OP: If you put a deltech cable (loaded in tension) on it, you'll eliminate the bending load on that hinge joint and place the frame tube and hinge into compression shear on the pin, eliminating a lot of the wobble. Torsion loads into the frame beam, like when climbing standing, will not be affected, but the hinge will be tighter. Any cable must be low-stretch; Steel (like Dahon's), kevlar, or UHMWPE like Spectra, Dyneema, etc. The line must be very taut when unfolded. Photo below is from before I secured the line ends. Knot must not slip, I used 8 bend with kevlar line which I happened to have some, kevlar is slippery so requires a good knot: