Calculating tire circumference

MacStacked

Calculate tire circumference accurately?

Trying to set up my bicycle computer properly and know my tire is 26" but Im struggling beyond that to distinguish between the options beyond that:

26" (650A)
26,5" (Tubular)
26,6" (700x25c)
26,8" (700x28c)
ATB 26x1,4
ATB 26x1,5
ATB 26x1,75

Tire is Innova and the markings on it say 54-559 26 x 2.0 A-2024-11

Any help appreciated thanks?

cabbott

easiest way, roll the wheel allong the floor, starting with the valve down, mark the floor, when the valve comes around again mark the floor, now measure the distance, easy.

Bunnyhopper

This can be handy for deciphering the different sizing systems:

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

Beasty

circumference = pi x diameter

cdaly_

MacStacked wrote: »

ATB 26x1,4
ATB 26x1,5
ATB 26x1,75

Tire is Innova and the markings on it say 54-559 26 x 2.0 A-2024-11

I'm presuming yours is an ATB 26x2,0 from the above chart. The difference is that a wider tyre sits higher and has a larger circumference.

MacStacked

thanks for speedy replies.

rottenhat

cabbott wrote: »

easiest way, roll the wheel allong the floor, starting with the valve down, mark the floor, when the valve comes around again mark the floor, now measure the distance, easy.

This is correct, but if you want an accurate measurement, you need to be sitting on the bike so it's easier if you have someone else to mark the floor for you. Make sure your tyres are properly inflated when you do it too.

alejandro1977

rottenhat wrote: »

This is correct, but if you want an accurate measurement, you need to be sitting on the bike so it's easier if you have someone else to mark the floor for you. Make sure your tyres are properly inflated when you do it too.

for improved accuracy do it for multiple revolutions (chalk mark on ground to start?) and divide my no. of revs (Junior Cert Science does come in handy)

victorcarrera

MacStacked wrote: »

Calculate tire circumference accurately?
54-559 26 x 2.0 A-2024-11

From this chart I estimate it to be 2009 mm.
Tyre size chart
Although the 54 tyre is not listed the 53 and 57 are.
This level of accuracy is not necessary as greater errors are recorded due to your position on the road and the fact that it is impossible to cycle in a straight line on the road. MTB's have so many other variables also like wheel slip obstacles etc.
The 2024-11 stamped on the tyre might also be a reference to the circumference although I've never seen the circumference stamped on a tyre.

blorg

That all sounds very complicated, surely it is easier to get a computer that figures out the speed by reference to an orbiting constellation of satellites 20,000km above your head? Seems the obvious way to do it.

doozerie

The most accurate way is definitely to sit on the bike while rolling it, as rottenhat suggests. That'll account for the compression of the tyres that your weight will cause. In reality of course, this level of accuracy is probably overkill, but you only ever have to do it once (per pair of tyres) so not a great deal of trouble to do. Going by the printed dimensions of the tyre can be unreliable because the tyres might not actually match them - I've seen people state that some (Michelin) 700x23c tyres actually measured 21mm wide, for example, although that's a bit of an extreme mismatch and probably very unusual.

cdaly_

doozerie wrote: »

In reality of course, this level of accuracy is probably overkill, but you only ever have to do it once (per pair of tyres)

Ah, not so. You have to take into account the changes due to tyre wear over time. That's going to change the circumference by 6.2857mm per mm of tyre wear...

doozerie

cdaly_ wrote: »

Ah, not so. You have to take into account the changes due to tyre wear over time. That's going to change the circumference by 6.2857mm per mm of tyre wear...

That's a good thing as your speedo will tell you you are going further and faster than you really are. You can exaggerate the effect even further by stuffing ever more Mars Bars in your pockets. Eventually your speedo will report that you have covered 150km in the distance from your shed to your front gate* and you can just turn around and go straight back to bed with a clear conscience knowing that your carefully calibrated speedo couldn't possibly lie.

*Distances may not be to scale...