Not to be confused with the time-trial and women's-specific 650C wheel, the 650B wheel is all about comfort, versatility, and speed over varied terrain. The key to this is all in the tires. A 650B rim with a 38mm tire inflated to 45psi can deliver an astonishingly comfortable ride over rough roads and loose surface materials. And yet, it is surprisingly fast as compared to a similar 700C configuration. This 650x38B setup is about the same diameter as a 700C wheel with a 19mm tire, so although there's a large tire on the 650B rim, the extra weight is located closer to the axle where it is easier to spin-up. And the 650x38B setup rolls better over uneven surfaces than a smaller 26inch road bike setup. The 650B is a nearly perfect, mid-sized wheel option.
