Edge 500 wheel sizes

aquanaut

Hi guys. Quick question. Edge 500 set for 700x23c wheel size. If I am swapping back tyre to a 25mm and 23mm on front, do I need to change wheel size to 700x25c on Garmin. Thanks

Beasty

The wheel size is only relevant if you do not have a GPS signal - it should pick up distance (and work out speed) from that, unless you have GPS turned off (in which case you would need to change the setting if that's the wheel the sensor is on)

Inquitus

If you have a speed cadence sensor it calculates your speed from the wheel rotations as this is far more accurate than the GPS for giving live speed information. GPS accuracy varies from 1-3m to 20-30m depending, and if you were to disable your speed cadence sensor you would see that your current speed would vary wildly at times even if you were maintaining a constant actual speed.

Beasty

I don't claim to be any kind of expert on the internal workings of the Garmin, but surely this would average itself out. When I analyse the ride data/plot there are no obvious material variations - I would guess that if the reading taken at one point is 20m out the chances are the reading at the next point is also

You may get the occasional spike in the post ride analysis, but even that is not apparant (to me at least) when riding the bike, and I'm guessing some smoothing takes place. There is also absolutely no doubt in my mind that the average speed calculated is very accurate

Inquitus

Beasty wrote: »

I don't claim to be any kind of expert on the internal workings of the Garmin, but surely this would average itself out. When I analyse the ride data/plot there are no obvious material variations - I would guess that if the reading taken at one point is 20m out the chances are the reading at the next point is also

You may get the occasional spike in the post ride analysis, but even that is not apparant (to me at least) when riding the bike, and I'm guessing some smoothing takes place. There is also absolutely no doubt in my mind that the average speed calculated is very accurate

It works out fine for post ride analysis I am sure as it is averaging 70 odd Km or more or distance with a variance at the start and end point of less than 20m at most, but for live speed readings on the bike, the gps value varies quite alot (@10%) even if you are maintaining a constant speed, whereas the speed / cadence meter calculation is always very accurate due to the nature of how it is calculated. It will be particularly noticeable when going from open road to tree lined avenue etc which will degrade the gps accuracy.

I use my gps for jogging also, and due to the slower speeds of that activity the variation in the speed displayed from second to second is even more noticeable.

seamus

I think it does some kind of combination. When I first fit the speed/cadence sensor, after about 20 minutes of riding a little message said, "Wheel circumference automatically calculated". So I'm guessing it used a combination of GPS to track my actual distance -v- the number of rotations of the wheel to figure out exactly what size wheel I have.

I don't know what happens though if the circumference is changed after that, as presumably the Garmin will calculate based on the original circumference and you will appear to be going slower or faster (for a larger/smaller wheel, respectively).

Maybe it will do it automatically - compare the expected distance via the sensor -v- the actual distance via the GPS receiver and correct when they're off.

aquanaut

Cool, thanks - it is set to auto anyway so should auto callibrate. I also cant imagine that a 2mm wider tyre would have a larger circumferance but could be wrong. Cheers lads