Originally Posted by Inertiaman

Since we're all at a place called bike forums, I hope most of us already understand how BB drop is defined.

27.5 wheels aren't necessarily 27.5 in diameter. The tire cross section has a substantial impact. 27.5 is the approx. diameter of a nominal 27.5x2.1-ish tire. But on this thread we are primarily talking about 27.5x3 tires, which coincidentally are pretty close to 29x2.2 in outside diameter. So switching the wheels as you note above may not impact BB height at all. Indeed, this is the premise behind the wave of new bikes that are "27.5+/29 compatible."

For reference, some radius figures (approx. dimensions, as exact dims depend on specific tire brand/model and pressures):
-29x3 is ~385mm radius
-29x2.3 is ~365mm radius
-27.5x3 is ~ 360mm radius
-26x4 is ~ 365mm radius

My main point was to quantify the discussion of viability of 27.5x3 wheels/tires on a frame originally intended for 26x4 wheels/tires. As you can see from the dimensions above, the radius is nearly the same, so effective BB height is essentially unchanged. The slightly larger radius of 26x4 is typically cancelled by the slightly lower pressures typical of 4" tires (and hence higher compression of the tire cross section) such that the effective outer diameters are, for practical purposes, equal. Practically speaking, differences in specific tires and pressures will yield more distinctions than the generic dimensions will.

What BB height is "needed" in Baja? Now that is a subjective discussion. My point is that the BB height of a Pugsley with 27.5x3 is objectively 5mm higher than an Advocate Hayduke with the same wheels/tires. So categorically dismissing the viability of a Pugsley running 27.5x3 is subjective hyperbole contradicted by the objective math.