[quote="DannoXYZ"][b]"For example, when viewed directly from the front the seat tube is largely if not completely hidden by the front wheel and head tube, yet air obviously still strikes the seat tube, adding to the drag. Similarly, the thighs are are partially hidden behind the arms, but nonetheless contribute significantly to the overall drag."[/b]
What the seat-tube affects the Cd even though it's not even included in the A. Aero seat-tubes lowers the Cd even though they don't affect A in any way. Another way to look at is that A is the particle-model of light and Cd is the wave-model. Combining them gives you a closer match to modeling the real thing.
[b]"Or you can measure drag force directly in a wind tunnel, and calculate CdA based on the air density and speed...but in any case, I don't understand how this addresses my point? All that matters to the individual rider is their CdA; estimating* A so that you can calculate Cd doesn't really tell you anything of value."[/b]
Well that's what rob is trying to figure out. Without the data at all, we have to way to determine if it's of value or not. By getting as much data as possible, the measured road power-output vs. speed, the calculated/measured Cd, A values, maybe even the wind-tunnel tests, but I don't think he has access to that. One you've collected enough data, you can start coming up with ways of analyzing and forming it into ways that may be helpful.
Like that one guy that graphed time-saved in time-trials based upon the vertical position of the forearms alone. The front-area certainly has not changed one bit, but there IS a difference in speed. Being able to quantify the data and putting a Cd to different configurations will help you make what-if predictions and maybe come up with a new set-up that you've haven't ever tried before. After all not everyone has a wind-tunnel for testing. And even those who use wind-tunnel testing, like auto- and aircraft makers, use it at the end to verify their calculations as a proof-of-concept confirmation. But where are you gonna get the numbers to plug into your calculations to begin with?[/QUOTE]
But your original position was that it is useful to be able to separate Cd and A when you already know the product. My argument is that, for a given individual, all that matters is the product, since that's what you wish to minimize. Being able to separate the two and, e.g., come up with prediction equations (several of which already exist) is an interesting academic exercise that may (or may not) contribute to our understanding of the determinants of cycling aerodynamics, as Rik O'Shea suggests. For an athlete who is simply interested in going faster, though, taking frontal photographs is only useful if you can't determine CdA via wind tunnel or field testing (e.g., due to lack of an appropriate venue).
