Hello All,
I have recently got this Garmin speed/cadence sensor (non-magnetic type) - https://buy.garmin.com/en-US/US/p/146897.
Problem I see:
I see that the Garmin speed sensor is not showing accurate distance. It always shows around 700m - 750m for an actual Kilometer of distance measured by a couple of GPS devices I used and my car odometer. This looks to be a huge deviation from the actual distance.
Things I have tried already:
1> Replaced the battery for Garmin Speed sensor with a brand new battery >> shows the same inaccuracy
2> disabled the cadence sensor and used only the speed sensor >> shows the same inaccuracy
3> Used the manual wheel size (2068mm for my wheel size that is 26 x 2.10) according to the Garmin website chart - http://www8.garmin.com/manuals/webhelp/fenix3/EN-US/GUID-DB0BC20A-DADF-41EB-A61C-14BEC158B43C.html >>> shows the same inaccuracy
4> Manually measured the circumference of the wheel using a measure tape when it finished one rotation, turned out to be very close to what Garmin chart above states, it is 2100, should be very close if I consider the pressure on the tyre when loaded >>> Shows the same inaccuracy
5> Used another device, my son's FR 230 for the same Speed and Cadence sensors, >>> Shows the same inaccuracy, in fact the distance showed by FR-230 is same as it shows on my f3 for very short loop around my building
6> Used the GPS to GPS mode in the app, moved it to UltraTrack mode and Turned off the GPS, but logically, this should not make any difference when f3 is just going to look at the speed sensor for distance when Auto wheel size calibration is completed OR when manual wheel size is set as it was in my case >> shows same inaccuracy
7> Comparing the distance with 3 devices for very short measured loop of 500 meters around my building, devices are 1>f3 with Garmin speed sensor, 2>Strava on my OnePlus3 cell phone, FR-230 using GPS for the activity. The results for more than a few rounds of this 500 meter loops show that FR-230 using GPS (GPS, high accuracy mode) and Strava on my cellphone shows same distance as it was measured by odometer for the loop/stretch with a deviation of around 10 meters in some cases. However, my f3 with this new speed sensor shows the difference of about 200 meters to 250 meters for a kilometer of stretch consistently, even with the manual wheel size set as per Garmin and per my measured wheel size - inaccurate results
8> I decided to use the Auto wheel-size (that is 2043mm) on my recent long trip to compare the performance with my short loops / rounds around the building. A couple of friends with whom I went, two of them using Strava and one of them using a cheap magnet based speed sensor shows the distance close to 98 kilometers, while my f3 with the Garmin speed sensor shows slightly over 80 kilometers for the ride - A screenshot that I'm trying to attach here should show how at certain places in the FIT file, I see the speed dropping to zero in the activity while cadence still shows certain value. I'm not sure if cadence sensor missing rotations since I may have not be pedaling at certain point, but speed sensor should surely show values when my cadence sensor is showing some!! Am I assuming it right? Am I missing something here??
Let me know if you guys have any ideas to try few more things? Or anyone has faced similar issues and if probable solution to make it work? I don?t think I should really tweak the size o my wheel to make it work, it may not provide me accurate distance if sensors are dropping dead in between / missing rotations in between!!
TIA.
Ketan
