noteon 

It's a fair question. The first frame snapped way before any trailer-hauling. The second snapped a couple of weeks after I started pulling the trailer.

After asking around and doing some research (I can't cite it--I wasn't looking to bolster my position in a debate, just making sure my kids were safe), it seems a trailer doesn't stress that part of the bike; it hitches directly to the rear skewer, while the breaks were up where the seat tube meets the top tube. (Both of the breaks--with and without the trailer.) I think it keeps breaking because their weight limits are, um, overoptimistic, I put more miles on it than they're gambling people will, and the BioLogic Post Pump doesn't extend far enough into the frame to prevent flex at that braze. But I'm just guessing, and I'm not an engineer.

But even if trailer-hauling does somehow increase stress on the seat tube, Dahon advertises this as a bike that "begs to be ridden hard," and says nothing about not pulling trailers. The marketing is that this is a rugged, 26" mountain-style BIKE. Period.

Dahon itself decided both breaks were fair warranty replacements, but their fulfillment has been dismal.