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Old 02-05-14, 10:45 AM   #1
FatBottomedGirl
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Garmin Edge 500 - speed sensor: how does it work?

Hi,

I was using for the past year the Garmin Edge 200.
My purpose was only to have a simple bike computer I could use on several bikes with no hassle.
It was working like a charm.

I just upgraded to the 500 for the cardiometer and cadencemeter (and the altimeter and the possibility to completely customize the interface,...).

I mounted the speed and cadence sensor on my race bike and kept my commuting bike extra-tool-free.

So far the 500 is working exactly as expected on the commuting bike, which means exactly like the 200 (except for the extra features...) meaning that I do have my speed purely GPS-based which is fine for this use.

My wondering is : how is this going to work on the racing bike with the sensor?
I have no worries for the cadence sensor which uses the sensor as only source...

But how is the speed sensor working? does it completely override the GPS data? does it combine its data with the GPS ones for better accuracy? can I start a workout before the satellites are aquired? can I ride fully indoor with no satellites at all?

And how does it work if I ride (outdoors, with satellites locked) and I lose the sensor (actually losing the sensor is not really possible, but the very possible situation is to have a puncture and just have the wheel replaced so I finish the ride on a magnet-less wheel)... Will I then have a smooth record of the ride (shall the pure GPS-based speed take over)?

I must say I do not fully understand how this speed sensor works - I mean how it interacts with the purely GPS-based speed aquiring?

Also, I found no menu to configure the wheel size which is a very typical setting on bike computers using spoke-magnet-sensors, so is the GPS used to balance and auto-fine-tune this?

Sorry if this is stupid, but I am new with this device which is both awesome and way more complicated than my old 200

Thanks!
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Old 02-05-14, 11:22 AM   #2
eja_ bottecchia
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FBG. I have a Garmin 510, but prior to that I had the Garmin 500.

When I use my indoor trainer, the wheel sensor takes over to give me a speed/distance reading (even indoors).

You can manually enter the wheel size or you can let the GPS do it for you.

The magnet will override the GPS data, so you can use the Garmin on both bikes with no problems

If you lose the magnet signal (because of wheel change, wheel magnet moving out of position or magnet falling off) you can resume your ride using GPS.
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Old 02-05-14, 02:19 PM   #3
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If the speed sensor is present (and working), the Garmin 800 uses that in place of the GPS-determined speed. (Presumably, the 500 works the same way.)

Quote:
Originally Posted by FatBottomedGirl View Post
And how does it work if I ride (outdoors, with satellites locked) and I lose the sensor (actually losing the sensor is not really possible, but the very possible situation is to have a puncture and just have the wheel replaced so I finish the ride on a magnet-less wheel)... Will I then have a smooth record of the ride (shall the pure GPS-based speed take over)?
The wheel speed sensor detects wheel rotations and, thus, wheel speed (the wheel has to be rotating).

The GPS detects position. GPS speed is determined by changing the position of the device (and noting the the time it takes to change position).

If the device isn't moving (like on a trainer, its speed is zero). The wheel doesn't have to be rotating at all (like on a rack on your car).

Last edited by njkayaker; 02-05-14 at 02:31 PM.
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Old 02-05-14, 02:27 PM   #4
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Originally Posted by njkayaker View Post
If the speed sensor is present (and working), the Garmin 800 uses that in place of the GPS-determined speed. (Presumably, the 500 works the same way.)
The only caveat I'll add here is you're much better off if you manually enter your wheel size in your Garmin settings rather than going with "auto calculate." I've found the mileage can be substantially off if you use auto calculate and if your wheel size is being interpreted incorrectly, it will impact your speed too.
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Old 02-05-14, 02:29 PM   #5
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Quote:
Originally Posted by njkayaker View Post
If the speed sensor is present (and working), the Garmin 800 uses that in place of the GPS-determined speed. (Presumably, the 500 works the same way.)
Yes, Speed sensor overrules the GPS for speed and distance. If it is take out then the GPS takes care of it which is a bit less accurate.
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Old 02-05-14, 02:41 PM   #6
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Yes, Speed sensor overrules the GPS for speed and distance. If it is take out then the GPS takes care of it which is a bit less accurate.
This is easily established by noting that the unit displays speed when the wheel rotates even with the bike not moving but outside knowing position.

(Some people have an idea that the Garmins are doing "fancy" stuff with both GPS and wheel-rotation sensor data to determine speed but there's no evidence to support that. I suspect it does the simple thing: the wheel sensor overrides the GPS for speed/distance.)

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Originally Posted by cafzali View Post
The only caveat I'll add here is you're much better off if you manually enter your wheel size in your Garmin settings rather than going with "auto calculate." I've found the mileage can be substantially off if you use auto calculate and if your wheel size is being interpreted incorrectly, it will impact your speed too.
That makes sense. GPS speeds are going to be less accurate for slow speeds (and small changes in distances).

Of course, it could be that the autocalculation (with the wheel sensor) is still better than just using the GPS.

The autocalculation of the wheel circumference is interesting. Without a circumference, the Garmins see the wheel rotation data but can't make sense of it. Thus, it measures the wheel circumference using the GPS distance and uses that (so it can go into the normal mode of having the wheel-rotation override the GPS speed/distance).

Last edited by njkayaker; 02-05-14 at 02:53 PM.
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Old 02-06-14, 09:07 AM   #7
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In my experience, auto-calculate works well as long as the first part of the ride (~1/2 mile) that it uses for calibration isn't tortuous.
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Old 02-06-14, 10:12 AM   #8
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In my experience, auto-calculate works well as long as the first part of the ride (~1/2 mile) that it uses for calibration isn't tortuous.
Worked OK for me too.
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Old 02-07-14, 09:35 AM   #9
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Does the auto-calculate happen with each ride? I.e., if I change from a 23mm to 25mm tire, will auto-calculate pick it up on the next ride, or do I need to force it some how?
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Old 02-07-14, 10:35 AM   #10
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I ride multiple bikes and whenever I change between bike profiles on the Edge and start a ride, it recalculates wheel circumference for that bike. There may be other ways to force it to recalculate, but I don't know them specifically. Perhaps forcing it to pair again with the GSC10 will force a recalculation, or switching to another bike profile momentarily and then switching back, or selecting manual wheel input and then selecting automatic, ...etc.
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Old 08-10-16, 05:37 PM   #11
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I went out to calibrate my Garmin speed sensor. First I used a tape measure and carefully measured four complete wheel rotations with my weight on the bike. I got 2277mm circumference. Then I tried the Garmin auto-calibration on the straightest street around. Garmin reported 2277mm also! So 2277 it is.
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Old 08-10-16, 05:39 PM   #12
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Quote:
Originally Posted by Looigi View Post
I ride multiple bikes and whenever I change between bike profiles on the Edge and start a ride, it recalculates wheel circumference for that bike.
You should not need to, since it will only use the sensor and auto-calibration on the bike with the speed sensor that it was calibrated with.
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