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Speed and distance reading low

During today's 2 hour ride everything was find for the first 1.25 hours. Then I noticed that the speed was much lower than expected. Cadence seemed OK. Battery was at 80%. It's paired to the older Garmin GSC10 Speed/Cadence Sensor that came with my old 705, but it's worked well the past couple of months. The magnet was aligned correctly.

Towards the end of the ride I know the exact 5, 4, and 2 mile points. I lost over a half mile during the last 5 miles.

To make matters more interesting, my cell phone was fully charged at the beginning of the ride and completely dead at the end. The phone and 520 are connected by Bluetooth.

Is there an issue with the phone, Bluetooth, and the 520 at times? Should I turn off Bluetooth on my phone when riding? If I remove the cadence magnet, will it default to GPS for speed/distance yet still have the cadence working?

0 Former Member over 8 years ago

Most likely it is a sensor issue. Check the gap between the magnet and sensor. It should be 2 to 5 mm. If the alignment and gap are good the next thing to try is a new battery in the GSC 10.

0 dgerard over 8 years ago

If I remove the cadence magnet, will it default to GPS for speed/distance yet still have the cadence working?

No. I had this same problem when the wheel speed sensor became intermittent in my wahoo blue sc sensor. My edge would act like it lost gps signal and at one point I lost miles of data that was not recoverable. The edge does not fall back to gps coverage when it loses the speed sensor data if there is a speed sensor paired to the device. Fixing my speed sensor issue fixed everything. I haven't lost a foot in weeks and what I had thought were gps issues have never recurred. It was all due to an intermittent speed sensor.

0 Former Member over 8 years ago

I always set the wheel circumference for my speed sensor instead of letting the Garmin automatically determine the circumference. The automatic calculation depends on the GPS and if you often drop GPS because of the area/terrain you are in (i.e. heavily wooded) it can change the wheel circumference mid-ride and this will cause your speed and distance calculations to be incorrect.

0 Former Member over 8 years ago

So what is the solution?

I always set the wheel circumference for my speed sensor instead of letting the Garmin automatically determine the circumference. The automatic calculation depends on the GPS and if you often drop GPS because of the area/terrain you are in (i.e. heavily wooded) it can change the wheel circumference mid-ride and this will cause your speed and distance calculations to be incorrect.

May I know more in details, e.g. what is the solution? I have the same problem which I lost my riding distance...

0 Former Member over 8 years ago

s.s.ong what type of sensor do you have? GSC 10 or the newer magnetless? Do you mountain bike in heavy tree cover or do you road bike?

0 jerrystl over 8 years ago

I've ordered the new magnetless speed and cadence sensors to replace my old GSC10. Hopefully this improves both readings.