This is something that has bothered me about most error bars I see on bike measurements, in the couple places that actually do it (I forget if this applies to the tririg testing), but when they just apply a round number percentage to all the results. First I doubt that actually uncertainty in a measurement would be a nice round number like 3%, because there are so many measurements that go into calculating drag, not just the scale, but also wind speed, temperature, pressure, etc. No way they sum up to a round number. Second the uncertainty is likely not the same percentage for every value. If you have scale that is +/-1% full scale accuracy, the uncertainty percentage of the measurement is going to be different for when measuring 500 grams versus 1000 grams. Simply if bike A has 10% more drag as measured in your test than bike B, the error bars on each should probably not be the same size. They may be similar size if something like the repeat ability of you position is much a bigger factor than the rest of the measurements, but I suspect it is not. I assume you guys have all the specs for the measurements you guys recorded and need to calculate drag?

All I know is testing where I have had to include uncertainty in the data, that has taken more time calculating than the rest of the results. Which makes sense when you start getting into it.

I am very excited to get to read about your results, but boy have you guys picked a very tricky thing to do. The majority of the work and time in this project is in front of you and the fun part is probably behind you, so I do not envy you guys.

I haven't really paid much attention to what you did, but it seems as if the only data you have that would lend insight into the precision of the measurements is n=2 values at each yaw angles, for one bike only? That's not a lot to go by, but why not just calculate the CVs and then average them? Unless you have data for the fixture drag, and/or reason to think precision varied with yaw angle, that seems as good an approach as any.

(BTW, the data look as tight as any I have seen, and if I understand correctly, reflect removing and replacing the bike, which is ideal, so good job.)

Right -- that was what I was thinking. But I also had another thought. Each run we hit zero yaw three times. So I also calculated an average based on those three data points (for each run) then checked what the relative (%) deviance from the mean was for the max and min value. It's tight. Really tight. I think both these approaches inform on the variability caused by movement from a live rider.



Average deviance from the control to the baseline across the board is +/- 0.4%. The alternative approach from the chart above suggests an average deviance of +/- ~0.7%.

From Tour Magazin, CdA of bike + pedalling legs


Shows that with huge resources and crazy designs you can make a bike that is significantly heavier but fractionally more aerodynamic for a limited scenario, while also worsening the braking performance and increasing service time.

Haha! That is great. I've not checked out Tour in a while are they still using the full dummy with floppy clothes or just using the legs now?

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

I think the visible minority at the top end change bikes regularly, but there is also a large number of us riding tri bikes that are quite old. My P2K was 10 years old when my P2 replaced it. I still have the P2K and only replaced it to get a more comfortable fit (it was slightly small for me.

I think that testing something like a P2K would be a great calibration tool. Afterall, Cervelo still use a P2K, with the DZ mannequin as their calibrator for their bikes in the wind tunnel. I know that modern bikes are more aero than a P2K, but it would be good to see how much better. Is if 50W? is it 5W? Are the advances with modern bikes (vs say bikes from 15 years ago) actually worth shelling out for a new bike or is it really just marketing and has limited real world value. If you can afford a new $10k bike each year, lucky you. If you have to make sensible long term bike choices, there really needs to be a valid reason for the upgrade. If I can save 50W over my P2k, then it's probably worth selling the kids, but if it's 5W, the kids can get a new pair of shoes instead of daddy getting a new bike.

I'm not saying we should sell our P5X's and get P2K's, but unless we do the comparative test, we never get to fin out how much $/W we're paying.

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TriDork

"Happiness is a myth. All you can hope for is to get laid once in a while, drunk once in a while and to eat chocolate every day"

I would like a comparison of Tom A.'s Cervelo S5 turned into a TT machine versus the TT superbikes.

Diabolo wrote:

I would like a comparison of Tom A.'s Cervelo S5 turned into a TT machine versus the TT superbikes.

That has been partially done
http://mobileservices.texterity.com/lavamagazine/201204?pg=20#pg20

Ditto!