All right, now we're doubling up on logical fallacies: both ad hominem and non-sequitur!

I never said all frames have to be triangulated. just that it's the most weight and stiffness efficient design.

If a designer wants to choose to emphasize something else, that's a free choice. Do you really think I don't know about the single top tube designs? Their designers have chosen that most likely for cost and simplicity over weight - a valid decision. The bikes with the fold in the top tube would have a more complicated fold mechanism alignment issue if the fold line extended down to a second pivot in a lower front tube.

Here's a cross section of one of the failed Tern frames.

Notice the massively thick side walls of the tube and its oval shape. Both are needed because that single tube is taking all the bending load. In a diamond frame that load is taken by two tubes that are more widely spaced. The stresses in those tubes vary inversely with a power relationship to the distance apart. That's why the diamond frames have smaller tubes with much thinner walls and are thus lighter.

Here's a Tyrell that's fully triangulated instead of the Castro open X:



Unlike the Castro, the bending load in the main triangle is not concentrated where the two top tubes cross. It's spread out over the cross section that runs down to the lower tube. Because of the power relationship to stresses, this makes the Tyrell much, much stronger and stiffer than the Castro.

Although I'd prefer a custom steel butted diamond framed folder, I made a cost decision to go with a heavy single top tube folder that met all my other requirements.

But the Castro is just an incompetently designed mess intended to look "kool." They had the tubes and the space to make a diamond, but instead chose an open X design. I've never, ever seen an open X used as a piece of real structure and you haven't defended it from an engineering point of view - you ducked the issue. And it's not me being pompous, it's Mother Nature saying so. I'll say to you what an aerospace colleague of mine once said about know-nothing managers making technically incompetent decisions: you may choose to ignore the laws of physics, but the laws of physics never ignore you. Enjoy your ignorance.