Originally Posted by
ShannonM
Tire width and tire height aren't the same thing... tires aren't cylinders. The same tire will have different dimensions when mounted on different rims. Different tires of the same nominal size will have different dimensions when mounted on the same rim. Many tires will not match their nominal dimensions when mounted on any rim.
No, a tire isn’t a cylinder. It’s a torus. But for a bicycle tire, the width and height are similar enough to be the same in every practical sense.
To get the exact diameter by calculation, you'd need to measure the height of that tire, when mounted and inflated on that rim... at which point you could just measure the wheel diameter directly and skip the math.
That depends on how “exact” a calculation is needed. Using badger1’s number of 692mm, the size of the tire is 27.2”. If the tire is 1mm taller the diameter is 27.3”. If it is 1mm shorter, it’s still 27.2”. A 10th of an inch isn’t going to cause any real issues with most calculations.
That’s also not taking into account the height difference from loading the wheel which could be substantial and entirely dependent on the pressure used in the tire.