Elliot wrote:... I am worried about a possible structural weakness in your trailer frame. ...
I’m talking about the spot where the triangular tongue attaches to the rectangular box. ...
Hmmm.
Good point.
This trailer started as a boat trailer, which is why the tongue is so long. The tongue is rather similar to figjam's image, only the tongue sides don't extend all the way to the side of the trailer, but shortly after going under the front of the box, they turn to the back with a weld to a longitudinal member, which runs along inside of the wheels.
I don't remember the exact details, but I had the same concerns, so I did examine it, and had someone jump on it while I watched to see what flexed where. If I remember some correctly, then:
- the outer sides of the triangle tongue are two "C" pieces of steel sandwiched together to make a "tube", which makes me think someone has already added to the original design
- the centre tongue member is a through-and-through, and rather robust, again two "C"s sandwiched, but looks original
(I was able to feed doubled extension cord through it to serve as new robust wiring for the lights - what else use for an extension someone's gone over with a lawn mower)
- the front of the box sits against a small piece of angle iron, welded across the three member tongue, as does the rear of the box
I though I had photos, but I can't find them right now.
As a utility trailer, this has had ridiculous loads in it, loaded up past the side supports (like we see in its last use when full of bikes, with bins of steel parts stacked at the front), with with much heaver loads of hardwood and scrap steel (previous owners, semi-rural setting, helped feed their kids by heading out every Sunday night to fill the trailer with scrap metal left out for the garbage trucks). We tried to break it once, but couldn't. Oddly, its taken the abuse.
When I adapted it for "living" use, for reinforcement on the inside I added 2x2 to near every seam with 1x2 up the sides. Under the box I added a near full-length piece of angle-iron to each side, bolted to the longitudinal steel member running inside of each wheel. I also discovered that the box was just sitting there, not bolted on, small screws rusted away... It's now bolted to the new angle-iron.
I'll have to take a much closer look at what goes where, once it's warm enough to do so.
Note: with the top section of 1/2" plywood being so heavy, and the triangle sides putting more weight to the rear, it's biased the whole balance to the rear. With the trailer empty, there isn't enough tonque weight, even with the spare tire. It has to have a load.
) 
