Originally Posted by
captnfantastic
How much bigger? Cut and unroll a 700c tire, and we have a
cylinder about 2124 mm long. The volume of cylinders of the
same length increases with the square of the diameter, so
the ratio is 23^2 to 25^2, or 529 to 625--the 25 mm tire has
about 18% more volume.
Here are the volumes for idealized tires:
If the diameter is 23 mm, the volume is (23/2)^2 * pi * 2124
= 882,470 mm^3
If the diameter is 25 mm, the volume is (25/2)^2 * pi * 2124
= 1,042,616 mm^3
Again, that's about 18% more air volume.
About 18% is correct but:
A tire is actually a torus where total volume is the area of a cross section of the tire multiplied by the circumference (where circumference is calculated using the toroid radius measured from the center of the hub to the center of the tire). The 2124 value is a tad high because it is based on the outer circumference of a tire and should not be a constant because the circumference of a 25mm tire is actually greater than that of a 23mm.
All great points though!