The great Sheldon says "If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated...but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls."
He then gives a table suggesting that the biggest tyre you should use on a rim with an internal width of 17mm, like the 217/517/717, is 37mm, and that it's best to use a 25mm rim for a 2"+ tyre.
Possibly he and I are (were
) the only two people in the world who think that, but I can console myself that I'm in excellent company.
Thanks for good thread and to One-eyed-Jim for nice link.
I guess the summary is that a 575mm rim is called a 559 (22") because a 559mm tyre fits into it. And the two combined are called a 26" wheel because everybody used to follow the British.