Two different CdA values

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BillOsler
Posts: 116
Joined: Mon Apr 04, 2016 10:10 am

Two different CdA values

Post by BillOsler »

I think I may have obtained interpretable data from the Aeropod. Finally.
Today I did a calibration ride and then 4 different tests on my TT bike. The 1st and 4th used the same setup as a measure of test-retest stability of the measurements. The out-and-back test run wasn't ideal (about 0.6 miles out, 0.6 back) and that may have affected accuracy.
The numbers suggest that using a wheel cover on the rear wheel lowers CdA by between 5% and 10% and that switching from an aero road helmet to a TT helmet reduces CdA by about 10%.
  • Trial 1 0.277 CdA on analysis - TT helmet, no wheel cover
    Trial 2 0.264 CdA on analysis - TT helmet, with wheel cover
    Trial 3 0.294 CdA on analysis - Road helmet, with wheel cover
    Trial 4 0.281 CdA on analysis - TT helmet, no wheel cover
The difference between the two control tests was a little under 2%. I had hoped for better reproducibility but I don't know what is reasonable.
I did the CdA analysis for each test by selecting the relevant portion of the ibr file in Isaac and saving just that section so each ibr file included only the data of interest for one test.
I am puzzled by a discrepancy between two numbers displayed by Isaac.
In both of the screen shots the entire out-and-back was selected in the CdA analysis display. In one case the CdA analysis result is almost identical to the CdA listed in the ride summary on the left but in the other analysis there is a large difference between the two. The relevant values are circled in the screen shots.
Two Different CdA values B.png
Two Different CdA values B.png (170.04 KiB) Viewed 2122 times
The attachment Two Different CdA values B.png is no longer available
This makes me wonder: What is the CdA in the ride summary supposed to represent? I'm assuming that the CdA analysis is based on whatever is selected in the CdA Analysis display and I would have guessed that the CddA in the ride summary was based on the entire ride ... but in that case if the entire ride is selected in the CdA analysis box shouldn't the two numbers agree?
I do have a suggestion/request for future versions if Isaac. When I select specific laps in Isaac and then choose CdA analysis from the Tools menu shouldn't the CdA analysis that pops up show a value for the entire selection instead of forcing the user to highlight a region of interest?
Velocomp
Velocomp CEO
Posts: 7793
Joined: Fri Jan 11, 2008 8:43 am

Re: Two different CdA values

Post by Velocomp »

Please post the .ibr ride file. If you have GPS in .tcx or .gpx format please post that too.
John Hamann
BillOsler
Posts: 116
Joined: Mon Apr 04, 2016 10:10 am

Re: Two different CdA values

Post by BillOsler »

I will attach the IBR and a screenshot showing the relevant laps in the file.
If the GPS data are really necessary can I PM them to you rather than posting?
The actual analysis was done on the the 4 separate files represented by the marked laps.
Attachments
TestRidesMarked.png
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BillOsler
Posts: 116
Joined: Mon Apr 04, 2016 10:10 am

Re: Two different CdA values

Post by BillOsler »

This shows the relevant portion of the GPS track and the elevation profile from the Garmin for the out and back section
Attachments
TestRidesMapProfile.png
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