CdA Seems a Bit Low

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AN2019
Posts: 21
Joined: Sat Oct 05, 2019 6:31 pm

CdA Seems a Bit Low

Post by AN2019 »

So, had some trouble with the Profile 4 calibration ride (seemed to work and then the next ride seemed to be counted as a calibration ride, I don't know). As such, I reset the AP, set it to Profile 3 for the sake of simplicity and did another calibration ride. Unfortunately, I live in a rural and rolling place, so not sure if it was ideal for calibration. The calibration file is attached.

After calibration, I did an 11-ish mile loop. Noticed that my CdA started out about where I expected (.34ish) while riding on the hoods. I got down as aero as I could and it shifted to .260ish. Nice! However, as the ride went uphill a bit and the tree cover got thicker, the CdA number did not increase even as I relaxed my position. In fact, I thought it got stuck for a bit. I rode most of the rest of the ride rather relaxed on the hoods. My overall CdA was about .320, which seems rather lower than I expected -- I'm a tall (and big for the moment, damn you COVID eating habits). Are things looking right?

Thanks!!

Attached is my calibration ride and ride after the calibration.
Attachments
Velocomp_09_11_2021_1601_1_Miles_CalRide.ibr
(13.44 KiB) Downloaded 5 times
Velocomp_09_11_2021_1605_12_Miles.ibr
(197.67 KiB) Downloaded 8 times
Velocomp
Velocomp CEO
Posts: 7328
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Re: CdA Seems a Bit Low

Post by Velocomp »

Your Crr rolling resistance is very high--0.0076. This is suitable for a mountain bike.

Your prior photos show you're riding a road bike. If your tires are fully inflated your Crr should be about 0.004

I readjusted your ride file to reflect a Crr of 0.004. This drops about 33W from your opposing power, thus raising your CdA.

Corrected CdA is about 0.367.

Corrected ride file and profile are attached.
Attachments
Screen Shot 2021-09-11 at 5.52.24 PM.png
Screen Shot 2021-09-11 at 5.52.24 PM.png (174.97 KiB) Viewed 369 times
Screen Shot 2021-09-11 at 5.52.03 PM.png
Screen Shot 2021-09-11 at 5.52.03 PM.png (276.13 KiB) Viewed 369 times
An2019_Corr.ibr
(196.92 KiB) Downloaded 8 times
AN2019.ibp
(413 Bytes) Downloaded 6 times
John Hamann
AN2019
Posts: 21
Joined: Sat Oct 05, 2019 6:31 pm

Re: CdA Seems a Bit Low

Post by AN2019 »

Velocomp wrote: Sat Sep 11, 2021 5:55 pm Your Crr rolling resistance is very high--0.0076. This is suitable for a mountain bike.

Your prior photos show you're riding a road bike. If your tires are fully inflated your Crr should be about 0.004

I readjusted your ride file to reflect a Crr of 0.004. This drops about 33W from your opposing power, thus raising your CdA.

Corrected CdA is about 0.367.

Corrected ride file and profile are attached.
Awesome, thanks.

Is there any way to update the AP with that Crr figure or do I need to do another calibration ride?
Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 8:43 am

Re: CdA Seems a Bit Low

Post by Velocomp »

AN2019 wrote: Sat Sep 11, 2021 6:58 pm
Velocomp wrote: Sat Sep 11, 2021 5:55 pm Your Crr rolling resistance is very high--0.0076. This is suitable for a mountain bike.

Your prior photos show you're riding a road bike. If your tires are fully inflated your Crr should be about 0.004

I readjusted your ride file to reflect a Crr of 0.004. This drops about 33W from your opposing power, thus raising your CdA.

Corrected CdA is about 0.367.

Corrected ride file and profile are attached.
Awesome, thanks.

Is there any way to update the AP with that Crr figure or do I need to do another calibration ride?
Load the .ibp profile I attached in my post. That will update Crr
John Hamann
AN2019
Posts: 21
Joined: Sat Oct 05, 2019 6:31 pm

Re: CdA Seems a Bit Low

Post by AN2019 »

Another dumb question: I can pull up my tires from Bicycle Rolling Resistance. Is it worth putting those numbers in? My front tire is a Conti GP Attack which BRR has listed at 0.00381 crr, with a Conti GP Force on the back listed at 0.00348 crr. I’m guessing one could average those and put that number in?
Velocomp
Velocomp CEO
Posts: 7328
Joined: Fri Jan 11, 2008 8:43 am

Re: CdA Seems a Bit Low

Post by Velocomp »

Yes, you can do what you suggest.
John Hamann
AN2019
Posts: 21
Joined: Sat Oct 05, 2019 6:31 pm

Re: CdA Seems a Bit Low

Post by AN2019 »

Having some odd issues:

On today's short (and pretty windy) ride, I wasn't really getting reliable CdA readings on my Garmin. Kinda seemed all over the place (dipped into the .29 range when riding on the tops, etc.). After uploading to Isaac, it seems to think that it was a calibration ride. After re-uploading it, it is now being treated as a normal ride. Very odd.

Why would it get read as a calibration ride?

Also, there remains a fairly sizable difference between Newton's power estimate and my DFPM readings. Is it possible that my DFPM is reading low?

Ride file attached.

It was a very easy ride to enjoy the weather, and I wanted to see if I could beat 1:30 on a short local climb. I did (1:20) and my Assioma Duo DFPM reported that I did 505 watts for the segment, 521 watts for my highest minute. By contrast, Newton reports 572 watts for my best minute. A short sprint also read about 300 watts higher than my DFPM.
Attachments
9-26 Short Easy Ride.ibr
(279.35 KiB) Downloaded 1 time
Velocomp
Velocomp CEO
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Re: CdA Seems a Bit Low

Post by Velocomp »

You are a very strong rider, even on an easy ride!

I think what is happening is that, in your sprints, your bike frame is flexing slightly, causing AP watts to be somewhat high. Where your readings differ in the sprints the differences are only for 1-2 seconds. This is enough to cause noticeable differences in Isaac, but it's impossible to tell if there is a problem, either with AP or your DFPM.

You are riding a route with a lot of varying terrain; this can cause some changes in reported CdA. This can happen when your body position shifts as you go up and down hills; this can slightly affect slope readings. Also, on downhills, if your ride position shifts AT ALL, CdA readings will be affected.

I think you're OK.
John Hamann
AN2019
Posts: 21
Joined: Sat Oct 05, 2019 6:31 pm

Re: CdA Seems a Bit Low

Post by AN2019 »

Velocomp wrote: Sun Sep 26, 2021 7:14 pm You are a very strong rider, even on an easy ride!

I think what is happening is that, in your sprints, your bike frame is flexing slightly, causing AP watts to be somewhat high. Where your readings differ in the sprints the differences are only for 1-2 seconds. This is enough to cause noticeable differences in Isaac, but it's impossible to tell if there is a problem, either with AP or your DFPM.

You are riding a route with a lot of varying terrain; this can cause some changes in reported CdA. This can happen when your body position shifts as you go up and down hills; this can slightly affect slope readings. Also, on downhills, if your ride position shifts AT ALL, CdA readings will be affected.

I think you're OK.
Nah, just a fatty from COVID, lol, not too strong until I get back to riding weight. Good to know on the above.

I should have prefaced all of this by saying I live in an AWFUL place to do testing. Rolling, traffic, inconsistent pavement, tree cover, etc. I took Aeropod up .25 miles at 15% the other day (peaking at 25%). I was doing 600 watts and going 5mph. Not an ideal area for wind testing.

Despite all of that, and using Profile 3, I think I have isolated my most aero road bike position at .26-.27 m^2 and my typical relaxed riding on the hoods at .36-.38 m^2. That is incredibly valuable information that I would’ve had to pay much more than the cost of the Aeropod to find out ONCE. Now, I potentially have a portable wind tunnel. Amazing.
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