carlton

Am I doing this right? Rim diameter + tire width + tire width = diameter? So a 700X23 would be: 622mm + 23mm +23mm = 668mm or 26.3 inches? Or is there a more accurate formula?

Bill Kapaun

Most accurate would be to run a tape measure around the tire and divide by PI.

Bike Gremlin

Mark it with a chalk, or something. Mark a starting point. Roll it on the ground for one full turn and mark the end (full circle).

Diameter = measured length / Pi
(Pi aproximates to = 3.14)

DiabloScott

That is the correct relationship... but it won't be accurate because tire widths are inaccurate and the bead seat diameter isn't where the tire "starts".

Some bike computers will want you to enter the diameter, and others will want you to enter the circumference... usually the example charts that come with the manual are close enough - maybe closer than your "formula". But lots of people like to get it down to the millimeter.




2105mm circ = 670mm dia so your estimate was pretty close.

carlton

Thanks for the quick answers guys. Don't know why I didn't remember about the old pi formula. I haven't used that method in over 50 years. When you get my age you usually more interested in eating pie than using it in a mathematical equation. And thanks for the chart Diablo.

fietsbob

3/14/17 was a year and 10 days ago..

Ghrumpy

Your concept is basically correct. But the accuracy of the calculation might leave a little to be desired. Clincher tires' nominal section width is measured across the sides, and won't necessarily equal the tire's height measurement from the bead to the crown of the tread.

If you can roll out the wheel and divide by pi, you can much more easily directly measure the diameter with a measuring tape or meter/yardstick. No need to calculate anything unless you just want to freshen up your math skills.

cny-bikeman

If this were 2015 you'd have pi to 4 decimal places. I'm old enough that when I learned Basic I had to memorize pi to 10 decimal places for coding punch cards.

gsa103

The bigger issue is that you typically want the "effective" diameter. Which it the tire of the when the contact patch is compressed due to weight. Typically that's around a 10-15% decrease in the tire height (tire only, not the wheel).