Dahon SUV D6 frame hinge I have a problem with the main frame hinge of dahon SUV D6 folding bike.
It is hard (stiff?) and the bike does not fold as smoothly as it should. I am trying to fix it.
But i do not Know how to UNASSEMBLE the hinge joint to fix it.
Is it possible to unassemble it? Can I fix it without unassembling it?
Either way, what tools and lubricants should I use?
I send some pictures
Oil it.
Your main hinge is a permanently assembled pin.  It is not a bolt and should not be removed.
I also notice your bike has rust marks - so I guess it has a hard life and is stored outside.  This will absolutely contribute to difficulties opening and closing the main hinge as well as shortening the overall life of your bike.
Are you following the recommended maintenance?  I expect it would say something about lubricate at least yearly, and more often in harsh conditions (like outside storage)