Bicycle Mechanics - Inconsistent speed with polar S520

I have a polar S520 and I'm recording speed and cadence. Most of the time, it seems to record the speed just fine but frequently the HRM will beep and show a speed of 0 only to resume in a few seconds with the accurate speed. Or, sometimes it will show I'm going 6 miles/hour when just a few seconds earlier I was going 15. I've tried adjusting the magnet and the sensor but I can't seem to adjust for this problem. Anyone else have a similar experience? :(

I hate digital stuff sometimes. I have found that a good place to start is to cut the power to something and then turn it back on. This has worked for me with everything from cell phones to CD players.

Does it do it in the same stretch of road. Could be power lines or outside interference. If it happens in the same area that is probably the problem.

I have a stretch of road that I have to edit in my software everytime I ride it

I had a similar problem with my Polar S720. Then a button broke off, so I sent the whole thing back (in warranty) - they claimed to make an internal adjustment. No cost, but a hassle to send it in & wait, assuming it is in warranty. Mine was pretty bad, it would record speed if I kept my wrist within about 6" of the pickup (pretty dangerous).

They are also sensitive to interference, either from power lines or electric fences (cattle, horses, etc). If it is on a certain stretch of road that may be the problem. If it is inconsistent I would contact Polar.

Had similar experience with the speed pickup. I had to adjust it many times. I changed the battery in the speed unit also without improvement.

I found the following to be true though trial and error.
The pickup will not transmit to the 520 if mounted on the left side of the bike, even with the 520 mounted on the left side of the bar. The distance for the pickup to the 520 needs to be less than listed in the instructions. (16" , if i remember correctly) The 520 must be on the bar (right side) in the same plane as the speed pickup. The magnet needs to be no more than 2.5 mm from the pickup. (closer is better) EMF's from electrical lines will make it malfunction. :(

The only time I've had a problem with mine is when I ride underneath streetcar power lines. Then my heart rate jumps to 240bpm, exploding my heart which should leave me for dead at the side of the rode. Miraculously I survive to ride out from under the lines resulting in a normal* HR reading

I have my speed pickup set up on the left side and have no issues.

(*normal being kinda high especially after a winter of getting fat and lazy. Indoor trainer what's that??).

I have a polar S520 and I'm recording speed and cadence. Most of the time, it seems to record the speed just fine but frequently the HRM will beep and show a speed of 0 only to resume in a few seconds with the accurate speed. Or, sometimes it will show I'm going 6 miles/hour when just a few seconds earlier I was going 15. I've tried adjusting the magnet and the sensor but I can't seem to adjust for this problem. Anyone else have a similar experience? :(


I just wanted everyone to know I solved this problem by moving my speed sensor to the same side of my bike as my HRM *and* moving the sensor closer, i.e. closer to the HRM *and* changing the orientation of the pickup so it would be even closer to the HRM. On my last two 50+ mile rides, it only dropped out once briefly.
I also had a problem after I crashed my bike with the HRM indicating twice the actual speed. I reset the HRM by pushing the small recessed button on the right side in with a paper clip and holding it and that solved that problem.