My wife is in a wheelchair, and I am in charge of its maintenance. Wheelchair wheels mostly have 540 mm rim diameters, 44 mm smaller than 650B. I also don't understand the need to drill for a valve stem since they are almost all pneumatic tires. The hubs use a push button through axle system with no provision for mounting any kind of hub or fixed gear. I am not sure how or why one would want to even try the conversion.