Join Date: Aug 2008
Location: Somerville, MA and Catskill Mtns
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To the OP: I am just nearing completion of a 58cm Soma Buena Vista 650B for my wife. I was inspired by yet another Soma BV 650B build that another forum member did (I'd have to find the link). 650B wheels do in fact work on this frame, but you need to have extra-long reach brakes (73mm). I went with Tektro R589 dual-pivot brakes. The only catch is that for the rear brake to reach adequately, the wheel must be positioned at the very front of the dropout. I was a little concerned about this since I'm running an 8-speed IGH on this bike, and IGHs place a huge amount of torque on the axle, something that derailleur gearing doesn't do. However, I've been told that it's simply not a problem, as long as the wheel is securely in the dropout.
Here are some pics (click to enlarge, and sorry for the crappy iPhone pics):
I also chose to go with really fat 650B tires (42mm Grand Bois Hetres) for their supreme comfort, versatility for riding on pavement, gravel, trails, etc, and amazingly low rolling resistance. These tires end up having the same overall diameter as 23mm 700c tires, so the reduction in toe overlap is minimal, especially with fenders. You might get an idea of this from my photos. However, since your wife's from would be significantly smaller, the toe overlap may be mmuch more pronounced than with the 58cm frame. If you go with narrower 650B tires, for example 35-38mm, you can then also go with narrower fenders and then you'll have a more significant reduction in toe overlap. But as mentioned above, toe overlap is usually not a big concern.
I'm a big fan of 650B conversions, as it allows you to have a much wider tire on a frame that otherwise doesn't allow it, while not affecting the bike's geometry.