Different distance Garmin and Powerpod in Isaac

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PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Wed Jul 27, 2016 11:55 am

I am riding with my Powerpod for 4 months (70 rides) on a road-bike and an MTB; using a Garmin Egde 800 and Garmin sensors 2.0 on both bikes.

It helps me on the rides and it gives an extra dimension and fun.

When I download the files from the Powerpod in Isaac the "distance" in Isaac is greater then the value in de Garmin. In the Garmin I have choosen for manually size the tire setting; for the roadbike (Schwalbe 700x35C; Schwalbe gives a value of 2185mm) I use 2168mm, the size Isaac gives for 700x35C. For the ride today I used in the Garmin 2177mm.
At the end of the ride the distance Garmin gave was 112,3km. In Isaac the Powerpod file gets a name ending with 112km so I think this is ok. But the distance in Isaac has a value of 119,4km. I had to use a 2040mm value in Isaac to correct the distance to 112,3km.

My experience is that the value for the distance in Isaac is (almost) for any file greater then the Garmin value (which in a software program is checked with GPS).

Certainly I am doing something "wrong". What must I do to get in the situation that the distance will (nearly) be correct in Isaac for a ride ?
Attachments
iBike_07_27_2016_0932_112_km.ibr
Importing the file it gets a name ending with 112km. The distance in Isaac is 119km. 112km is correct.
(681.27 KiB) Downloaded 59 times

texmurphy
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Re: Different distance Garmin and Powerpod in Isaac

Post by texmurphy » Wed Jul 27, 2016 5:56 pm

You likely have your garmin set to auto-calibrate the wheel size. As a result, the garmin may use a different wheelsize on every ride.
The computed wheelsize will be dependent on your gps track for the first 1 km of riding each time you start an activity. If that track is under tree cover, or not straight, or your 800 has not fully acquired satellites then the wheelsize will be incorrect.
On your 800, go to Bike Profiles->Bike Details and enter a Custom Wheel Size - obviously use the same size as your Powerpod/Issac setting.

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Thu Jul 28, 2016 5:02 am

Thanks.

I used 5,5 bar in the tire. I did a short ride (open area, good asphalt) with the Garmin on "automatic". It gave a size of 2177mm.
I use this size and gave it in manual (no "automatic") in the Garmin for the ride.

Schwalbe Surpreme 700x35C, Schwalbe gives a size of 2185mm. After 13.000km of use and not inflated to 6 bar, the tire is a little "smaller" :-)
The value in the Powerpod is 2168mm (the default for 700x35C).

Normal I use 2168mm in the Garmin and in the Powerpod/Isaac. But this does not explain the difference in the file when downloading it in Isaac. The file name gets an 112km "extension" in Isaac, in the file the distance is 119km.

Velocomp
Velocomp CEO
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Re: Different distance Garmin and Powerpod in Isaac

Post by Velocomp » Thu Jul 28, 2016 9:05 am

I looked at your ride file.

The actual distance measured, from beginning to end, is 119.37KM. So, Garmin and PP agree

The ride file name does indicate 112KM. Why the file name is different from the ride length is a mystery to me. You can rename the ride file, with the correct length, if you wish.
John Hamann

PaulK32
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Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Fri Jul 29, 2016 9:34 am

Thanks for the reply John. But the situation is something different.

The Garmin gives 112,3km.
Download of the Powerpod file in Isaac gives a (standard) file name "Ibike_date_time_amount_km's". The amount is 112km. So far so good I thought. But then ...
The data inside the Powerpod file gives 119,3km.

So I did a ride of 112km (sure I did this same ride several times in 2015/2016) and the tire-size in the profile is 2168mm. This is also in the Powerpod file. Why gives Isaac the file name 112km and inside the file there is 119km ?

I am sure I am doing something wrong, but what am I doing wrong ?
Can it be a mismatch of the profile I use in the Powerpod and the "actual" profile in Isaac when downloading the files from the Powerpod ?

Velocomp
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Re: Different distance Garmin and Powerpod in Isaac

Post by Velocomp » Fri Jul 29, 2016 7:42 pm

Measure the circumference of your wheel manually by marking exactly one wheel rotation on your pavement.
John Hamann

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Fri Sep 23, 2016 11:23 am

Good afternoon John,

I have made a lot of fine hours on the mtb, now made some rides on the roadbike.

I experience the same "issue" as I had earlier. The ride on the Garmin gives 112km and when I download the PP-file in Isaac it gets the extension 112km. But Isaac gives in the screen values 118,7km.

It is not the value in mm of the tire (Schwalbe 700x32C). After measuring set via hand set in the Garmin and Powerpod on 2129mm. I really still do not know why this happens in Isaac. But I am curious enough to find it out. Possible I found a thing to look at. In the details screen of Isaac the "Gear ratio" is shown. I did this ride on 39-15 (chainring-sprocket) and avery 20 minutes a short moment out of the saddle on 52-15.
The Gear ratio in Isaac gives 2,79 for 39-15 and 3,74 for 52-15. The Bikegearcalculator app gives 2,79 for 39-14 and 3,71 for 52-14. It gives 2,60 for 39-15 and 3,47 for 52-15.

The Gear ratio will be a "result" field form a set of "data" fields. It is a indication, that one of these data fields is not giving the right value. I hope you give it a look at. I send you the PP-file, the PP-file incl GPS and the Garmin file.

Thanks, I am still very enthousiastic in using the PP. Gives a lot of extra fun during the ride.
Paul
Attachments
PP_2016_09_19_1242_112_km_in_isaac_119km.ibr
The PP-file after opening in Isaac and incl Garmin GPS data.
(1.13 MiB) Downloaded 27 times
PP_2016_09_19_1242_112_km.ibr
The PP-file
(683.25 KiB) Downloaded 26 times

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Fri Sep 23, 2016 11:38 am

The .fit file from the ride.
Attachments
1369149000.zip
The Garmin file
(244.2 KiB) Downloaded 27 times

Velocomp
Velocomp CEO
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Re: Different distance Garmin and Powerpod in Isaac

Post by Velocomp » Fri Sep 23, 2016 2:18 pm

This is quite mysterious!

Please send your .ibr file as downloaded, not the .ibr ride file that is merged with the Garmin file. I'd like to see the data that is coming out of your PowerPod.
John Hamann

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Sun Sep 25, 2016 2:52 pm

The mystery will be solved :-)
Attachments
PP_2016_09_19_1242_112_km.ibr
Downloaded file from PP
(683.25 KiB) Downloaded 23 times

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Sun Sep 25, 2016 2:59 pm

After the mtb ride of today I downloaded (and erased) all the files on the PP. In the download-list I saw the file now with another extension of 119km. I attach this file.

I hope you can find what causes this.
Attachments
PP_2016_09_19_1242_119_km.ibr
Downloaded 25-09-2016. Now this file has a extension of 119km in stead of 112km.
(683.26 KiB) Downloaded 25 times

Velocomp
Velocomp CEO
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Re: Different distance Garmin and Powerpod in Isaac

Post by Velocomp » Mon Sep 26, 2016 10:19 am

I have investigated this some.

PowerPod firmware counts the number of revolutions of the bike wheel, then at the end of the ride file computes the length of the ride file by multiplying the number of wheel revolutions by the circumference. This ride file length is stored as part of the ride file name "iBike YYdate_xx length" As long as the number of revolutions stored in the counter isn't "off", then the ride file length xx will be approximately correct. There are situations where the ride file length counter is corrupted, for example, if the ride file isn't terminated properly. So, I have seen examples where the ride file length header is incorrect.

Isaac software receives speed, cadence, power data, and a lot more. Isaac software uses the speed data (reported once per second) to figure out the distance traveled. This is a completely different way to compute distance and, because there are a large number of data points, is less prone to error. Note however that for distance to be correct, speed has to be correct. Speed is determined by the circumference stored in the profile at the time of the ride.

How your Garmin computes ride length is not known. If it is using GPS data, that can be notoriously unreliable. If it is using speed/cadence sensor data, then it should provide a number comparable to PowerPod, as long as the circumference is the same as used in PowerPod. However, I know that Garmin has some kind of system that adjusts circumference based on GPS data. So, who knows what circumference it is actually using?

You have a whole series of PP rides that show that this particular ride is 119KM. This length is based on Isaac calculations and thousands of data points, both of which I trust. It does not bother me that the ride file header length in PowerPod is somewhat random; a single error in that counter can result in a mistake. Furthermore, the only purpose of that header number is to make it easier to find a particular ride file in a long list.

The only thing I can think of doing is some kind of map investigation, where you lay out the route in an app and have the app tell you what the ride length is.

I don't have any additional insight to provide beyond this.
John Hamann

PaulK32
Posts: 26
Joined: Sat Mar 26, 2016 10:05 am

Re: Different distance Garmin and Powerpod in Isaac

Post by PaulK32 » Mon Sep 26, 2016 1:58 pm

Good afternoon John,

Thank you for the support and effort.
Enough time spent on this subject; we must be on the road enjoy riding and the views.

Paul

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