If I'm getting it, that's the diameters of 700 and 650B wheels respectively. So, the difference between the radius of the two is 3/4" or, 19.05mm. Accordingly, if using my plastic rule from college days, if the rear brake pad is at the top of the stop on the 105 calipers' arms on my Z85, I could swap out the 700 wheel (that cannot be fitted with 28 mm tires due to frame-impingement problems) for a 650B (11 spd compatible hub) and the rim brakes would still work if the bottom of the stop on the brake's caliper arm is within ~20 mm. Then, I could have a road bike that'd accommodate 28mm or larger rear tires. But alas... arms look to be a mm or two short.
