So why would you care about 650B mountain bikes? Well, there has been a lot of debate about wheel size in the mountain bike industry. The basic premise of the wheel size debate is that we came to our current standard of the 26" wheel somewhat arbitrarily. The standard of the 26" wheel size was established long before mountain bikes came around. So nobody can say that 26" wheels are and always will be the perfect size for mountain bikes.
If this all sounds familiar, it should. This is the same argument the 29" wheel crowd has been using for years.
So you might wonder why we don't actually know what wheel size we should be using. Well, in most cases it comes down to cost. It is very expensive to make new tooling for different size tires and wheels, so you can't just try anything out whenever you want.
Then there is the establishment issue. Nearly all of the advancements in mountain bike geometry and technology have been based on 26" wheels. If you just change the wheel size, nothing says that all of the old established standards with 26" wheels will still work. As with most engineering problems, there are both positives and negatives to almost every option. So, new design optimization may need to take place for each wheel size.
So why 650B? The people behind the 650B movement claim that with 650B tires you get all of the same advantages of the 29" movement (lower rolling resistance, better traction, smoother ride, etc.) with less of the disadvantages (geometry limitations, toe clearance issues, higher center of gravity, suspension travel limitations).
Much of this may be true, but as I always say, you should get out on a bike and see for yourself if it works for you.
One cool thing about these 650B wheels is that some fork manufacturers are now giving them the OK to run in their standard 26" forks. This will take the 650B movement a long ways down the road to longer travel without other sacrifices.
I find the idea of looking into different wheel sizes appealing, but I think it may be a long time, if ever, before we as an industry can say what wheel size is best for any type of riding and any type of rider.
If we take the arguments of both the 650B and 29" movements to extremes, we will end up with custom sized wheels, tires, and frames for each and every rider.
I think in the end here, the bike industry will learn some lessons from all of this and we may end up with some better options for different sized riders and different types of riding, but don't expect wheels to go through a rapid evolution. There is way too much invested in the 26" wheel for it to go away anytime soon.