I recently was able to do a couple of time trials with a power meter and put all of the data into bestbikesplit including the exact weather, power, etc. and when I teased out the CdA it came out quite a bit lower than I expected (0.215). Has anyone had the chance to compare actual windtunnel CdA numbers to race CdA numbers using bestbikesplit? Wondering how accurate it is. I could be off on the rolling resistance too (.002475) but not sure how to validate that.
I think the imprecision of the air density and wind information is the biggest source of error. Did you take those into account?
If you are really interested in your number, the best method is Chung’s virtual elevation protocol. My experience so far is that the VE figures and the BBS predicted figures are not close.
I thought that BBS took those factors into account when you plug the location, date, and time and get the weather from that day. At least temperature and wind direction and speed. Not sure about density.
I would say this, but also add that tunnel CDA for me at least is likely a lot better than real world CDA. I am tested at .229 but in a perfect setting for 3-6 minutes. Not drinking, moving around, avoiding potholes etc etc.
Maurice
I would say this, but also add that tunnel CDA for me at least is likely a lot better than real world CDA. I am tested at .229 but in a perfect setting for 3-6 minutes. Not drinking, moving around, avoiding potholes etc etc.
Maurice
Right, which is why you don’t see athletes fresh out of some wind tunnel work gaining 15 minutes or something in an Ironman, as the best case drag reduction math might suggest. Drinking, moving around, avoiding potholes and the like is how we actually ride IRL, so why not test that way?
when I teased out the CdA it came out quite a bit lower than I expected (0.215). Has anyone had the chance to compare actual windtunnel CdA numbers to race CdA numbers using bestbikesplit? Wondering how accurate it is. I could be off on the rolling resistance too (.002475) but not sure how to validate that.
I don’t think you will get near .0025 Crr on a real road. Which means your CdA is even lower than .215.
Which PM are you using, has it been calibrated, and what was your average power and speed for the TT?
Actually why don’t you give all the relevant details because there is no way to be sure without the whole shebang.
I thought that BBS took those factors into account when you plug the location, date, and time and get the weather from that day. At least temperature and wind direction and speed. Not sure about density.
Yeah, I know they do something with the “advanced weather” estimator, but I don’t know how sophisticated they are with integrating dew point and and air pressure into the algorithm for CdA. I’m sure Ryan will come on here to tell us, though
It is a brand new stages and yes I calibrated it prerace. I went 54:10 for 40k on 282 avg, 284 NP most recently. It was very humid and hot but I was bummed about how low my power was so wanted to take something positive away from it hence the CdA question. Thanks for any comments. Oh and I’m 6’ 150.
It is a brand new stages and yes I calibrated it prerace. I went 54:10 for 40k on 282 avg, 284 NP most recently. It was very humid and hot but I was bummed about how low my power was so wanted to take something positive away from it hence the CdA question. Thanks for any comments. Oh and I’m 6’ 150.
No offense but this is probably by far your biggest issue.
Brand new as in maybe 2 weeks old. I have a good idea of power and perceived effort and it has matched up exactly with what my powertap normally reads but I haven’t run the 2 together.
Drinking, moving around, avoiding potholes and the like is how we actually ride IRL, so why not test that way?
Think about that. It’s too hard to be repeatable and accurate test after test. The noise in the tests would be great, the data shit…at best, with no real conclusions.
You’ve got to anchor time savings somehow so you anchor them to baseline in a controlled environment.
Imagine if they did research as you suggested for things like ABX or missiles.
I think that’s fair. But nobody is doing themselves any favors by taking an absolute wind tunnel CdA number and inputting it into the BBS model to make inferences about race day timing expectations, which is what I assume the intended use case is here.
What number should they put in then? What’s the fudge factor? .001? .005?
I would say this, but also add that tunnel CDA for me at least is likely a lot better than real world CDA. I am tested at .229 but in a perfect setting for 3-6 minutes. Not drinking, moving around, avoiding potholes etc etc.
Maurice
Right, which is why you don’t see athletes fresh out of some wind tunnel work gaining 15 minutes or something in an Ironman, as the best case drag reduction math might suggest. Drinking, moving around, avoiding potholes and the like is how we actually ride IRL, so why not test that way?
This has been hashed out a gone over a million times. I realize you think wind tunnels are relatively useless, but some dude from MIT would like to disagree with you.
Mark Cote back in 2008:
"track power testing clearly validates wind tunnel results and reinforces the importance of aerodynamics on the test rider. But since power testing lacks measurement precision and takes a lot of time to execute properly, rider positioning and equipment testing should primarily be conducted in a good wind tunnel. "
The modeling and research is linked here.
http://a2wt.com/research.pdf
I can provide you with several other sources if you’d like. And also I don’t think at any point we’ve said X will save you 15 minutes in an IM. That would be a net drag reduction of close to 450 grams or .045 in CdA. Maybe if the person came in without aero bars on a round tube bike, but the people who have come in have already done a lot of their own homework. 5-10 minutes isn’t outside the realm of possibility. We’ve seen this validated from results from those we’ve tested.
As I implied, I think they should put in the number and assume the noise/variation output from sets of runs completed by outdoor VE testing.
I don’t think the testing is useless at all, I just think it’s directional. The original question in the thread was how good is one model’s output (BBS) vs. another (wind tunnel) in terms of real life application. My response was that I think a third modeled output (VE) could perhaps serve as an even better input for real world application (what BBS tries to do). Do you or the guy from MIT disagree with that?
It is a brand new stages and yes I calibrated it prerace. I went 54:10 for 40k on 282 avg, 284 NP most recently. It was very humid and hot but I was bummed about how low my power was so wanted to take something positive away from it hence the CdA question. Thanks for any comments. Oh and I’m 6’ 150.
Calibration would involve hanging weights and measuring torque. And with Stages you still have the one sided error. A CdA of ~.20 is certainly possible, but it would be rare if you achieved that easily.
What was the power you expected?
I don’t think the testing is useless at all, I just think it’s directional. The original question in the thread was how good is one model’s output (BBS) vs. another (wind tunnel) in terms of real life application. My response was that I think a third modeled output (VE) could perhaps serve as an even better input for real world application (what BBS tries to do). Do you or the guy from MIT disagree with that?
Since I’ve done extensive testing in the wind tunnel, on the track (alphamantis/VE) as well as field testing using the Chung/VE/AeroLab protocol, I can tell you that they all correspond very closely with each other. The VE/Chung model takes quite a bit of time to figure out a good location as well as a lot of runs to figure out how to get repeatable data. So yes, it is a good way to get an input if you have a lot of time and experience doing it, but the numbers from the tunnel or track are even better because there is a greater degree of control of the data.
So in summary Tunnel ~= Track Aero ~= VE/Field Testing, If you are controlling the variables properly then it doesn’t really matter which method you use as they all should end up being essentially the same. If you took time to read the research you’d know this.
Sorry I zeroed not calibrated. I was expecting around 310 as my road bike ftp is around 330 and I haven’t had much time in the tt bike. I think the heat took a heavier toll than I expected. Anyway thanks to all for contributing.
Challenge accepted. In the piece you linked to, Mark Cote says one of the disadvantages of wind tunnel testing is the difficulty convincing riders of actual real world improvements. As an advantage of track/road power testing, he says:
Tests are conducted in near realâ€world race situationsRiders generally are convinced of the results more than wind tunnel testing
Consider me one of those riders. That’s really all I’m saying.
The other advantages and disadvantages he lists are those you and Brian have cited and that I don’t dispute. So maybe we are closer to agreeing than not.