Using CdA from AP?

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kq6up
Posts: 57
Joined: Sun Jan 13, 2019 9:58 pm

Using CdA from AP?

Post by kq6up »

I used my AeroPod to carefully measure my CdA on my commuter bike. I am not concerned with aerodynamics on this bike, but I thought I would have an accurate measurement of power with my PP if I used the CdA derived from my AP. This doesn't seem to be the case. In order to have the data be believable I have to arbitrarily lower CdA if I have a WSF that makes the wind data work. In other words, I can't have my cake and eat it too. Why could this be?

Thanks,
Chris Maness
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Re: Using CdA from AP?

Post by Velocomp »

I don't understand you post. Please rephrase.
John Hamann
kq6up
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Re: Using CdA from AP?

Post by kq6up »

I post in steps.

1. Take a CdA measurement of my Surly CdA=.600 (this is very slow commuter bike with 38mmx700c tires)

2. Transfer that measured CdA to my PowerPod.

3. Calibrate the wind scaling factor on the PowerPod.

4. Adjust the CdA on the Surly profile to use on the PP for a stand alone power meter.

The power meter reading around 300Watts coasting downhill (while soft pedaling to keep the cadence something other than zero). I had to manually adjust the CdA for the surly to make the power believable on that ride CdA=.400.

My questions is: Why does my AP value for CdA not work on my PP?

Chris
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Velocomp
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Re: Using CdA from AP?

Post by Velocomp »

1) Your WSF is pretty high, and the large headwinds on downhills are suspicious. I'm not confident your wind calibration is correct. Excessive headwinds would cause high watts, forcing you to lower CdA to correct.

2) Your ride file shows significant changes in slope correction on downhills. This might be due to bike flexing, which also would cause changes in CdA.

I would first work on getting the wind speed dialed-in.
John Hamann
kq6up
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Joined: Sun Jan 13, 2019 9:58 pm

Re: Using CdA from AP?

Post by kq6up »

Velocomp wrote: Tue Apr 09, 2019 1:13 am 1) Your WSF is pretty high, and the large headwinds on downhills are suspicious. I'm not confident your wind calibration is correct. Excessive headwinds would cause high watts, forcing you to lower CdA to correct.
I have adjusted the WSF=2.0 to make this ride work, but another ride yesterday I had to adjust to WSF=2.72 to make it work, and that was on a dead calm morning. This PP tracks the wheel speed more consistently between cruising and bombing down hills compared to my old one, but I still have issues with the WSF jumping around on different rides. I am wondering if this setting is sensitive to temperature (more so than just accounting for change in air density).
Velocomp wrote: Tue Apr 09, 2019 1:13 am
2) Your ride file shows significant changes in slope correction on downhills. This might be due to bike flexing, which also would cause changes in CdA.

I would first work on getting the wind speed dialed-in.
This is very doubtful. This was my Surly Disk Trucker. It is a 45lbs 4130 steel bike made for bike packing. It is incredibly stiff unloaded (and it was unloaded). It is not a typical "lively: steel frame. It feels dull, heavy, and stiff unloaded.

Regards,
Chris
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Re: Using CdA from AP?

Post by Velocomp »

I don't have anything else to add to this conversation.
John Hamann
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