Two different CdA values
Posted: Sat Dec 29, 2018 4:24 pm
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%.
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.
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?
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
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 (170.04 KiB) Viewed 2834 times
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?