slightly. Its constancy is why it is a crucial. When validated with personal aero meters, lowering your CdA can uncover speed you never knew you had with the same watts.  

These numbers are the result of recent Notio-led wind tunnels tests at the National Research Council in Ottawa, Canada – part of our work to refine Notio software and make aero testing simpler and available to more people.  

Increasingly we are working with athletes and spending time inside wind tunnels to deepen our understanding of speed, and more specifically how to measure it. By looking at what parameters we can play with to improve aerodynamics, we will simplify how riders lessen the impact of shape and frontal area – obstacles that stand in the way of going fast.  

Typical measurements   

Body size, shape and the environment around it can make a dramatic impact on CdA measurements. With the environment, for example, there can be a 1.5 km/h difference in temperatures ranging from 25°C to -5°C. Winter clothes can decrease average speeds by 1km/h, too. The typical CdA of a bike + rider is 0,250 m2 (0.25). An improvement in aerodynamic speed requires only a slight decrease in those numbers, say from 0.25 to 0.22, to go faster – a potential gain of 1.8km/h (on a flat road at 300W).   

On a TT bike, the Influence of an athlete’s position on aerodynamics can generally (but not always) be found in modifications to changes in extension angles, variations in stack height under armrests, variations in the width between armrests and in extreme rider positions (that might not be sustainable). Modifications however won’t be universal for every athlete.  

Measuring and then experimenting with these variables is the value of understanding CdA. While some modifications will yield greater results than others, it is the broader canvas that must be painted for typical measurements firstly to be understood, then modified, so riders can find their fast.  

 More to come. These wind tunnel test form part of Notio’s ongoing work on improving user experience – enhancements slated for 2020.