Originally Posted by Pompiere
So, why 650B instead of the slightly more common 650A, otherwise known as 26x1 3/8? There is only a few mm difference and in the pictures, they look the same.
That's a good question and I'll try to answer that although I'm no expert on the subject.
650B was the tire of choice for the fine and expensive hand-built French "constructeur" (spelling) touring cycles of the '40s and '50s such as the Alex Singer and Rene Herse. This was the golden age of European touring and raundoneuring. Also this was the days of the so-called post-war technical trials which the constructeurs built and showed off thier finest bicycles, racks, lights, etc. This was at the time when gasoline in Europe was scarce and before the automobile became popular among the masses. Today millions of utility bikes in Europe and Japan still use 650B but that size has never been popular in North America so it is new to many of us Americans. 650B tires traditionally have been fatter for a smoother ride over rough post-war European roads.
650A (26 x 1 3/8) is I believe a British invention and mainly used of course on the millions of English 3-speeds and in North America on cheap 10-speeds of the '70's and other assorted kids bikes. Usually the only tires available were cheap black and gumwall tires in 1 3/8 or 1 3/4 (?) found in K-mart or whatever.
Within the past couple of years quality rims and tires have now become available for both 650B and 650A. Millions of 3-speed bikes are still in service and some people have been wanting to update and upgrade these bikes for sentimental value, to make them more ridable, etc. Utility and practical commuting bikes are becoming more popular now so I hope both wheel sizes take off. 650A is cool, too.
Hope that helps. I wish a real expert like Jan Heine of Vintage Bicycle Quarterly can chime in here.