I have three 650B conversions in the current fleet. My most recent is a Fuji 12-S. It was quite straightforward in that it had a ton of clearance for 650B x 38mm tires, and I used Weinmann 750 center pull brakes. I've only taken it out for one real shake-down ride, so I haven't yet decided if it's going to be a keeper:
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The 650B conversion that I've put the most miles on of any in my fleet is a '97 Lemond Buenos Aires. I had to dimpled the chain stays a bit to get it to fit 38mm tires and use Tektro long-reach dual-pivot brakes.
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The third is an early 70s St. Etienne frame powder coated gloss black and equipped with a porteur rack. This one was originally fitted for 27" wheels, but Weinmann 750 brakes give me enough reach:
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