Related
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Forerunner 910xt + Speed/Cadence sensor gsc 10. Cadence correct, speed not!

Former Member
Former Member over 9 years ago
Hello,

I've just purchased and installed a new Garmin GSC-10 Speed/Cadence sensor for my bike. I paired this sensor with my Garmin 910xt watch and because I'm training indoor, I swithed GPS off and adjusted the wheel size manually (= wheel circumference). I use a tyre which has size 700x23 written on its side wall. From the conversion table (https://support.garmin.com/support/searchSupport/case.faces?caseId=%7B8c7b7b10-0424-11de-76c5-000000000000%7D) I found a value for wheel circumference which is 2096 mm. I measured the circumference also by myself. I got a result 2100 mm which is "same" compared to this conversion table. Finally I gave this number to my 910xt in Bike 1 -settings (2096 mm).

When I start pedaling, the speed shows me something like 90 - 150 km/h, although I'm using first gear and I'm pedaling constant slowly. If I start pedaling faster, the value increases naturally. However, the value is totally in correct as you can see. What have I done wrong?

The cadence information is correct, however!

I've checked the magnets, that they are close enough for the GSC-10 sensor (the magnets almost touch the sensor). I've also checked that the green and red LEDs flash correctly when the correct magnet passes by. Only thing is that Garming 910xt calculates or scales the speed wrong? How do I fix this?


Best regards,
Joonas
  • 0 over 9 years ago
    That sounds as if it's about a factor of 10 too big. Are you sure the 910XT wants mm and not cm?
  • Former Member
    0 Former Member over 9 years ago
    I was thinking the same, but there's unit of "mm" in the settings (910xt display) and every tutorial say that give the circumference in "mm" which is obtained from Garmin's conversion table ( 700x23 => 2096 mm). Does people give the value in mm or in cm when adjusting the wheel size?
  • 0 over 9 years ago
    For the Garmin devices I have, it mm, but I thought I'd ask because of the factor of 10. That's odd. The only thing I can think of to do is to try auto calibrating outdoors on the road if you can, and if the 910XT does that. It takes about a km of riding. See what number it comes up with using GPS.
  • 0 over 9 years ago
    Just grabbing at straws here, but does the 910xt give you a choice of what type of sensor to pair the GSC10 as? It should be paired as a "spd/cad" sensor not a "speed and cadence" sensor. Although my expectation if it was paired as the latter, is that you wouldn't get any speed reading from the wheel and instead would get GPS speed.

    Maybe posting a .fit file from your device would give some clues if someone wanted to look in it. I don't have the software to do that though.

    For certain I don't think it's a GSC10 issue unless your crank magnet or something else is also triggering the speed sensor, which did happen for me once. Possibly it's a bad reed switch, but I've never seen a post of one failing like that. You can double check that the crank magnet is not triggering the speed sensor by either removing the wheel or the magnet and turning the crank while the GSC10 is in test mode. My crank didn't fire it every time, just once ever dozen or so passes.
  • 0 over 8 years ago
    Speed and Distance off on 910XT, but 1000 seems to be correct

    I've got a similar issues: I'm using my CompuTrainer and have my Garmin 1000 computer and 910XT all running. The CompuTrainer and the 1000 seem to be very close, but the 910XT which has always worked well seems to be 5mph off from the other two now. Possible issues?
  • 0 over 6 years ago
    Just a quick remark on this, as someone else may navigate to this topic:
    I was having issues where my 910 was autopausing a bit, when riding on rollers. Or else suddenly giving out a super low reading though I was maintaining the exact same speed.

    The obvious fix was to change the sensor's battery. But as I had none at that exact moment, I decided to boost the magnet signal by popping a small rare-earth magnet onto the wheel magnet. It worked somewhat.

    Today I replaced the sensor battery prior to my roller session, and started to get skewed speeds, that were roughly double what they should have been. After messing around with a bunch of things I removed the little rare-earth magnet and everything is now fine.

    BTW I use 2 separate ANT sensors that aren't made by Garmin, and they've always worked well. And the new battery fixed the auto-pause issue.
  • 0 over 6 years ago
    Most often, when spurious data is transmitted by a magnet triggered speed or cadence sensor, it's because the magnet passes along the center of the magnetic switch and that causes double triggering. Most of these sensors will have a line or groove molded into the sensor body, that indicates the preferred location where the magnet should pass the sensor.

    For a sensor like the GSC10 specifically, that groove is about 3/4 of the way towards the tip of the arm. You should set your magnet so that it passes along this groove, or if anything, further toward the tip of the arm. Otherwise you may experience double-triggering of the sensor. The same applies to the cadence detector - there is a molded line in the main body of the sensor, and the cadence magnet should pass along that line, or further towards the edge of the sensor body, not toward the center.