Tire Size and Air Volume

Air volume was calculated in MS Excel with the following formula (this example is for 650B/584ISO with 38mm tires):

2*PI()^2*(584/2+38)*38^2

This will give results in cubic millimeters. Volumes above have been convert to liters (dividing by 1 million), rounded to the nearest tenth.

Some variances may be introduced from rounding, unit conversions, and other factors.

This is all white coat, clean room, armchair nerdery stuff only. Real life measurements of tires can vary based on many factors including miles, rims, tubes, production batch, etc.

Tire weights are manufacturer published weights, except for Schwalbe which were 100% fabricated for my own amusement.

Sources:

https://en.wikipedia.org/wiki/Torus

http://www.metric-conversions.org/volume/cubic-millimeters-to-liters.htm

4 comments

  1. I could be wrong, but as I understand in your example 38 is the “small” diameter not the radius. I think your formula should be: 2*PI()^2*(584/2+38/2)*(38/2)^2. Maybe I’m missing something…

  2. Ernesto, good question.
    First, I should point out that this calculation uses tire width as a proxy for tire height. In the real world, this isn’t a perfect assumption.
    The calculation is based on the radius (of the rim + tire; which is 1/2 * bead seat diameter + 1 * the tire width) and the tire width.
    Another way to think about is that the (584/2+38/2) is fundamentally incorrect because the tire is on the outside of the rim, so if you were to calculate the diameter, you would add the tire width twice (since the a bike at rest will have tire at the top of the rim; e.g. under a fender…and tire between the rim and terra firma). This can also applied to the disprove the *(38/2)^2 portion of the formulae because the tire height is the full height of the tire; not half of it.
    I hope this helps!

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