burnthesheep wrote:
Speaking for outdoors, with elevation changes, not velodrome and as a less educated sensor user: In general this gets back to a gripe of mine about accel/decel. The formulas with sensors (as I'm aware) penalize CdA or CRR for the sins of accel and decel in laps. In reality, if you're in your position for the test and there isn't a massive outside variable like a changing yaw wind you're testing on purpose..............the calc should penalize elevation correction with accel/decel. Not CdA. You're holding as constant a speed/power as you can, your power or speed increases or decreases largely due to elevation changes. Not huge wind changes, if so, try again another day.
I've no idea how you fix the formula or what logic in the programs would be necessary, but it's almost like "if lap is test lap, assume CdA = almost constant". Then use the last 1/4 second's datapoint of accel/decel figure to correct the next point's elevation.
It's why I'm obsessive about finding/using circular laps that even if it has an elevation problem I can do enough laps in short enough physical exertion time that the screwups of accel/decel occur over a same known spot every lap and it's usually only one spot. But with longer out/backs, you're praying that the dozen or so little pieces of change in the road "work out" in the math somehow.
So, the circular laps may look like: where I'm doing a 1/3 mile lap of a roundabout with a small elevation change on it.....
__^__^__^__^__^__^__^__^
But the out/back laps will look like: where it "misses" or treats the bumps/accel/decel differently over laps
______________^____^______
____^_____________________
What you might be seeing is your accelerations/decelerations are actually throwing the barometer off, and making it look like excess or a shorter of altitude change, that works out to a CDA error. If you want to prove/disprove this to yourself, simply look if barometer elevation and VE match since VE does exactly what you say, it assumes "CDA is constant"
I have seen data from at least 3 vendors that exhibit this problem. It's the consequence of relying too much on a barometer. To be clear a good barometers plays a critical role in the solution but it's shortcomings need to be compensated for.
But it's very easy to diagnose thanks to VE.