Chime in from small wheel world.. Bike Friday uses a seat post mast,
of extended length,
but the tube OD is oversize, straight gage, with a machined Al shim at the top
to go down to fit the seat post standard size of 27.2.



BiFri folds.. Bene: the low step over. I swing my leg
in front of the saddle, in dismounts.


I note brace tubes and gussets added in some situations,
like X frame MTB's..