Why don't I see many cross supports for the 3d printers designs.

Why don’t I see many cross supports for the 3d printers designs. Triangles have more strength. Rectangles wobble.

Triangles make the build area smaller than with a square shaped reprap. Mostly in the height.

Well triangular side supports may work. I found out about the build height thing via irc. Larger joining blocks would help too. Mainly for the top.

The Mendel90 design uses right angle panels of instead rods, so no truss structures are necessary. I can vouch that it’s absolutely rigid.

Indeed. That does look sturdy.

Would it be easy to convert a prusa to a square shaped printer without too much cost?

How about this one. The original Mendel uses a triangular frame, as does this derivative that uses extruded aluminium. http://www.reprapbcn.com/catalog/index.php/full-kits.html

Geoff that is triangular only on the side. More accurately, it is a simple tent shape.