Friday, February 13, 2009

CdA- what it is and why it’s important to cycling!

CdA, otherwise referred to as Coefficient of Drag, is the product of the Drag Coefficient- Cd (dimensionless) and Frontal Area- A (m2). It is a common metric used in the automobile industry where designers try to achieve a low coefficient to improve aerodynamics for maximizing fuel efficiency.

The typical frontal area for a bike/rider is between 0.4-0.7 m2. Let’s say 0.6 m2. And, the typical drag coefficient (Cd) for a bike/rider, riding with hands on the hoods, is 0.6. (BTW, a sphere/ball has a Cd of 0.5). Therefore, the overall CdA= 0.6 x 0.6= 0.36 m2 . Since CdA is a product of Cd x A, in order to reduce the overall CdA (our goal- the lower the better) you can either reduce Cd or A, or optimally- both. How do we do that? And, how do we measure that?

First, let’s answer: how do we reduce both Cd and A. We can reduce the Cd by paying particular attention to our equipment. i.e. TT bike (or road bike w/ aero bars), aero helmet, tight clothes, disc wheels, bottles mounted on frame, etc. And, we can reduce the Frontal Area-A by assuming a more compact/aero position on the bike. Now, how do we “accurately” measure our CdA? Well, falling short of renting time in a wind tunnel, and blowing close to $1000, you can do this with a Power Meter at your local Track (Velodrome), if there is one nearby, or you can buy an iBike Aero from Velocomp Sports and do it on the roads in your neighborhood. The iBike Aero, I like to say, is the poor man’s wind tunnel. It will compute a “snapshot” CdA as you ride. If you also have a Power Meter, like an SRM or Power Tap, with the new ANT+Sport Wireless protocol, you can compute a “continuous” CdA as you ride. Pretty neat huh? For more information on the iBike Aero, go to: Or, you can email me about it: and I’ll tell you more about it. The question I get asked most often about the iBike Aero is: Is it accurate? The answer: Heck yeah..I have the data to show and prove it.
How is CdA used? Here’s a quick example to highlight it’s application/use. The force on an object (cyclist in our case) due to aerodynamic drag can be calculated using:

F= CdAp[v^2/2]

F = aerodynamic drag force [N]
Cd = drag coefficient
A = frontal area [m2]
p = density of fluid [kgm-3]
v = velocity of object relative to fluid [ms-1]

So, you might ask: What is the force on rider if, Cd=0.55 and A=0.6 m2, w/ Air Density= 1.2 kg/m3 and Velocity= 11.2 m/s? (These are typical values for someone riding in the hoods at 25 mph):

Force= 0.55(0.6)(1.2)[(11.2)2/2]= 24.8 Newtons= 5.57 lbf

Alternatively, we know that Power= Force (N) x Velocity( m/s), therefore:

Power (to overcome wind resistance)= 24.8 N x 11.2 m/s= 278 watts


1 comment:

boydjohnson said...

Awesome post Rob! I actually just talked to Brian and he told me that you have been loving your iAero. I just wrote about the same benefit of using an iAero for lowering drag coefficient. I tried a few things and lowered mine by .02 (effectively getting 30 seconds faster in a 10 mile time trial). If you can use your iAero to give people bike fits, it's going to be the best way to make them fast during time trials.

As for how accurate it is. . .I do all my rides with two power meters to answer that, and we even let everybody download the data to see for themselves.