Not that much, the 650b in 42mm is the same circumference as a 700c in 23mm, so if you want to test the BB height, simply fit a 23mm tyres if you can.
I'll try that.
@psee started
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Not that much, the 650b in 42mm is the same circumference as a 700c in 23mm, so if you want to test the BB height, simply fit a 23mm tyres if you can.