Speed seems to drop to half

My Garmin 520 has seemed to work fine for the few weeks I've had it, but then today on a long ride the speed tended to drop to half then back up. So I'd see things like 16 km/h then a second later 32 km/h and back down to 16 and so forth.

My heart rate and cadence seemed to be accurate (I don't think there's any ANT+ problem).

Here's the ride - the problem is worst at around 2 hours and 13 minutes. https://connect.garmin.com/modern/activity/1043269445

(There are other issues with this like the calories, distance, and power figures, but I think those are separate).

Any ideas?

i had this happen on my old bike with magnetless sensors. i also only had the issue with speed occasionally. assuming the sensor is on your rear wheel, the recommendation you will receive is to try the sensor on your front wheel. it might be missing a wheel revolution every once in a while which could cut your speed in half.

If you have the new magnetless sensor it could be a battery contact issue. Try rotating the battery cap between open and closed a number of times. That may get the contacts that are in the cap seated better. If that doesn't fix it try putting something behind the battery so it presses tighter against the negative contact. That's in addition to trying it on the front wheel.

I used the Fit File Repair Tool to correct the Edge distances using GPS data.

https://www.dropbox.com/s/kux00jivt7wo7vl/activity_1043269445-fixed.fit?dl=0

My Garmin 520 has seemed to work fine for the few weeks I've had it, but then today on a long ride the speed tended to drop to half then back up. So I'd see things like 16 km/h then a second later 32 km/h and back down to 16 and so forth.

Exactly the same issue. Happens only with magnetless speed sensor. Seems to be some flaw of Edge 520 design, as my Fenix3 never had this problem.

Did you buy your Edge recently, or it was one of the first units shipped? I have a suspicion that a first batch of units had this problem.

Hmmm....the Garmin is new. It is a magnet speed sensor. :confused:

Will try it on the front or see if there's logic behind it somehow only picking up every other revolution (like if it's wobbling?)

Thanks.

Most likely the magnet is poorly aligned. For a GCS 10 it should be aligned with mark on the speed arm with the gap between magnet and arm being 2 to 5 mm.

I have similar with mine with magnetless sensor. Happens occasionally, but seem to happen in specific locations. These locations seem to have a concrete barrier with metal railing near the path, but could be a coincidence. Seems to be a loss of ANT signal or something. Not a big issue for me but would be good to sort it. I suspect some smarts in the software that could identify when the sensor speed is less than 60% of GPS reported speed with good GPS signal and speed above say 10kph, and switch over to GPS speed under these conditions would alleviate the problem for most.

I have similar with mine with magnetless sensor. Happens occasionally, but seem to happen in specific locations. These locations seem to have a concrete barrier with metal railing near the path, but could be a coincidence.

It may not be a coincidence. The speed sensor senses the earth's magnetic field so something creating a magnetic filed could interfere.

I am a bit surprised to hear it detects via magnetic field. It seems pretty obvious it will be effected by magnetic fields produced by electronics/motors etc. I would have thought they would use an accelerometer and sense gravity. There are not many devices that can produce a gravitational field to compete with the Earth!

Just out of curiosity I would like to know why it works that way. The cadence sensor is based on accelerometer measurements. There don't seem to be any significant issues with interference other than maybe with magnetic trainers.

dropping by to say you are not alone. mine just picked up this speed drop in half "feature" 3 rides ago. twice before 5.3 update and today it's still the same. weird that it drops in half and not to zero. i guess it's picking up every other wheel revolution. cadence works fine, so I don't think it's a magnet issue. I have more problems with this than any other garmin I have owned. real disappointed