rruff wrote:
RChung wrote:
You know, if you have wind, speed, gradient, and weight right, then speed dependence in estimated Crr and CdA can sometimes be a symptom of inaccuracies in power. (Typically, speed dependence is a symptom of inaccurate wind measurement; that's why I said 'if wind is right.')
I'm referring to a shallow climb (little over 1% grade) with a strong tailwind where Crr dependence was observed. I can't rule out another cause, but I had a lot of experience on this course, and there wasn't another easy explanation. CdA dependence comes from Dan Bigham's testing, and pro teams who observed the same and have worked to optimize skin suits for different speeds. One advantage of being really fast is that CdA tends to drop, but Crr goes in the opposite direction.
So couple of things in the thread :
I asked BurnTheSheep why he goes full tilt in testing and we got on the conversation of Crr/Cda dependency on speed. We can come back to that. My personal belief on "speed to test at" is to go at a pace where fatigue will not be the limiter of how much you test. I will get better data with more runs at lower intensity that being toast after a few runs. Your comment that the more "aero contribution" the better is true. But you can get very good results without having to go close to or over threshold. Also I agree on your comment of apparel possibly being more speed dependant. If I was fine tuning two TT skinsuits, maybe I would confirm A vs B differences at higher speeds. But 90% of my testing is done at HIM watts. Personal opinion there
Yes, Crr/Cda will be impacted by speed. Dan showed this in a thread not so long ago and was right. IIRC his charts were well over 50km/h. Robert explanation is in line with what I have seen with tunnel testing at various speeds. I also saw people explain other speed dependant anomalies with this theory at what I called "mremortal speeds" (40ish km/h), rather than Dan's super-human speeds. They were wrong IMO. In one case it was airspeed sensor calibration issues.
I saw another track case where they were unable to explain speed dependant anomalies so they attributed it to change in speed dependant Crr/Cda. As soon as you properly factored banking into the equation, the problem went away. Alternate sensors helped figure this out.
What I am saying is sometimes (this is not necessarily your case), we need to get to the bottom of what is causing the anomaly rather than use the Crr/Cda theory. For this, the more underlying sensor data you have the better to figure this out.
Im my case the degrading CDA number at higher speeds with tailwind is 98% of the time a windspeed calibration issue.
I can reproduce a similar thing on my favorite section of road. BTW, "favorite" to me means the most challenging to figure out. I have a section of road I love to test on because it creates all kinds of problems. There is a dip and rise in altitude, in a big arc going from West-East to North South, completely exposed, with the wind coming typically from North East and 2 expansions joints that cause a thump-thump in 2 spots. You get huge swings of head wind to high yaw to tail wind in a very short period of time. You see huge swings of instantaneous CDA. You can go from 48km/h tail wind to 34 km/h head wind in 30 seconds. And you can do it out and back to get really interesting results. Doing it in no wind and huge wind conditions allows to pick apart the different issues.