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Inaccurate speed reported using magnetless speed sensor and Garmin 520

DeltaBCDad
over 8 years ago
I have a Garmin 520 computer with the magnetless speed and cadence sensor, but I'm having some difficulty with it. Specifically the speed readings I'm getting are very frequently obviously incorrect when using the speed sensor. For example, it'll display speeds ranging between 10 km/h to upwards of 100 km/h, but my actual speed is probably closer to 30 km/h. These errorneous readings will hover around an incorrect value for some period of time (seconds to minutes) at which point the speed will bounce around a bit and then settle at a new reading, which may or may not be correct.

I've replaced the battery in the speed sensor, but that hasn't helped. I've changed the orientation of the speed sensor (pulling the rubber loop over the top from the front or the rear). I've tried forcing the diameter of the wheel to the actual measured value- but none of these things have worked- I continue to get strange readings, to my dismay.

I've read that these devices can be very sensitive to the presence of magnets on the bike. I don't have any magnets on my bike but it is a steel framed bike, could that be the problem?

I'm very frustrated because for the most part I'm very happy with it, but the crazy speed readings are maddening. I can rely on the GPS speed sensor, but I like how much faster the wheel based speed sensor recognizes starts and stops.

Does anyone have any suggestions? If I could buy a GSC 10 I'd do that because it'd probably solve all the problems I'm having. I'd even buy a Wahoo speed sensor if that'd solve the problem.

Thanks,
Brad
  • 0 over 8 years ago
    Magnetic fields can be a problem. Chains can become magnetized and cause problems when the sensor is on the back wheel.

    If the problem consistent or does it just randomly occur? If is is consistent try taking the sensor away from the bike to see if the problem goes away. You could remove the wheel and spin it. Or take the sensor off and rotate it back and forth in your hand.
  • 0 over 8 years ago
    Magnetic fields can be a problem. Chains can become magnetized and cause problems when the sensor is on the back wheel.

    If the problem consistent or does it just randomly occur? If is is consistent try taking the sensor away from the bike to see if the problem goes away. You could remove the wheel and spin it. Or take the sensor off and rotate it back and forth in your hand.


    In this case the sensor is on the front wheel so magnetized chains shouldn't be an issue, plus it's a brand new chain. Although I'm using disc brakes, and the rotors are old. Anyway of checking to see if the rotors are magnetized? Maybe the skewer?

    The problem isn't entirely consistent- it's really, really odd. On an 8 km stretch of flat road under a constant cadence and without shifting the speed sensor will indicate a speed of ~ 20km / h for a while, then up to 80 km/h for a while, then down to 45 km/h for a while, then down to 26 km/h for a while, and then perhaps back up to 45 km / h. Sometimes the speed seems reasonable, sometimes it seems totally crazy. And it happens all over the place, not just on that road. It's so frustrating.

    Brad
  • 0 over 8 years ago
    I really don't know much other than the chain is known to be a potential problem. The rotor is something to consider. Demagnetizers are sold but I don't have any idea about the cost or how well they work. You could try moving the sensor off center away from the rotor if it will stay there. Normally is is recommend to have the sensor on the front wheel since it is closer to the head unit and away from the chain but it wouldn't hurt to try it on the rear.
  • 0 over 8 years ago
    I really don't know much other than the chain is known to be a potential problem. The rotor is something to consider. Demagnetizers are sold but I don't have any idea about the cost or how well they work. You could try moving the sensor off center away from the rotor if it will stay there. Normally is is recommend to have the sensor on the front wheel since it is closer to the head unit and away from the chain but it wouldn't hurt to try it on the rear.


    I'll try it on the rear wheel and see if that helps and I'll try taking it off the bike entirely, although if that proves to be the problem I'll have to buy a speed sensor from somewhere else. Maybe I can find a GSC 10 or Wahoo Blue SC somewhere. :|
  • 0 over 8 years ago
    Do you have any other devices (LED lights, other Garmin sensors etc) mounted near the 520?
    I once mounted a Garmin Tempe near my 810 and experienced a similar thing, I put it down interference from the Tempe blocking the signal from the speed sensor.
  • 0 over 8 years ago
    Having same issue.

    DeltaBCDad, having same issue here. You're not alone. MTB with disc brakes here.
  • 0 over 4 years ago

    I am having the same issue after installing a brand new KMC chain. If the chain is in the lowest speed (closer to the sensor), then the sensor will fail. If the chain is in the highest speed (far from the sensor) it works perfectly. The sensor also works perfectly in the front wheel. I've changed the sensor in the rear wheel and I am experiencing the same issue. Any clue? have anyone fixed this?

  • 0 over 4 years ago in reply to fgelfo

    It sounds like the chain is magnetized. The only options are to put the sensor on the front wheel away from the chain or run the chain through a degausser.

  • 0 over 4 years ago in reply to aweatherall

    Thanks... yes, when I am outdoor I use the front wheel... however, when I am in the indoor bike trainer the only option is to use the smallest gear in the cassette. I've tried using a magnet to demagnetize, however, it does not work.

  • 0 over 4 years ago in reply to fgelfo

    Maybe it is some other component in the drive chain that is magnetized. You might be able to install a phone add that will allow you to find where the magnetic field is coming from.