Inconsistent distance measurement between PP and Garmin

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doib
Posts: 14
Joined: Mon Nov 11, 2019 11:25 am

Inconsistent distance measurement between PP and Garmin

Post by doib » Sat Jan 04, 2020 2:01 pm

Hello John,
I found a big difference between the distances measured by my powerpod 3 compared to those measured by the Garmin 820. This happens, however, especially with the MTB (on which I have the sensors without a magnet both on the wheel hub and on the crank, original Garmin). On the road bike (where I have a garmin gsc-10, which in theory should work badly with PP) the readings are different for a few tens of meters, so I would say consistent. I attach the file ibr and the link to the same race today, extracted from the PP and Garmin Connect, to highlight the differences (46.14 km PP versus 50.77 km garmin). From the graph in isaac, however, it does not seem to me that there are problems in the readings of the speed sensor.

Couldn't it be a problem related to the circumference of the wheel? I had entered the size shown in the isaac choices (29x2.3 inches, the size of my front wheel on which the sensor is mounted. The rear one is 29x2.2, slightly narrower).

Other question, is the crr entered in the profile adequate? The readings seem to me sometimes high (for example the 886 Watts of an uphill sprint...), even if the average values are absolutely consistent with my standards.
Thanks so much.

https://connect.garmin.com/modern/activity/4398732306
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iBike_01_04_2020_0928_52_km.ibr
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Velocomp
Velocomp CEO
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Re: Inconsistent distance measurement between PP and Garmin

Post by Velocomp » Sat Jan 11, 2020 7:22 am

Sorry for the slow reply; your post was somehow not flagged for me.

You've got some really low speeds on your route, due no-doubt to the steep slopes you're climbing.

When you are going slow enough, such that your wheel revolutions are less than one wheel rev per second (around 4 mph) your unit will think you have stopped pedaling (you're walking) and it may stop recording.

There are some places where speed drops to zero on your climb. It's possible that these are places where the unit stops recording. This would result in lower than actual distances.

If you do a lot of this kind of riding, where slopes are steep and your speeds are slow, there is a way to workaround this, but you will need to use a magnet-based speed sensor:

1) Attach the magnet-based speed sensor to your MTB
2) Place TWO speed magnets on your bike wheel, approximately 180 degrees from each other
3) Pair the speed sensor to your MTB
4) Measure the circumference of your wheel in mm (mark a spot on your drive, move the bike forward one wheel turn, then measure the distance in mm)
5) Connect PP to Isaac, and go to "Edit/Edit Profiles/Extract from Device" to get your profile for the new speed sensor
6) In the "Basic" window you will find a place where you can select "Tire Side Wall Marking" and adjust Tire Circumference. Select "Custom" for Tire Sidewall Marking
7) Take the measured circumference in mm you measured in step 4), DIVIDE IT BY TWO, and enter it into the Tire Circumference box.
8) Click the "Send to Device" button at the bottom of the page.
9) Do a new calibration

These steps will create a system that measures speed two times per wheel revolution, effectively making your setup provide speed information at very low speeds. The ½ tire circumference you entered in step 7) will correctly account for the two measurements per revolution.

Let us know how it goes!
John Hamann

doib
Posts: 14
Joined: Mon Nov 11, 2019 11:25 am

Re: Inconsistent distance measurement between PP and Garmin

Post by doib » Sat Jan 11, 2020 6:24 pm

No problem and thanks John,
for the very clear and convincing explanation. In fact on certain off-road climbs I go almost slower than on foot .. In fact, on very slow climbs even the garmin has problems calculating the values (especially the slope, for the speed it may be that it uses the GPS data). The points where the speed goes to zero are probably points where I stopped to wait for friends. Maybe the frequent stops fooled the device. Today, however, despite having made a climb with very steep parts, there are no big differences between the measures of the garmin and those of the PP .. on the contrary, the PP recognizes me about 500 m more. But I was alone and I only made a stop at the top of the hill. So I will monitor the situation a bit and then maybe I will follow your advice and equip myself with sensors with magnet, although I would prefer to continue using the one I have.

On the other hand, in today's ride the power values are much higher than normal, definitely not credible. Even the NP is almost double my FTP ... Looking at the graphs, I noticed that in some places the slope is really high compared to the speed (it is certainly overestimated, from the 35th minute to the 38th minute approximately) .. it is actually off-road parts, on the plain or slightly uphill. Could you kindly take a look at the file to see what the problem is? I got an idea, but I'd like your expert opinion..
Thank you very much, see you soon.

Davide
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iBike_01_11_2020_1052_32_km.ibr
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Velocomp
Velocomp CEO
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Re: Inconsistent distance measurement between PP and Garmin

Post by Velocomp » Sun Jan 12, 2020 10:34 am

Your PP was bumped at the 35 minute mark. It took a few minutes for it to automatically recalibrate itself.

If you delete this section of the ride the rest of it is fine.
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doib_01_11_2020_1052_31_km.ibr
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John Hamann

doib
Posts: 14
Joined: Mon Nov 11, 2019 11:25 am

Re: Inconsistent distance measurement between PP and Garmin

Post by doib » Sun Jan 12, 2020 12:17 pm

Thanks John, this morning before going out I checked the fork pressure .... it was very low, even when blocked it went down by 2 or 3 cm..I believe that the reason for the readings was too high was that. After restoring the pressure in the fork, everything is back to normal and the values are now correct, with no peaks above my ability :-). I did not believe that the PP was so sensitive to these variations, even if you have always highlighted it. Does it depend on dynamic crr? I read that the PP should be able to "feel" the ground and adjust the readings. Of course the bicycle must be well adjusted to have realistic readings, not like mine yesterday ...

Velocomp
Velocomp CEO
Posts: 6351
Joined: Fri Jan 11, 2008 8:43 am

Re: Inconsistent distance measurement between PP and Garmin

Post by Velocomp » Sun Jan 12, 2020 2:45 pm

doib wrote:
Sun Jan 12, 2020 12:17 pm
Thanks John, this morning before going out I checked the fork pressure .... it was very low, even when blocked it went down by 2 or 3 cm..I believe that the reason for the readings was too high was that. After restoring the pressure in the fork, everything is back to normal and the values are now correct, with no peaks above my ability :-). I did not believe that the PP was so sensitive to these variations, even if you have always highlighted it. Does it depend on dynamic crr? I read that the PP should be able to "feel" the ground and adjust the readings. Of course the bicycle must be well adjusted to have realistic readings, not like mine yesterday ...
PP will measure slope changes as little as 0.05%--that's ½ foot elevation change vertically after traveling down the road 1000 feet!

PP also detects road surface roughness, though that effect is not nearly as impactful as slope change.
John Hamann

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