A brief summary of hinge problems on Bike Friday New World Tourist. I ordered this bike in December 2014, received it in late March 2015. From the week I received it, I have had issues with the lower hinge routinely coming loose, as well as excessive flex (ie, the top of the chainring moves several millimeters left and right under normal pedaling).
I am fully aware that folding bikes are intrinsically very flexible, which is no problem. But the folding hinge continues to come loose on a weekly basis, suggesting a serious defect in the frame.
First, note the play in the hinge barrel, welded onto the front subframe. The nondriveside has very little play, while the driveside has over .020":
Did the barrel ovalize with use? Was it oversized out of the factory? Not sure, but the frame has less than 200 miles on it, and I've been diligent about keeping the hinge bolt tensioned (despite regular loosening from riding).
Does a loose hinge barrel compromise the structures rigidity? If the plates of the lower subframe are clamped tightly enough on the front subframe hinge barrel, it shouldn't matter. But the hinge bolt must be left loose enough to fold the bike. I tightened the hinge bolt such that there was drag in the hinge, but the frame was still foldable. I mounted a dial indicator on a magnetic stand and mounted it to the rear chainstays:
I then placed the indicator on the plates of the rear subframe immediately outside the hinge barrel. Any deflection of the indicator should be from flex in the rear subframe between the magnetic base mounting location and the indicator location.
Alternately loading the driveside and nondriveside pedals:
We find displacement with no hysteresis, ie, the indicator needle returns to the same location after each loading, regardless of load direction. Next, I move the indicator a few millimeters over to the barrel. Now, indicator movement can be attributed to frame flex and sliding of the rear subframe with respect to the front:
Alternatively loading each pedal:
We get similar indicator displacements, but with hysteresis, indicating that the rear subframe is sliding with respect to the front.
It looks like the play in the front subframe hinge barrel permits sliding between the front and rear subframes, and I have a hunch that this motion occuring with every pedal stroke is the problem causing the hinge to come loose periodically. Note, the hinge locking nut is new, and the hinge bolt is sufficiently long (I can see threads coming out of the locking nut).