My Garmin Edge 500 shows an “auto” set circumference of 2156 my measured circumference is 2196. Does the G500 continually calibrate the circumference setting or should I manually set it at the measures amount? I have been in the “auto” setting for years however I have very long event using cue sheets and would like the most accurate measurement setting.
Thanks for your help.
Josh
Make sure the GPS is on. Use that. Use auto. As far as I know, auto basically takes the GPS calculated distance, figures out how many times your wheel has spun, and does the math to calculate a wheel circumference.
Thanks!
Make sure the GPS is on. Use that. Use auto. As far as I know, auto basically takes the GPS calculated distance, figures out how many times your wheel has spun, and does the math to calculate a wheel circumference.
Chances are the discrepancy is coming from your roll out distance being unweighted, and the auto calculation number from the garmin being calculated under load.
If my math serves me correctly, the wheel circumference your garmin is giving you correlates to a wheel diameter of about 12mm less than your roll out circumference. As I said before, probably the difference in unweighted vs weighted. However, 12mm does seem like a rather large amount of tire flattening…is your tire pressure correct? But in the end, it is a GPS calculated number and being different by only 12mm is pretty good in my book!
That’s a 2% error. You can do way better than that with a good rollout measurement. Find some good pavement and make sure your tire is properly inflated. Put your bike on the pavement so the valve stem is bottom dead center, and mark that on the pavement with a Sharpie (it needs to be a narrow mark to make the line width narrower than your measurement uncertainty). Now roll your bike in a straight line, through five (or more) wheel revolutions while sitting or leaning on your bike such that the tire has a nominal amount of compression. Mark the location of the fifth recurrence of the valve stem hitting BDC on the pavement. Then take a good tape measure that is calibrated in mm and measure the distance between the marks, then divide by the number of revolutions, and enter that number in your Edge as a custom cal.
The auto cal feature on Edges is good, but you never know if it will sometime decide that your wheel diameter has changed, based on missing a wheel revolution or two, or during sketchy GPS conditions, and then it will throw your distance measurement off. Custom cal only takes maybe ten minutes to set up, and it won’t go bad.
If you have several wheelsets or tires of different diameters then you will have to manage that accordingly… still less risk to your pacing strategy for the work involved than potentially getting a bogus auto-cal at a bad time.
The tires are on a gravel bike so the inflation is correct at 35 psi. I did sit on it as I sccoted ahead. However I only went one complete revolution. Going 5 revolutions then dividing might give a more accurate number. I’ll do that later today. Thanks for that idea!