# tyre circumference



## spennie (10 Jan 2012)

Silly question i know, I'm just trying to see if my tyre circumference is right for my cateye strada computer, i have on a pair of schwalbe durano 20x1.10 and make the circumference 1560 mm, can anyone tell me if this is right or wrong reading.


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## wheres_my_beard (10 Jan 2012)

I never thought maths from school would ever have a real world use, but it does.

*c/d=π*

Circumference = c
Diameter = d
Pi (3.14) = π
To work out circumference, you need to measure from axle to floor (the radius) and multiply by two to get the diameter.

Multiply this by 3.14. The resulting number is your circumference.

From what you say, you know c already, so:

1560mm/π = d

So d, according to your calculation is

Your diameter according to your calculations should be 496.8mm (19.6") (making the radius approximately 248.4mm, which is about 9.8"). This sounds less than you would expect from a wheel which is apparently 20" without a tyre on.

I hope this helps.


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## mcshroom (10 Jan 2012)

It's probably easier to use the table values: -

http://sheldonbrown.com/tire-sizing.html


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## wheres_my_beard (10 Jan 2012)

You could always use a reference point on the tyre, like the valve in relation to the fork, and roll the wheel along the ground and measure the distance covered to make one full revolution.


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## Scilly Suffolk (10 Jan 2012)

True up to a point WMB; as a minimum I'd go for eight decimal places:* 3.14159265.*


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## wheres_my_beard (10 Jan 2012)

Jimmy The Whiskers said:


> True up to a point WMB; as a minimum I'd go for eight decimal places:* 3.14159265.*


At which point did I vere off, not withstanding your preference for 8 decimal places? 

I was going to give the first 4 decimal places but that seemed a tad over accurate.


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## arallsopp (11 Jan 2012)

I tend to:

put in a best guess, 
ride over a known distance (ie: alongside someone you trust to have set their computer correctly, or a long ride on straight(ish) roads whereby the error will be marginal),
work out how wrong I was as a percentage of the distance it logged.
reduce/increase the circumference on your computer by that percentage.
I set mine by comparing a GPS track to my computer over a 35 mile ride. It seems to agree within a few feet most of the time.
Andy.


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## Scilly Suffolk (11 Jan 2012)

wheres_my_beard said:


> At which point did I vere off, not withstanding your preference for 8 decimal places?
> 
> I was going to give the first 4 decimal places but that seemed a tad over accurate.


Just pulling your leg mate.


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## BlackPanther (22 Jan 2012)

I just used a dressmaking tape measure all the way around the tyre. Failing that, use a piece of string and then measure that with a normal measure. It's important that you get the circumference measured exactly, as the speed/distance will then be really accurate. Most cycle computers have a manual circumference input as well as the usual 26/700 etc 'standard' sizes.


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## Riding in Circles (22 Jan 2012)

Put the wheel on a flat surface like the pavement with the valve at the bottom, mark the valve location on the flat surface, roll the wheel one complete revolution, mark the surface again, measure between the two marks, it isn't rocket science people.


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## BlackPanther (22 Jan 2012)

Jimmy The Whiskers said:


> True up to a point WMB; as a minimum I'd go for eight decimal places:* 3.14159265.*


 
Naaaah, you need at least 1,000 places.
3.
1415926535 8979323846 2643383279 5028841971 6939937510 
5820974944 5923078164 0628620899 8628034825 3421170679
8214808651 3282306647 0938446095 5058223172 5359408128 
4811174502 8410270193 8521105559 6446229489 5493038196
4428810975 6659334461 2847564823 3786783165 2712019091 
4564856692 3460348610 4543266482 1339360726 0249141273
7245870066 0631558817 4881520920 9628292540 9171536436 
7892590360 0113305305 4882046652 1384146951 9415116094
3305727036 5759591953 0921861173 8193261179 3105118548 
0744623799 6274956735 1885752724 8912279381 8301194912
9833673362 4406566430 8602139494 6395224737 1907021798 
6094370277 0539217176 2931767523 8467481846 7669405132
0005681271 4526356082 7785771342 7577896091 7363717872 
1468440901 2249534301 4654958537 1050792279 6892589235
4201995611 2129021960 8640344181 5981362977 4771309960 
5187072113 4999999837 2978049951 0597317328 1609631859
5024459455 3469083026 4252230825 3344685035 2619311881 
7101000313 7838752886 5875332083 8142061717 7669147303
5982534904 2875546873 1159562863 8823537875 9375195778 
1857780532 1712268066 1300192787 6611195909 2164201989


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## Scilly Suffolk (22 Jan 2012)

I'm only impressed if you typed that yourself, rather than cut & paste...


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## gaz (22 Jan 2012)

Don't forget you should be on your bicycle at the time, as the rolling circumference of bicycle without your weight is different to with your weight.


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