In this video, I carry 85 lbs of firewood in the trailer - bringing the total to 120 lbs with the weight of the trailer itself.
I strapped a camera to the box, to check how the brakes work under load. The front plate flexes too much for my taste: being aluminum, it's bound to fail with metal fatigue at some point if I don't stiffen it. So that's what I'll do next.
The surge brakes work very well: I can't feel the trailer at all when I stop. I can feel it when I pedal however... :)
