Im late to the party. I got here while looking for info to offer some BMX-related corrections for the Sheldon Brown tire sizing systems page.
Originally Posted by
Viich
Because bike manufacturers hate us they also claim that, for no reason they've disclosed, OS20 at 451mm is not compatible with the fractional sizing 451mm.
Its a narrow rims and tires versus wide rims and tires thing. Same 451 BSD, but mating wide tires to skinny rims (and vice-versa) seems like a bad idea, assuming it works at all.
Originally Posted by
Viich
They're labeling the tires as 20x2.1" but marking it OS20. This is because if they call it 22" then it can't be used in BMX class racing, only cruiser.
Actually, the dividing line between class an cruiser is drawn at 22-1/2. The USA BMX rule book says:
Bicycles for competition in the Intermediate / Expert/ Girl Expert / Pro classes must be equipped with wheels having a total diameter, inflated tires included, less than or equal to 22 1/2 (57 cm).
Bicycles for competition in the Cruiser / Girl Cruiser shall have wheels where the total diameter of the inflated tires included shall measure more than 22 1/2 (57 cm) with a 2 cm variance to accomodate 24x1 tires.