CdA Seems a Bit High (for once)

Post Reply
Posts: 37
Joined: Sat Oct 05, 2019 6:31 pm

CdA Seems a Bit High (for once)

Post by AN2019 »

Especially when manipulating crr to around .0004 (typical settings).

I'm riding heavy right now at 95kg (and dropping) and tried my favorite 10 mile rolling testing route today.

Bike is an Aeroad with 62mm deep wheels, Enve 25c on the front and Enve 29c on the back, pumped to about 85psi. The route has OK pavement and about 450 feet of climbing. Not as much traffic today thankfully. I went near FTP (about 302w normalized). Finished the course in 26:23, at about 22.8mph (about 37kph).

Issac is telling me that my CdA was about .353 which seems very high since I spent the bulk of the time in the drops. Is this accurate or am I missing something?
(145.67 KiB) Downloaded 168 times
Velocomp CEO
Posts: 7845
Joined: Fri Jan 11, 2008 8:43 am

Re: CdA Seems a Bit High (for once)

Post by Velocomp »

Lots to explain:

1) AeroPod has more updated CdA measurement algorithms than Isaac, so I look first to AP CdA data

2) Your CdA data is in the HR channel; for profile 3, CdA = (HR*4)/1000

3) When you hit the lap mark AP records your average CdA for the just-completed lap. You'll see it as a flat line in the HR file (mile 8 in your ride file). For the first lap your average HR was 72, so your CdA was 0.288. Note that this includes the spurious first 1 mile, where AP was auto adjusting itself.
Excluding this portion your AP CdA was 71.5 * 4 /1000 = 0.286

4) For the second portion of your ride the AP CdA was 0.285

Note that all of the above is with Crr set to 0.006, the value you had in your profile. And AP CdA values are are based on the profile values set by the cyclist.

After the ride, how do you adjust AP CdA for a different Crr?

1) Use the command Analyze/Check Calibration

2) Click the box that says "Use DFPM"
Screen Shot 2022-06-04 at 3.27.56 PM.png
Screen Shot 2022-06-04 at 3.27.56 PM.png (279.25 KiB) Viewed 4117 times
3) You'll get a value of CdA (0.314), with the original Crr value of 0.006 for Crr
Screen Shot 2022-06-04 at 3.39.11 PM.png
Screen Shot 2022-06-04 at 3.39.11 PM.png (226.7 KiB) Viewed 4117 times
4) Click the Estimated tab in the lower left corner, and manually enter the Crr value 0.004. Then, click anywhere in the window. A new value of CdA will be computed, based on the Crr value of 0.004. In this example, the CdA value is 0.341
Screen Shot 2022-06-04 at 3.30.15 PM.png
Screen Shot 2022-06-04 at 3.30.15 PM.png (270.83 KiB) Viewed 4117 times
So, reducing Crr from 0.006 to 0.004 INCREASES CdA, from 0.314 to 0.341--an increase of 0.027. This is logical; when your Crr goes down, frictional watts go down, and must be compensated for by increased aero watts. Increasing CdA by 0.027 will increase aero watts bye the amount "saved" by lower friction.

5) Your AP CdA, with a 0.006 Crr, is 0.285. With a lower Crr of 0.004, I estimate your AP CdA would be 0.285 + 0.27 = 0.312

Of course, the easy way to do this next time is to adjust your profile so that Crr = 0.004 :-)
John Hamann
Post Reply