I have the same issue with my Dahon Formula S18. Every few rides or so, I have to tighten up my post as well. I keep a narrow profile PARK 8mm wrench in my bag. It just takes a quick turn of that nut in the stem (like 30 degrees or so) and it's tight again. It doesn't take much movement of that nut to make things loose. I've thought about putting some Loctite on it but I've just gotten used to using the wrench.

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