Cfrtjryk

I need a chassis for a box trailer. For now I'm focusing on the chassis itself, as once that's built I can use it to haul the rest of the things needed for the box. I'm no steel-worker though, and thanks to where I'm located getting a mobile welder out here won't be cheap. I've designed a simple ladder chassis in 50x50x3mm box section steel, and want to know the viability of simply bolting these parts together like so:

I am aware that my bolt-holes will need to be very accurate, and to prevent slippage I intend to use nylon-lined nuts, shake-proof washers, and locking fluid on bolt threads. Where the tongue meets the draw-bar two 5mm steel plates sandwich the join to prevent twisting.

The axle, 550kg suspension units, and hubs are being purchased as a single unit from a fabricator. Each part will be individually painted after machining, and the interiors treated with rust preventative. If you want to have a look at the model it's here.

I'm aware that welded joints are generally stronger.
My question is will this design be strong enough without welding?

In my analysis your design is not going to be adequate. Here are the main points:

1. Box steel with just 3mm thick walls can deform when squeezed under bolting pressure or when subjected to torsional pressure when bolted through the box.


2. Carriage bolts with the square tang under the head are meant to be used with wood. Unless you intend use dozens of hours filing round holes to square the carriage bolts are not suitable. If you used a hack to make the round hole big enough diameter to accommodate the square tang then you have lost the precise alignment that you mention in your question.


3. For a bolted construction I see a severe shortage of triangulation designed into structure to keep it square.


4. The cantilevered cross members should have some type of edge interconnection to define the outer perimeter of the frame. The need for this would become much more obvious as you address building the next levels of the box trailer.

With the time and materials that you would spend building this you would be better off just considering purchasing an electric arc welder and doing some practice welding on scraps of steel to brush up your technique to weld this frame construction yourself.

I think it's probably adequate for the task. 550 kg is a pretty light weight trailer. I think I'd maybe go with a heavier wall thickness or taller section on the longitudinal members, and a pair of angle braces at the rear to help prevent racking. Deck will help with that also.

How are you mounting the suspension? I do have some concern with just bolting that up to the thin wall longitudinal members without some sort of gussets or such. I have a small, lightweight (M416) trailer that developed cracks in the frame around the rear spring mounts - had to weld in doubler plates.

Basic welding with a MIG or flux core unit is pretty easy to learn, and it opens up your options for alot of stuff. I'd look into it.

Another thing to consider is how you'll be using the trailer. If it's primarily for occasional light use to the home improvement store on good roads, this is probably fine. If it will be heavily loaded, used daily and sees rough roads then you're going to see wear - as in the bolt holes wearing larger, bolts wearing and everything getting loose & sloppy until failure.