Originally Posted by
soma2x
27" to 650b should work very well. 27" inch wheels are 630mm in diameter whereas 700c wheels are 622mm. You'd only need 4 more mm of reach from the brakes to convert from 27" than you'd need to convert from 700c. Tektro R556s have something like 55 mm to 73mm in reach. If you're riding a 27" bike that uses short reach brakes (39mm to 49mm) the Tektros should work OK with the pads all the way at the bottom of the calipers.
I'm thinking exactly the opposite. It's hard enough to find brakes with long enough reach going from 27" to 700C. Now add another 4mm and it becomes even harder. Basically you're going from 630mm to 584mm. That's 46mm in dia or a difference of 23mm smaller in the radius. That's almost a full inch, 25.4mm