Question about wheel and tyre sizes

[Yellow Fang]

I was a bit surprised when setting up my new cycle computer this morning. I had to input a tyre size. Since the default was over 2000, I guessed it meant wheel circumference. However, when I divided it by Pi it came out very approximately at 650. I expected the default would be 700, but never mind. I measured the diameter if the wheel on my new bike, multiplied by Pi and put it into the computer. Then I noticed a table of tyre sizes in the instructions. The number for my tyre (25-520) was less than the number I had worked out. However, according to the table, the number for 700mm wheel (with tyre) was less than 700 x Pi. I thought 700 referred to the diameter of the wheel in mm, in which case how can the circumference of the wheel + tyre be less than 700mm x Pi? The bicycle is an Airnimal Chameleon with 24" wheels, only they are not 24". They are more like 22" in diameter with another inch for the tyres.

 

[S-Express]

Just put a chalk mark on the tyre and measure the distance over one complete revolution. There's your 'actual' roll out.

 

[simon.r]

Here's a link that pretty much explains the vagaries of tyre / rim sizing: http://www.sheldonbrown.com/tire-sizing.html

 

[mjr]

Wheels for 700c tyres are 622mm diameter. Roughly, the c denotes an old French tyre size (38mm IIRC) and 700mm was the outer diameter of the tyre on the wheel and I don't remember the measuring oddity that explains why it doesn't quite add up