Originally Posted by
70sSanO
I’m no expert on vintage tire sizes, but here is how I understand it.
26” refers to the outside diameter of the tire. The actual beam/rim diameter can be anywhere from 22” to 23-1/2” (559mm to 597mm)
27” tires also refer to the outside diameter of the tire. I believe the standard beam/rim diameter was around 24-3/4” or 630mm.
In the British Empire and it’s former colony across the pond, the Imperial system of inches flourished for many years. It is so ingrained that people ride 29ers with the same bead diameter as a European road bike (622mm).
So far so good. What you are missing is that the “standard” tire differs for each of those sizes to reach the magic nominal tire diameter. A 559mm rim, for example, is 22” in diameter. To reach 26” outside tire diameter means that a 2” tire is used. For a 622mm rim, the diameter is 24.5”. To get to a 28” diameter, a 1.75” tire (a 44mm). Not many people now, or even when the tires were first made, would use that wide a tire.
Meanwhile the French, and I suppose other countries, have a size called 650. It’s diameter was 650mm outside diameter or 25-1/2”. There is an alphabet soup of bead diameters that ranged from 22-1/2” to 23-7/16 (571mm to 590mm).
The French sizing system is even goofier. There are 700 and 650 rims but the size is based on the width of the tire designated A to D with the A being a narrow tire and the D being a wide one. The 700 series rim sizes are 642mm (A), 635 (B), 622 (C), and 583 (D). The tire width needed would be 29mm (A), 32mm (B), 39mm (C) and 58mm (D). A similar system existed for 650 series rim/tire size. Obviously we aren’t really using the tire size that was meant for the French system with 700C tires. Some are but, for the most part, people have traditionally used narrower tires on those rims.
So yes 650b is historically a 26” tire, except for mountain bikers. No one will buy a 26+ mtb, so the logic is a bigger modern mountain bike tire is 27.5” in diameter. Likewise a 29” outside diameter tire can’t be called a 700mm outside diameter because it is so much beefier. But we won’t talk about 700c X 40 tires.
The 26” tires that were originally used for mountain bikes are a left over of where mountain bikes came from. The converted balloon tire bikes used in the early ages of mountain biking were really closer to 26”…a 559mm tire with a 50mm (2”) tire…then a 700C is to 700mm. The 27.5er (584mm) and 29er designations are due to the true tire size which is what you get when you put a 2.3” (58mm) wide tire on 622mm rim (for example).