Garmin Edge 500 - Inaccurate speed reading when cold
Discussion
When I start a ride in cold weather my Garmin Edge displays some crazy speeds for the first 2-3 miles or so before settling down.
It is on a fixed speed bike with no cadence sensor so it is just using GPS. I always switch it on and leave it outside for 5 minutes or so while I get ready so that it has a chance to get a satellite fix.
The average speed seems accurate enough (16mph for first mile or 2) but the instantaneous speed is crazy - 26-35Mph which is faster than I can spin my legs.
After a couple of miles it settles down. This does not happen when the weather is warmer (above 10°C).
I know I need a few miles to warm up but what is happening with the Garmin?
It is on a fixed speed bike with no cadence sensor so it is just using GPS. I always switch it on and leave it outside for 5 minutes or so while I get ready so that it has a chance to get a satellite fix.
The average speed seems accurate enough (16mph for first mile or 2) but the instantaneous speed is crazy - 26-35Mph which is faster than I can spin my legs.
After a couple of miles it settles down. This does not happen when the weather is warmer (above 10°C).
I know I need a few miles to warm up but what is happening with the Garmin?
I used to have the Garmin Edge 500 and my old Cateye cycle computer running together on my MTB.
I find that the instantaneous reading on the Garmin is affected by many factors, mostly tree cover and low temperatures. It was often as much as 5mph out when compared to the Cateye. It can take an age to catch up with significant changes of pace too, but if I settle into a nice rhythm it usually settles down too. The Cateye was the wired kind, accurately calibrated to the wheel circumference, and even if it was slightly 'out' for distance, it would still record the same comparative error for a given speed, whereas the Garmin can be both over-reading and under-reading at different points. The thing is, when I stop and cycle through the data on the Cateye at the end of a ride, it's always as near as makes no difference the same figures as the Garmin.
I dumped the Cateye to make space for bar lights, but I think that the issue with the Garmin is that the inner gubbins are interpreting data to artificially create a 'current speed' yet when you upload the data it all settles out and speed is worked out from a trail of accurately plotted GPS coordinates. I don't worry about it too much, as I only really use it to log my map trace data and total mileage. My biggest worry with my Garmin is that the battery life indicator is permanently at 100% these days. I took an enforced 5 month break from riding last year, and the 'fuel gauge' would fall as expected prior to putting it away. Then, since I started riding again, it has never once dropped below 100% battery life available.
I find that the instantaneous reading on the Garmin is affected by many factors, mostly tree cover and low temperatures. It was often as much as 5mph out when compared to the Cateye. It can take an age to catch up with significant changes of pace too, but if I settle into a nice rhythm it usually settles down too. The Cateye was the wired kind, accurately calibrated to the wheel circumference, and even if it was slightly 'out' for distance, it would still record the same comparative error for a given speed, whereas the Garmin can be both over-reading and under-reading at different points. The thing is, when I stop and cycle through the data on the Cateye at the end of a ride, it's always as near as makes no difference the same figures as the Garmin.
I dumped the Cateye to make space for bar lights, but I think that the issue with the Garmin is that the inner gubbins are interpreting data to artificially create a 'current speed' yet when you upload the data it all settles out and speed is worked out from a trail of accurately plotted GPS coordinates. I don't worry about it too much, as I only really use it to log my map trace data and total mileage. My biggest worry with my Garmin is that the battery life indicator is permanently at 100% these days. I took an enforced 5 month break from riding last year, and the 'fuel gauge' would fall as expected prior to putting it away. Then, since I started riding again, it has never once dropped below 100% battery life available.
Main reason for that behaviour is probably down to how instantaneous speed is calculated vs time on distance.
Generally GPS receivers calculate instantaneous speed by doppler principle - the frequency shift in an incoming signal. Don't ask me how the implementation actually works. Once you upload to connect etc., the speed numbers are coming from time on distance. Any positional jitter is going to even out over the long term.
Generally GPS receivers calculate instantaneous speed by doppler principle - the frequency shift in an incoming signal. Don't ask me how the implementation actually works. Once you upload to connect etc., the speed numbers are coming from time on distance. Any positional jitter is going to even out over the long term.
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