Fixing a wobbly Dahon frame hinge

How to fix a steel Dahon frame hinge that wobbles, even when tightened

The closed and clamped hinge. Grab this and push it up and down to see if it wobbles

Between our family and our friends, we use Dahon folding bikes a lot. My heavily modified (including electric hub motor) Dahon Boardwalk has done around 20,000km of heavy riding, carrying my 80kg self plus panniers on our steep (and some rough) mountain roads (here's my page about my electrified Dahon Boardwalk, here's my post on the Xiongda 2-speed motor I'm using on my Dahon, here's my post about fixing a broken Dahon handle post). Our friend Bradley also does heaps of distance on his Dahons, mostly 2 Speed TRs (one is also electrified). It is tremendously useful to us to have folding bikes, especially living on a mountain where getting a lift in a friend’s car can save a lot of time and effort.
On the other hand, Dahons have also caused us a lot of bike mechanic grief. It seems to me that the culture of marketing and “innovation”, with almost annual “new models”, means that the designs aren’t refined, but are simply shuffled. All of the Dahons I’ve known have had serious design flaws, providing fairly major bush engineering challenges to keep them on the road: splitting seat tubes, frame and steerer hinge problems, headset problems (especially with threadless). We’ve only bought steel-framed Dahons, very conscious of the repairability of steel, as well as how fragile and unreliable aluminium frames are in diamond frames, let alone under the greater stresses of folding bike geometry.
We’ve had to repair nearly all the frame hinges that have done reasonable distances. This is what happens:
Even with regular tightening of the hinge adjusting bolt, frame hinges tend to eventually get a wobble. This is an up and down wobble, found by grasping the hinge of an unfolded bike, and lifting it up and down while holding the rest of the bike down.
What seems to be the problem is that the mating faces of the hinge are manufactured flat, so any tiny wear at top and bottom allow a small rocking movement, regardless of how tight the hinge is. A little rocking brings huge pressures on the surface at top and bottom and leads to more wear, making more wobble even when tightly adjusted. Once worn slightly convex, clamping pressure only acts on the middle of the hinge faces. What is needed is to spread the clamping pressure back to the top and bottom of the faces.
On our steel Dahon frames, we have found a remarkably simple remedy: laying a very small pad of stick welding at the top and bottom of the male mating face, filed smooth (I use a 2.6mm mild steel rod, at 130A). This creates pressure points at the very top and bottom of the hinge mating face, and leaves a slight gap in the middle - where the faces were rocking. The raised pads are perhaps a little more than 1mm high after filing.

The welded pads are on the left side hinge face, top and bottom. At the top of the right side face you can see the worn area which has caused the wobble

Before welding, the hinge clamp is undone by removing a small circlip from a vertical pin and removing the pin so that the clamp can be folded out of the way. 

What this achieves is to distribute the hinge clamping pressure onto 4 discreet points on the perimeter of the hinge faces: top and bottom welded pads, hinge pin on one side, clamp on the other.
I’ve done this repair on (I think) 4 Dahon frames, with excellent results.