I'd be interested to know if both your Dahons failed at the same point. I'm favouring a Dahon at the moment due to price and those small wheels on the Brompton.
Not at exactly the same point, but both on the seat tube.
Both were Dahon Matrix bikes. One was a 2008 model, with the big hinge, and one was the 2009 model with the lockjaw system.So I guess the stress points were in different places.
This is a current Espresso, but has the same 2008 model frame design:
And this was the Lockjaw version.
What I didn't realise, and no one at
Evans even mentioned it when I ordered my bike, is that there is a weight limit to them of 95kgs.
Now, I'm a little over that on my own. Add in a rucksack with a change of clothes for work, a lock, tools, and a camelback with two or three litres of water, and I'd probably be over by a considerable margin. So maybe it was material related and maybe it was 'user error' but I only found out about the weight limit after the problems.
Looking at your profile picture, it doesn't look like you'd be anywhere near that.
I'd just do some googling to see if quality's improved.