Originally Posted by
fuzz2050
The Most of the time a 650b conversion doesn't actually drop the bottom bracket to much, 650b tires are usually pretty wide, so the difference in rim diameter is usually made up by tire diameter. A typical conversion from 700c tires drops the bottom bracket by a few mm. A 27 inch tire might pose a problem though, you already have a fairly hefty tire, and you have a few extra mm of rim to deal with.
It might not be a good idea, but I think you could probably get away with it.
27" = 630 mm
700c = 622 mm
650b = 584 mm
630 - 584 = 46 mm
46 mm / 2 = 23 mm = 2.3 cm
That is a pretty big difference, enough to make pedal strike a big issue as well as mess up handling.
Also, no options for brakes, bike will still be too big.