Originally Posted by
acidfast7
When using the ETRTO, the overall height is not 2 x FN + SN (first number and second number). In the case of ETRTO the first number is the width of the tire rather than the height.
Abbreviations like ISO and ETRTO are all well and good, if one uses them carefully, which is why I was careful to NOT use them. And I have now edited the quote to clarify which part of the post I was addressing. The numbers OP used, and which I quoted, were just numbers. Of those, 559 refers to an actual measurement of a nominal "26 inch" rim, and 40 refers to an actual measurement, the width (which is approximately the same as the height) of the tire. So to turn those numbers into an overall diameter of the wheel (wheel being defined as a rim with a tire on it) you really do need to count the tire twice.
The "26 inch" rim that measures 559 mm, for what it's worth, assumes a 2.0" (50 mm) tire to add up to 26 inches; so 559+50+50=659 which is, of course, just a little shy of 26 inches, but you get the idea.
Originally Posted by
acidfast7
Sorry for the pedantry.
no worries, let's just get this right and not confuse anyone!