I took out the rear wheel from 1 of the bikes and it is right at 5 inches .
My experience is that you do not want to force the frame
open wider than the relaxed state, the tension on the
frame can lead to it snapping.
There is not really a good time for the frame to break, but
there are definitely worse times, like on the side of the
road so the bike pitches you into traffic, or 10 miles down
a 20 mile backwoods trail...