Super low CdA vals -- bad calibration? too much wind?

[evokesiron]

Was seeing super low CdA values. (E.g., mile 9.5 - 10, get average CdA of ~0.11).

Was it too windy, or some other calibration issue?

Thx!

[Velocomp]

I agree these are very low values.

What kind of DFPM are you using?

[evokesiron]

sram red quarq dzero. Power looked reasonable on trainer.

One thing maybe to note, is that I started the calibration ride and noticed I had not disabled the DFPM, so both the Aeropod and DFPM were enabled. I disabled the DFPM on my Garmin (and AP was still enabled), so I should have been in a good state at that point with power reading only from AP on my Garmin. I restarted the calibration process (got the red/green flashing again) and then commenced and got the steady count up to 50, 75, 100 as expected. So it seems like I was ok, but just wanted to note that.

[evokesiron]

Another thing I noticed: During the ride, the power that was displayed from AP seemed quite low compared to my perceived effort. Was it getting low power from the DFPM and that is what is causing the super low CdA calcs maybe?

[Velocomp]

It sounds like your AP calibration was correct. You got the correct wattages during your cal ride, and your AP watts afterwards looked OK.

Yes, your Garmin displays DFPM watts. Your DFPM watts were really low in your ride file, and that would drive-down CdA value.

I would do another ride, making sure your DFPM is properly calibrated.

[evokesiron]

So the power displayed in the Garmin is exactly what my DFPM would show, just relayed via the AP, right?

Anyway, one thing I noticed is that my Crr was likely set too high. My profile had 0.006, but I actually used fast tires and looked up their supposed Crr, which should be closer to 0.004. Cool feature in Isaac to be able to tweak the CdA/Crr! However, I wanted to tweak the Crr and have it adjust the CdA rather than the power estimate. I got around this by copying my profile, adjusting the the Crr to be 0.004, and then did Switch Profile After-the-Ride. Then I went back to tweak CdA/Crr, and adjusted the CdA until I got the same power as from before (effectively updating the CdA with this new value of Crr). Anyway, that's cool, exactly what I wanted (in a bit of a round-about way, assuming I did that optimally )

Once I had accounted for the difference in Crr, the CdA didn't look so crazy low for the particular selection, however still unlikely. So probably the power meter was still off. But I could tweak the power up to a value that might be more likely (based on perceived effort), and that at least gave me an upper bound on the CdA. So still got some value out of the data..

I was going to try and work some of this out myself, but I was so happy to discover Isaac had this capability (phew)

[evokesiron]

One issue I noticed: In Isaac, if I select a portion of the ride, and Keep Selection. All values in Tweak CdA/Crr/... update appropriately according to the selection, except CdA. It still retains the CdA value for the entire ride, even after saving and reopening.

So my process I described above seems to work, except it cannot be applied to a selection of the ride. Not sure how to get it to recalculate the CdA to just the selection like the other values (distance, power, etc)

[Velocomp]

sram red quarq dzero. Power looked reasonable on trainer.

One thing maybe to note, is that I started the calibration ride and noticed I had not disabled the DFPM, so both the Aeropod and DFPM were enabled. I disabled the DFPM on my Garmin (and AP was still enabled), so I should have been in a good state at that point with power reading only from AP on my Garmin. I restarted the calibration process (got the red/green flashing again) and then commenced and got the steady count up to 50, 75, 100 as expected. So it seems like I was ok, but just wanted to note that.

Yes, that sounds correct.

[Velocomp]

Another thing I noticed: During the ride, the power that was displayed from AP seemed quite low compared to my perceived effort. Was it getting low power from the DFPM and that is what is causing the super low CdA calcs maybe?

The power you were seeing on your Garmin was from your DFPM. If that power was low then your DFPM wasn't calibrated. That would cause CdA to be low.

[Velocomp]

So the power displayed in the Garmin is exactly what my DFPM would show, just relayed via the AP, right?

Anyway, one thing I noticed is that my Crr was likely set too high. My profile had 0.006, but I actually used fast tires and looked up their supposed Crr, which should be closer to 0.004. Cool feature in Isaac to be able to tweak the CdA/Crr! However, I wanted to tweak the Crr and have it adjust the CdA rather than the power estimate. I got around this by copying my profile, adjusting the the Crr to be 0.004, and then did Switch Profile After-the-Ride. Then I went back to tweak CdA/Crr, and adjusted the CdA until I got the same power as from before (effectively updating the CdA with this new value of Crr). Anyway, that's cool, exactly what I wanted (in a bit of a round-about way, assuming I did that optimally )

Once I had accounted for the difference in Crr, the CdA didn't look so crazy low for the particular selection, however still unlikely. So probably the power meter was still off. But I could tweak the power up to a value that might be more likely (based on perceived effort), and that at least gave me an upper bound on the CdA. So still got some value out of the data..

I was going to try and work some of this out myself, but I was so happy to discover Isaac had this capability (phew)

Lowering Crr would cause CdA to rise some.

Yes, it sounds like you've mastered a very cool feature of Isaac!

[Velocomp]

One issue I noticed: In Isaac, if I select a portion of the ride, and Keep Selection. All values in Tweak CdA/Crr/... update appropriately according to the selection, except CdA. It still retains the CdA value for the entire ride, even after saving and reopening.

So my process I described above seems to work, except it cannot be applied to a selection of the ride. Not sure how to get it to recalculate the CdA to just the selection like the other values (distance, power, etc)

What I do is highlight the section of ride I want to tweak. I look at the average DFPM power for that section only, then tweak CdA until I get AP power for the section equal to DFPM power for the same section.