Let’s first start by explaining CdA, the coefficient of drag (Cd) multiplied by the frontal area (A). Two important points about frontal area: when a cyclists is pedalling the frontal area is constantly changing as the pedals rotate: the frontal area is not all one surface, it’s a combination of all the surfaces; changing the frontal area may have a positive or negative effect to the drag coefficient of your torso in particular.
There are two methods of measuring your CdA. You either go to a wind tunnel or you conduct some real world aero testing.
How do I measure my drag / CdA in the real world
As a cyclist your CdA is proportional to the amount of speed you achieve for the power you produce. The two cycling parameters that you need to measure and record to enable the calulations therefore are power and speed. You also need to know the air density at the time of the test runs. The CdA is constant: air density affects the amount of power you need to produce a given speed. The thiner the air the faster you will go for a given power.
You then need a peice of software that will process the speed and power data for you and produce a CdA number. Fortunately there is a free app that will do this for you. It’s call Aerolab and can be found in Golden Cheetah.