Garmin Sensor Puzzle

[GScot]

Anyone seen this? New Garmin Edge Speed Sensor, the one with no magnets, purchased in bundle with Cadence Sensor in January. Paired with FR620 watch and Garmin USB 2.0 dongle for use with Zwift. Worked flawlessly on carbon and aluminum bikes. New steel Black Mountain Cycles Monster Cross and suddenly the speed reported is double. Makes me a badass on Zwift but not what I'm looking for. Followed all of the troubleshooting from Zwift and Garmin which amounts to change the battery. Problem remains.

Here is the kicker. Moved sensors back to a carbon bike and it works normally. Then back to the steel frame and it continues to report double speed.

Sensor has to be on the hub shell so relocation is out. Any experience or ideas? I did try with the watch right by the wheel, no change.

Edit: I am off to try a different wheel.

[ptourkin]

Do you have different bike profiles on your watch? I'm guessing you looked, but if you didn't make sure wheel size is the same on both.

[EDS]

Do you need to input different wheel size (due to larger tire)?

[GScot]

I'm working on it currently. The watch auto calibrates this sensor by period comparison to GPS data. I have checked the value and it is within a few mm of what I would manually input. Zwift requires no calibration as I understand, it assumes every wheel is a 700x23. So far Ultegra 6800 wheelset has the problem and a RS81, they are pretty much the same other than the rim. I am trying a campy record hub DT rim wheel next.

[GScot]

Put the third wheel on and it looked OK for about 30 seconds then started jumping back and forth between real and double speed. Took it out and let it auto calibrate on the street and it now correct or at least apparently correct with the watch display. Zwift is still out of whack but at least I have a good data set to provide on a problem ticket.

[kramnnim]

Bizarre. What happens when you ride the steel bike outside? Maybe the GPS speed overrides the doubled speed...

[guido]

I use a Garmin new style speed sensor on the trainer with both a steel and a titanium bike with the edge 1000 and have no issues (though I don't use zwift). I even put the sensor on a powertap hub and have no abnormal behavior. Works like a charm... I do a lot of trainer miles...

[GScot]

It's bizarre alright. After riding outside, just a short ride but I removed the sensor on the Garmin and added new sensor. Less than a mile and it calibrated.

It's currently a you gotta be ****tin me situation. It auto calibrated the wheel size as 1088mm. That is just about half size so now the garmin reads right indoors or out. Of course Zwift is still double because it defaults to 700x23.

I put the wheel on a carbon bike and of course it reads half speed on the trainer in the garmin, Zwift is correct again with the carbon frame. ***:help:

LOL I guess I can double my FTP on Zwift to get a workout in at warp speed.:banana::banana::banana:

[parallelfish]

Proof that steel is faster.

[kramnnim]

So it sounds like the sensor itself is faulty?

[GScot]

So it sounds like the sensor itself is faulty?

Seems like it to me. Although Garmin devices compensate with GPS comparison so I would never know if I wasn't using Zwift. The USB Ant+ on my laptop is from Garmin so I'm going to contact Garmin for suggestions.