Wheel/Tyre circumference and CATEYE Cycle computers.

[GuyBoden]

No need to complicate things with geometry (not everybody's strongest point)

I apologise for bringing maths into the forum, I will try not to do it again.................

 

[Tim Hall]

I don't have a computer on any of my bikes. I don't think I'm missing anything.

 

[GuyBoden]

I don't have a computer on any of my bikes. I don't think I'm missing anything.

 

[Ajax Bay]

I apologise for bringing maths into the forum

The point you seem to miss is that your maths depends on a measurement and the possible error in your suggested measurement is of the same amplitude as the (many) circumference measurers advocate. So your error will be more than 6 times greater than theirs. This maths (theory) approach is in this case weaker than the engineering approach.
I still say: take a punt eg from some database (say 2100), do a trial compared directly with GPS and reset circumference calibration to get the readings to agree. This also sidesteps any percentage error present in the cyclocomputer but does rely on the GPS reading to be accurate (so a decent distance or gather several data and average).
Mine reads to within 0.2% (I have other fields displayed on the GPS) - which is useful to read off in conjunction with audax routesheets where kilometres at each turn are provided. I can't think of other reasons why you'd want such accurate reading display but maybe others can.

 

[oldroadman]

All very fascinating. Different tyres will have different aspect ratio (so some are taller than others), even at the same "size". This all seems a lot of messing about when a GPS can do it for you if it has the necessary functions. No sensors on forks, just a nice little satellite working away and information going up and down.
Quite how we all managed before computers I'll never know....

 

[snorri]

Quite how we all managed before computers I'll never know....

Have you forgotten the Lucas Cyclometer?

 

[gilespargiter]

Then of course it goes without saying that if you want it to read in the far more sensible units of miles then; divide the circumference of your tyre measured in metric by 1.609 Your device will then have a read out which is in miles and times related to miles.

 

[oldroadman]

[QUOTE 4238261, member: 43827"]Yes, but how does the GPS know the correct circumference of my tyres?[/QUOTE]
Er, why would it need to? Just watches progress from high above. Mr Garmin seems to be able to tell you all you want to know (or not!) about distance, speed and route.