QUOTE(fatani @ Apr 30 2018, 02:37 PM)
im no sifu, but from my experience:
1. Yes. the main cause is probably road vibration. that nut on handlepost clamp always need re-tightening after some time. we can notice this when try to lock and unlock the clamp. if the clamp feels a bit loose, it's time to turn that nut anticlockwise a little. my dahon do suffer this minor issue.
2. yes. probably caused by the frame flex, and over tightened case.
3. no idea. but i guess it should be ok. all i know is that it should be greased.

Thx Fatani,
1) So overtime, if we keep tightened, will comes a point we can't tighthen anymore the screws..what then?
2) I dont have the skills to dismantle the headset, thus thought some slight oil for quick fix..only to find out its the frame joint that creaks.
So, most of the time when the handle feels a bit loose, its the handle post that needs tightening, not headset, right?