Crr

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Aeropodster
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Joined: Thu Nov 01, 2018 9:01 pm

Crr

Post by Aeropodster » Fri Nov 02, 2018 7:46 pm

I have some questions about Crr in Isaac that I'm hoping someone may be able to help with.

1. Does Isaac assume a constant Crr for an entire ride, or can it vary?
2. If it can vary, does it vary formulaically (e.g. with temperature), or is it solved, as with CdA?
3. Also if it can vary, is there any way to see the selected Crr for a given segment along with the CdA in the CdA Analysis window?

The context here is using an Aeropod with a DFPM to estimate CdA, and I'd like to understand what's happening with the Crr at the same time.

Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 9:43 am

Re: Crr

Post by Velocomp » Sat Nov 03, 2018 6:37 am

We hold Crr constant when doing CdA measurements. (We say so several times in the instructions). :-)
John Hamann

Aeropodster
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Joined: Thu Nov 01, 2018 9:01 pm

Re: Crr

Post by Aeropodster » Sat Nov 03, 2018 6:44 am

Thanks John, I'm sure I'd read that at some point, but forgotten.

The reason this came up is that I did a set of out and back runs over an hour, during which time the temperature dropped by 5 degrees. The CdA in Isaac drifts upwards, by an amount which looks like it offsets the increase in Crr I would expect due to temperature.

Is there any way for me to compensate for this in Isaac, or is it something I'll just have to adjust for outside the software?

On a similar topic, the CdA estimates in the CdA Analysis window don't seem to change when I adjust the Crr for the ride as a whole quite materially. Is this what you would expect?

Velocomp
Velocomp CEO
Posts: 5355
Joined: Fri Jan 11, 2008 9:43 am

Re: Crr

Post by Velocomp » Sat Nov 03, 2018 7:56 am

Please post the ride file.

There is a tricky way you can use Isaac to play around with the effect of Crr on CdA. It's a bit cumbersome but it works; I will try to explain

1) Open the ride file

2) Select "Analyze/Tweak CdA, Crr, Cm"
Screen Shot 2018-11-03 at 6.58.43 AM.png
Screen Shot 2018-11-03 at 6.58.43 AM.png (158.86 KiB) Viewed 240 times
3) This window will appear (your data will be different):
Screen Shot 2018-11-03 at 7.00.54 AM.png
Screen Shot 2018-11-03 at 7.00.54 AM.png (145.33 KiB) Viewed 240 times
4) You will see the values of CdA and Crr used for this ride file. Change Crr to whatever you would like it to be; click Accept. In this example I've adjusted it to be 0.0045:
Screen Shot 2018-11-03 at 7.23.21 AM.png
Screen Shot 2018-11-03 at 7.23.21 AM.png (126 KiB) Viewed 240 times
5) Click Accept. Isaac will recalculate AP watts using a Crr of 0.0045. You've now adjusted the ENTIRE ride file to a new Crr of 0.0045 (that is, AP watts are calculated on the basis of a Crr of 0.0045). The next steps will figure out the corresponding CdA with the tweaked Crr value of 0.0045

6) In the main window, check the "View DFPM" box, and highlight the section you'd like to analyze for CdA.
Screen Shot 2018-11-03 at 7.25.13 AM.png
Screen Shot 2018-11-03 at 7.25.13 AM.png (190.56 KiB) Viewed 240 times
7) Look at the "Average DFPM Pow" number in the Selection Stats. This is the average power measured in the interval selected (233.6W):
Screen Shot 2018-11-03 at 7.29.47 AM.png
Screen Shot 2018-11-03 at 7.29.47 AM.png (55.31 KiB) Viewed 240 times
8) Uncheck the View DFPM box and look at the Avg Power. This is the average power measured by AP, using the assumed CdA value of 0.254 AND the tweaked value of Crr = 0.0045. (Both CdA and Crr are seen below the NP 251W in the selection stats window). In this example average power is 237.7W
Screen Shot 2018-11-03 at 7.32.28 AM.png
Screen Shot 2018-11-03 at 7.32.28 AM.png (52.59 KiB) Viewed 240 times
9) The governing principle of CdA measurement is that opposing AP watts should equal applied DFPM watts.

10) In this example, AP power (237.7W) is HIGH compared to DFPM power (233.6). Therefore, the assumed CdA of 0.254 is too high (the higher the CdA, the more watts AP will report).

11) So, in this example we will need to tweak CdA DOWN somewhat, until the AP watts are reduced to 233.6W, the DFPM watts

12) Go to "Analyze/Tweak CdA..." and type in a lower value for CdA (this is where you have to do some guessing). The applied/opposing watts values already pretty close, so as a first guess, I reduce CdA to 0.251,
Screen Shot 2018-11-03 at 7.43.44 AM.png
Screen Shot 2018-11-03 at 7.43.44 AM.png (148.08 KiB) Viewed 240 times
13) I click "Accept", and Isaac recalculates AP power with the new, assumed values of CdA = 0.251 and Crr = 0.0045. It reports 235.6--down 2 watts from the previous 237.7.
Screen Shot 2018-11-03 at 7.45.58 AM.png
Screen Shot 2018-11-03 at 7.45.58 AM.png (53.53 KiB) Viewed 240 times
14) AP Watts dropped; we're moving in the right direction, but we still have 3 more watts to lose! a 0.003 adjustment reduced AP power by 3W, so for my next guess I reduce CdA to 0.248
Screen Shot 2018-11-03 at 7.50.34 AM.png
Screen Shot 2018-11-03 at 7.50.34 AM.png (150.59 KiB) Viewed 240 times
15) Click Accept: AP watts are now 233.5--dead on with DFPM watts of 233.6W!
Screen Shot 2018-11-03 at 7.51.42 AM.png
Screen Shot 2018-11-03 at 7.51.42 AM.png (54.85 KiB) Viewed 240 times
------------------------

So, for this example, when we reduced Crr from 0.0054 to 0.0045, the associated CdA that balances opposing and applied watts is determined to be 0.248

Though this is a somewhat cumbersome process, you can do a lot of playing around to understand the effect of Crr and CdA on watts.
John Hamann

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