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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:

Trailer frame diagram showing material dimensions etc.

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.

Detail of the nose of the trailer

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?

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    I'm voting to close this question as off-topic because it isn't about home improvement as defined by this site. – isherwood Dec 3 '18 at 14:18
  • Thanks for the input anyway :) Can you suggest a more appropriate location for it? – Alex Dec 3 '18 at 14:33
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    My $0.02 ... it's an interesting question and in the realm of things a home DIYer tackles. How else do you get the drywall home :) Or we could take it out to make room for one more question about wiring a two way switch! – CoAstroGeek Dec 3 '18 at 16:48
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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.

  • Regards the carriage bolts I'd read in product descriptions that they were intended for wood or steel, so assumed (silly me) you could hammer them home into steel like you would wood - I've now read up and indeed it's recommended you file a square hole. I feel like I could sacrifice carriage bolts so long as the other methods (nyloc, threadlock, s/p washers) were used though? I was hoping with two bolts per joint, plus the triangulation offered by the tongue would be enough. Would it perhaps help to extend the tongue to begin behind the axle, rather than in front? – Alex Dec 3 '18 at 14:20
  • I think the A-frame of the tongue provides enough triangulation on the horizontal plane, no? Then there's the deck, even if it's wooden. – isherwood Dec 3 '18 at 14:20
  • Also on the subject of the perimeter: this will be part of the box structure. Until that is added, only the space between the hubs will be used as a load bed. – Alex Dec 3 '18 at 14:27
  • Bolts apply tension in one dimension only. In the other two dimensions, the holes weaken the structure, and paired holes are worse. Non-circular holes require special care to avoid cracks starting at inside corners. – amI Dec 3 '18 at 19:24
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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.

  • Suspension arrives as a 50x50mm box-section with swing-arm units that mount on the plates at each end. I haven't drawn in the suspension units themselves, but have drawn in the 'axle' bar - you can see its flange sticking out at the very bottom there. I think I'm just gonna have to nut up and learn to weld aren't I? Looking for small machine to learn on now as I have a neighbour I should be able to borrow a larger one from (as well as grab some pointers hopefully...) – Alex Dec 3 '18 at 18:45
  • Any reason why not ARC welding? MIG seems a lot more expensive to get into. – Alex Dec 3 '18 at 18:49
  • I think stick welding is more difficult in general - and even more so on thin material like this. But that may be my own bias and lack of experience. I did a little stick welding as a kid, and never got very good at it. But picked up MIG pretty easily later on. – CoAstroGeek Dec 3 '18 at 21:04

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