s5100e wrote:
codygo, you seem to really understand this stuff. For those of us less knowledgeable, how does Cd change with speed? My weak understanding is Cd is a dimensionless coefficient, and if so why would that coefficient change with speed? I can certainly understand that there is some aspects of fluid properties that may be speed related, but the Cd change thing has me baffled. I guess in truth I am curious what in the drag equation is speed dependent since speed is already accounted for in the basic equation. My math skills are less than stellar, but if 2 variables in an equation change with one input change that is one weird curve. Any insights would be appreciated.Cody knows his stuff, hopefully he can dumb it down so I can understand it :-)
Someone kind of derailed the thread with the "at what speed" to which I would answer "at the speed of a front of the pack triathlete". I am guessing, but the OP means draft illegal triathlon so the answer is probably 40ish km/h / 25mph.
Measured Cda will vary with speed and there is a whole conversation of the difference in how fabrics and head tube behave differently and if you are testing between 36 and 48km/h you probably have MUCH bigger things to worry about when trying to get accurate numbers.
RChung said it best with :
'I think the important question isn't "are CdA and Crr speed-dependent?" We think they are. The important question is "how speed-dependent are they over the range of speeds we experience as cyclists?"'
The answer : not much
I would add another dimension to Burn, DD and others input.
A "aero conscious" MOP may be .24ish. These are the kind of people DD sees. They go see him because they are aero conscious.
But the real MOPer who's level of consciousness is "I need Zipp wheels to go fast" is probably .27ish.
At the pointy edge more people are aero conscious athletes. If I go to sportstats, pick Oceanside, take bib numbers, look at finishpix, I would say the FOP guys are .23ish-.24ish. Not in the tunnel where they put their head in a perfect position but where they are on race day.
I have seen a ton of guys north of .25 but a lot of them are aero unconscious.
Aerometers don't sell much because there is a whole lot of aero unconscousness/incompetence out there (along with a bunch of other problems)