Dahon handle post hinge loosens too quickly I noticed that the handle post hinge on my Dahon Speed D7 now requires attention much more often than it used before. Unlike main frame hinge which remains tight for long after adjustment, this one now requires tightening at least once week, otherwise handle post becomes shaky.

Red arrow on the image below shows the 10mm nut I have to tighten to move that metal "foot" above a bit closer to the edge to the to secure the handle post when bike is unfolded.



(don't know if I managed to explain the process clearly enough, but Dahon bike owners should get the idea, I believe)

This is an old bike, but until last 4 or 5 months I only had to tighten that nut once a month or even less often. Now it's much worse and getting worse with time.

I have marked one of nut's sides to determine if the metal "foot" started to wear out quickly or the nut is somehow loosened moving down the screw. Turned out it is, something forces the nut to rotate at 60 deg or so every week, so I'm not actually tightening it but returning it where it belongs.

To be honest I don't understand the mechanics of this process (what makes it to unscrew) and wonder if this is something I can fix or I'll need a new handle post soon.

Thanks!
As with any critical fastener on a bike, the vibration and flexing forces from riding will eventually work threaded parts loose. Normally this is the sort of thing that occurs over a few months--not every week. Fortunately, there are a few options that are applicable to this and any other situation where threads work themselves loose. (Before you do anything, though, make sure that the rest of your steering components are safely adjusted.)

The easiest thing would be to buy a tube of Loctite (or comparable) thread locking compound. It is cheap and easy to find. You'll want to get a formula that isn't permanent so that you can make adjustments down the road. Back the nut off until you see some threads and add one or two drops those threads. Adjust the tension where it needs to be and let it sit for a little while. (Loctite is safe for threads mostly everywhere on a bike!) In less critical applications, beeswax is a great natural compound for securing threads and preventing corrosion.

A second option would be to replace your 10mm nut with a lock nut. These are nuts with a nylon (or similar) insert in the aft part of the threads which prevents it from working loose. Similarly, you could procure a lock washer which will prevent the nut from backing out when tightened properly. (It doesn't look like you have enough space for it, but a second nut behind your primary nut (called a jam nut) will keep the adjustment stable as well. Jam nuts are typically narrower than a standard nut so they use less threaded area.) Any well-stocked hardware store will have suitable metric hardware. 

If those options fail you, you may need to take more permanent action. Either chemically (with a permanent thread locking solution), or mechanically by installing a castle nut. A castle nut has notches along one side to mesh with wire or a cotter pin that is drilled through the threaded bolt. The wire prevents the nut from backing off, but the downside is that this requires drilling a tiny hole straight through the bolt (which would require you use a drill press and lots of diligence). I really don't think you'll have to resort to this last option, but it works.

In any case, continue to be vigilant in inspecting those parts for wear and adjustment.