Speed sensor affected by Mag dumb trainer

Hi, I set up a dumb mag trainer (Decathlon InRide 100) with Garmin Speed Sensor 2 + Cadence Sensor 2 set. Worked great for some months, clocked up 475km in Zwift, then speed sensor started mucking up. Initially it would slow down and die after 5-10 minutes into a session, then on later attempts would ‘connect’ but show zero speed.
Changed sensor batteries, reset speed sensor, tried different devices (phone/computer) and figured the speed sensor was dead. The cadence sensor continues to work perfectly, and changing the phone/computer location made no difference - so it wasn’t a signal loss problem.
Eventually got myself a new sensor (cheaper IGPSport) - same deal.
I read somewhere that these ‘magnetless’ speed sensors (the type that wrap around the hub) use the earth’s magnetic field for orientation, and can be affected by other magnetic fields (eg mag trainer). So yesterday I put the sensors on my MTB on the stand outside - both Garmin and IGPS speed sensors worked perfectly. Took the old bike off the trainer, and onto the stand outside - worked perfectly. Put the old bike back into the trainer while the wheel was still spinning and connected in Zwift - the first time the watts/speed dropped to zero when I put the back wheel near the mag trainer. Moved the mag trainer outside and tried again, this time the speed kept going properly for some time when i dropped it into the trainer. However when I re-started Zwift again it was back to connected/zero speed readout scenario.
Clearly something on/in the mag wheel-on trainer is creating a magnetic field which is interfering with the speed sensors - BUT NOT ALL THE TIME. Why did it work perfectly for months, then cut out? Why did it work again (briefly) after I had picked up the mag trainer and moved it outside? I suspect it might have something to do with grounding/earthing - since the trainer sits on rubber feet.
Would “grounding” the trainer frame make a difference? Would insulating the bike axle from the mag trainer frame help?
Any experience/ideas/solutions?

Hi @Gilbert_Appleby Welcome to the Zwift community forums!

The answer to your question is a very complex one, there are so many variables to consider when dealing with issues like this it is impossible to get a perfectly clean signal between your sensors and receivers. The best we can do is try to limit the amount of interference that is ocurring.

With the understanding that yes, magnetic fields will impact a wireless signal, and that those magnetic fields are constantly changing, let’s take a look in some other areas to see if we can limit the amount of interference from other sources.

A great place to start is our basic wireless interference article, which can be found here.

I also encourage you to look for any other wireless devices in your immediate area that aren’t necessary. Also, distance from your Wi-Fi router is important if you are closer than, say, 10 feet (3 meters) this can cause additional interference.

Please keep us updated on how this goes, I have a few other tips up my sleeve if needed.

I’ve got Zwift going again by getting a combo speed/cadence sensor that uses the old-school magnet-on-spoke option. I’m wondering if with wear the mag trainer is now somehow producing a larger magnetic field?? Or has magnetised the axle?? Dunno - but it works again and I now have 2 redundant speed sensors…