Wheel Circumference

[slowmotion]

If you want to get an accurate measure of circumference, ride tyres inflated to one's preferred pressure for a bit, and ride for a mile or two outside. The clever consumer electronic device will derive accurate speed from GPS and corresponding rotational speed from the 'speed sensor' on the rear hub or attached to the LH chain stay (+ magnet on a spoke) and auto-calibrate the actual (and accurate) effective wheel/tyre circumference. To find out that figure, go in to the Garmin's 'settings' to find out.

However this measurement is not useful, in this context. The speed displayed will be per GPS (unless disabled or in a tunnel) or if the bike is ridden on the turbo, speed = 0 and whatever's displayed as speed really doesn't matter.

Have you actually done this @slowmotion ? How confident are you that your result is accurate within a tolerance of (say) 1% (22mm)?
What did you do with the information?
Temperature change is going to have a negligible effect on tyre height (pressure increase is about 1psi every 3 degrees C) btw.

Have I done this?? Of course not! Why should I care about an accurate speed or distance?

 

[Ajax Bay]

If you want to get an accurate measure of circumference, ride the for a bit under prevailing ambient temperature, inflate them to a known pressure, and do a roll-out test with you in the saddle. Ignore the clever assumptions built into a piece of consumer electronics.

Have I done this?? Of course not! Why should I care about an accurate speed or distance?

Well you offered a way of determining this and I thought the method you suggested was difficult to carry out (in the saddle), unlikely to provide an answer accurate enough to be useful, and advocated ignoring the simple GPS approach; that's all.

 

[slowmotion]

Well you offered a way of determining this and I thought the method you suggested was difficult to carry out (in the saddle), unlikely to provide an answer accurate enough to be useful, and advocated ignoring the simple GPS approach; that's all.

I wasn't being entirely serious but never mind.

My faintly jaundiced view of GPS cycling computers dates back to somebody on here claiming that the ride from the carpark at Ditchling village to the top of the Beacon contained downhill stretches........because his computer told him it was true.

 

[Bristolian]

Well, this has been a most educational and, at times, highly amusing thread. I realise (now) that I was bothering about something that really doesn't matter all that much in real life. I have removed the speed sensor from my road wheel and am relying on the GPS for speed and distance measurement - seems to work okay.

The speed sensor is now fitted to the wheel I use on my turbo trainer for no other reason than I can

 

[gbb]

Well, this has been a most educational and, at times, highly amusing thread. I realise (now) that I was bothering about something that really doesn't matter all that much in real life. I have removed the speed sensor from my road wheel and am relying on the GPS for speed and distance measurement - seems to work okay.

The speed sensor is now fitted to the wheel I use on my turbo trainer for no other reason than I can

I liken your conclusion to mine regarding chain life. After trying many different methods, studiously recording mileages achieved against chain manufacturer vs method.,,,I came to the conclusion it made very little difference. So I stopped, i now buy a typical chain, oil it when it needed it, replaced it when it needed it and spend more time thinking about, or doing, more important stuff.