Is Watts/CdA a good predictor of Time-Trial Speed?

Is Watts/CdA a good predictor of Time-Trial Speed?

A recent Scientific Triathlon podcast argued that the best predictor of average bike-speed on a flat course is watts per CdA.  If your watts per CdA is higher than the rider next to you, you will beat that rider in a flat time-trial. [This is in contrast to watts/kg, which is the best predictor of climbing speed.  See my blog on the Tron Bike for a discussion on watts/kg.]

Most of us triathletes do power-based workouts on the bike.  We work on increasing our Functional Threshold Power (ftp) and have a good feel for the average wattage we can produce during a race. But do any of us have a handle on what our CdA is? And how do we reduce it?

What is CdA?  CdA is the coefficient of aerodynamic drag multiplied by a relevant area. Area(A) is usually taken to be the frontal area of the object, but in reality, Cd and A are rarely treated separately. CdA is a measure of resistance as a body moves through a fluid. It is a function of both the size and shape of the body and density and viscosity of the fluid. If you are a bigger person, your shape is bigger, so you may have a higher CdA than a smaller person.  On the other hand, if the larger volume of muscle generates more watts, it likely will overcompensate for the additional "area" of that muscle.

Back to the title question: Is Watts/CdA a good predictor of Time-Trial Speed? The answer is 'yes', but it is almost worthless in practice, because no one knows their CdA!! Even most CdAs of professional cyclists reported in the literature are not directly measured but are calculated from their speed and watts.  You can do the same. Go to the chart below and find your average moving speed for a recent time trial (or a flat bike leg of a triathlon). Find the watts/CdA that corresponds with that speed using the dashed curve. Divide your average watts for the time-trial by that number. That is your CdA x1000.

[Example: If your speed was 20 mph, then Watts/CdA x 1000 is 0.5. If your average watts for that event was 160, then 160 watts / (1000 x 0.5) = 0.32 m² for your CdA.]

The points [blue squares] on the chart above were gathered from the internet and from friends who shared their watts, speeds, rolling resistance data, and air density. [A slightly more sophisticated estimation of watts/CdA can be done with this additional data.] The pink line is the theoretical watts/CdA if one ignores everything but aerodynamic drag (no power losses from drivetrain, rolling resistance, elevation gain/loss, wind, etc.) Are you surprised at how irrelevant the other factors are in predicting race speeds on these flat courses?

Note especially that nowhere is rider weight or bike weight part of the equation! Weight only effects the speed when there are changes in elevation or accelerations. At constant speed on a flat course there is no penalty for a heavy bike! If the extra weight is due to aero features of the bike, a few additional kilos is an advantage on a flat course.

Now that you know your CdA, how do you reduce it? That will be covered in Part II.