CRR and CDA recommended values for MTB

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TuRu00
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Joined: Wed Jan 04, 2017 9:34 am

CRR and CDA recommended values for MTB

Post by TuRu00 »

Hello!

I am ready to ride with PowerPod and my MTB, and reading the forum noticed that the CRR and CDA (m2) values should be changed for MTB. I read that .0082 and 0.049 should be fixed values. Is that right? Should I change this before or after an O&B calibration? Any advices about calibration with a MTB bike?

Bike its a 29er 2.3 back and 2.1 rear with 2 bar.

Thanks!
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

Those are the correct values.

I would change the values prior to the O&B ride; that way, after the calibration is finished you will be ready to go!
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Yesterday I did the steps but something wrong happened. Today I will try again.

1) Created a new profile as shown on Cal Ride Video. Changed CdA and CRR.
2) Attached PP to MTB.
3) Paired sensors.
4) Green and red flashing.
5) Pushed button. Started flashing orange.
6) Riding... 1,2,3...49,50,51,52... 56. What the hell?
7) Stopped and tried to turn back, 57,58... what the hell? I pressed 4 seconds central button. 50 stucked (i stopped many times showing 0 on my speed sensor in order to let the PP to know that i was turned my way).

after my tries I pressed 10 seconds central button and it asked me to re-calibrate. One more cal-ride. It went from 0 to 100 without stopping on 50. This time i did not stop on the 50s... After that something like a reboot and started to show power.

Another behavior I don't like: during the ride on MTB (after the unsucesfull cal ride) Garmin showed 0w a lot of times, but the ride on PP memory it didn't. May be is because of the stones and "jumps"? PP and Garmin are a few centimeters far away.

Thanks
TuRu00
Posts: 69
Joined: Wed Jan 04, 2017 9:34 am

Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Another question for today. Should I calibrate it on the road or gravel with my MTB?

Thank you!
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

I would calibrate on the road; it's easier to do!

Please send a ride file taken after the cal ride.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

I made a hard reset using isaac. It was impossible for three times to get the O&B calibration. Don't know why but when counter gets to 50 it continues.. no red lignt and no 50 stall. No idea. Files attached.

Now I changed CRR and CDA values after the cal and on the profile and i am going to test if they are consistent or not.
Attachments
MTB_27_06_2018_1716_1_km_HiDef.ibr
(13.92 KiB) Downloaded 261 times
MTB_27_06_2018_1705_4_km_CalRide.ibr
(106.21 KiB) Downloaded 213 times
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

It looks like your turn-around point was on a hill. That's OK, but if you happen to be going slow enough, PP will think you've stopped and you won't get the turn-around cue.

I looked at your post-cal ride file and it looks OK.
John Hamann
TuRu00
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Joined: Wed Jan 04, 2017 9:34 am

Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Sorry John but i think that the cal-ride was not ok (for 3 times) because of any hill. The third time was the same track that i used may times with my road bike successfully.

In any case, after an hour of riding i think that the calibration is pretty good. Specially on the second part of the ride. A little bit high over readings may be because i think that NP 163 and IF 0,86 is too high for today. I am quite impressive with results on MTB. I attach the file for any fine-tuning, if needed.

By the way, why PP temperature is 25 degrees when here in Spain we are at least at 30?

Thanks!
Attachments
MTB_27_06_2018_1732_16_km_HiDef.ibr
(496.95 KiB) Downloaded 219 times
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

This does look good.

I don't know why temperature would be "off", but in any event it is not used in any calculations.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Hello John! Do you why it says that I was drafting between 38:00 and 40:00 (in minutes) while descending? I wasn't. Just descending.

Thanks!
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

You had a strong tailwind during your descent. If a tailwind is strong enough, the software guesses that drafting was involved.

Sometimes the guesses are wrong...
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Hello!

I would appreciate very much you to review my IBR file, because I think that is over reading. My IF for today was 1,47 and it wasn't.

I've noticed some things:

* On Isaac tracks are a little bit shorter than Garmin.
* My Tires are 29x2.0 (there is no option for this on Isaac).
* Elevation is always different. Today on Isaac was about 840m, real elevation is 920m.
* On the beginning of the ride, watts were pretty high. Also the slope was around 22% and it wasn't.
* Attached TCX (from Garmin) and IBR (original, no merged)
* Isaac says that I made 239watts for 10 minutes, that is too much for me now. (267 for 5 minutes is also too much).

Any help? I don't want PowerPod to fake my numbers during the season (not now, but in the short future) and I would love to make some base in MTB.

Thanks very much for your kind help and support.

Arturo.
Attachments
MTB_16_07_2018_1756_26_km_HiDef.ibr
(860.27 KiB) Downloaded 213 times
activity_2856484712.tcx.zip
(275.2 KiB) Downloaded 197 times
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

1) For 29 x 2 , you can figure out in millimeters the circumference of your wheels, then enter that manually in your profile

2) PP measures barometric pressure, then converts that to elevation. Weather changes cause changes in baro pressure, so that accounts for the elevation changes. You can correct the starting elevation in Isaac: "Analyze/Analyze Device Settings/Starting Elevation"

3) During the first 90 seconds PP recalibrated itself. Prior to that the watts were high

4) In the section where your watts are high, I suspect your front fork was not locked out. If you lean back on a flexible suspension, the reported slope (and power) will be exaggerated. The best way to get stable readings is to lockout the front fork on climbs.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Thank you John !

1) Ok. Just changed.
2) Regarding to this.. it is better to load elevation from GPS (i guess that calcs change) or to correct elevation the way you point out?
3) I think that the fork was locked. I tried yesterday one more time 100% sure of this. Watts were pretty close to my limits, I will try this weekend before reporting.

Thanks!
TuRu00
Posts: 69
Joined: Wed Jan 04, 2017 9:34 am

Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Hi again,

I am definetely sure that my powerpod does not measure distance properly. Don’t know why but in a 37 km ride it counted 29. Wheel circumference was changed to right values. any idea?

Also it is still over reading while uphill, even with locked fork because my IF and Power is too much.

Today I made hard a reset and another o&b cal ride (it did not stopped on 50). I will tweak cda and crr values step by step until I get it more accurate.

Any advice on this? I need PP to measure properly on MTB. Please let me know if you need me to enhance your algorithms.

Thanks.
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

Please post your ride file.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Thank you John.

IBR file is not the "original" because I merged the Garmin GPS data (in TCX format). Look at the differences while merging.

By the way, It seems like Isaac does not support FIT 2.0 file format, is that right? It does not read FIT from Garmin.

Thanks!
Attachments
Captura de pantalla 2018-07-24 a las 9.48.20.png
Captura de pantalla 2018-07-24 a las 9.48.20.png (138.17 KiB) Viewed 13917 times
MTB_21_07_2018_1032_29_km_HiDef.ibr
(1.34 MiB) Downloaded 215 times
2861730413.fit.zip
(94.41 KiB) Downloaded 208 times
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Kind reminder :-)
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

Your speed is showing a lot of dropouts. What kind of sensor do you have? Where is it attached?
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Magene (https://www.amazon.es/210-Ant-velocidad ... B077NG5N5H) magnetless speed sensor on front wheel.
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

This sensor is working very, very poorly. I would replace it with a Garmin sensor.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Thank you jhon. I will try with another one.
TuRu00
Posts: 69
Joined: Wed Jan 04, 2017 9:34 am

Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

After changing to Garmin Speed Sensor no problems anymore. Thank you John. Now results are pretty close to my normal numbers. The only thing is that there are some differences between Garmin records and PP records (about 10-12 watts average). Should I change the smoothness on PowerPod?

Thanks!
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

Hmm...numbers recorded by PP and Garmin should be close to the same...Garmin is (theoretically) "seeing" the same numbers as PP!

Please provide a ride file that illustrates this difference.
John Hamann
TuRu00
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Re: CRR and CDA recommended values for MTB

Post by TuRu00 »

Smoothing to 3 sec is pretty pretty close (max different in 5 sec effort 488 watt in PP, 459 in Garmin). I will set to 0 sec again and uload the IBR file to show what i mean.
Velocomp
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Re: CRR and CDA recommended values for MTB

Post by Velocomp »

There's a 99% chance that the differences are due to different smoothing algorithms between Garmin and Isaac. I doubt this is anything worth worrying about.
John Hamann
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