Those hinges look handy Stuee, but what are they made of ?
If stainless steel then your going to have to use dissimilar metal rods to weld them.
My hinge uses an 8mm ground dowel pin in a reamed hole size and size polished to fit.
As for my fold being simple that's because I am simple
incapable of a complex thought.
BUT and this is a big BUT, the tricky part is where to position the hinge so it folds with all the gubbins ,i.e chain, cables etc still stay in place and that nothing collides when folding.
In my case the hinge is at 45 degrees across the spine but angled back 8 degrees to get the needed clearance,, I should add I built an animated 3D model of the complete trike before slashing into steel to prove it out, so its well worth a bit of thought before cutting.
regards
John