Calibrating my bike computer. Help, opinions and gentle tutting welcome.

[Cyclopathic]

Ok here's the thing, the calibration chart that comes with the comp' gives the circumference figures to enter into the comp' for various wheel sizes.

The 700C goes up to 700 x 32C, giving a circumference reading of...2155mm

My tire informs me that it is 700 x 38C, for which there is no calibration.

Unhappy with this I did the calculation myself. The diameter was indeed as near to 700mm as could be so I multiplied this by 3.141 and got the figure 2200 (near as damit)

I checked the circumference again with a piece of string and came up with a smidge over 2200 but not enough to matter.

So at last my question is should I use this 2200 figure as my calibration number despite it not apearing on the calibration chart, bearing in mind there was no calibration on the chart for my exact wheel size as proscribed by the tire info?

I've tried to put this clearly so I hope it's not too messy and someone can decppher it and tell me the best number fro calibration.

At the moment I have split the difference between 2155 and 2200 and used 2180 but Im not happy about it.

Also are discrepancies of this magnitude really enough to matter very much. If the figure the comp is using is lower than the real circumference then I'll really have gone faster and further than it says, but not much. Aahhhhhhhgggggghhhhhh. my head hurts.

(Should I just go back to using one of those speedos that had a wheel drven off the front wheel and a cable atatchment?)

 

[barnesy]

An option would be to pump the tyres up to their normal pressure and sit on it, mark on the ground beside the air valve and roll forward one full rotation and make another mark, now measure the difference between these too marks

 

[numbnuts]

what he ^^^ said

 

[Norm]

I think that's no different to what cyclopathic said he / she had already done with the string.

If you have measured the actual circumference of your tyres, that's a much more accurate figure than any chart based on generic tyre sizes.

To put the differences in context, the difference between 2155 and 2200 is equivalent to around 2%, or 1 mile in 50.

If it's any consolation, the old speedos driven from the front wheel spindle would be in heaven before they got to an accuracy of 2%.

If you use the actual circumference of your tyre, whether that's measured by string or chalk marks on the floor, then you can rest assured that your speedo is more accurate than those who use the calibration charts.

 

[Cyclopathic]

Thankyou all very much. I think you have answered my question.

And to think I am supposed to be cutting down on pie.