4WD box trailer 1.4t dwgs attached.

So a single 75x50x2.5 RHS aligned to be taller vertically has a moment capacity of ~1440 Nm at a relatively safe stress of 120MPa.

To use this strength with the worst case of a point load, you're talking ~400kg of capacity per 75x50 RHS beam.
You have 7 of them which gives your deck the strength to completely safely carry ~3 ton spaced right down the centre of the deck with a safety factor of ~3 on yield.
It won't start to take a permanent bend until you've got ~9 ton spaced down the centre of the deck.

If you consider a distributed load, it would be ~840kg per beam or ~6 ton in the deck still with a FOS of 3 on yeild of those beams. You could put ~18 ton of sand in it before the beams took a permanent bend.

What I'm trying to say is, the beams you have under the deck are way out of proportion to the axle underneath and the cladding on top.

Dougal,


I wish I had you engineering powers. This is good information. The only way I design is guesswork!


So before I lighten the design (which I agree is a bit heavy), I think your calcs might be centered around dead loads. If the trailers has a ton on board, and is travelling over humps and bumps at speed, could there be forces that equate to 3 or 5 times the weight of 1 ton pulling and pushing and forcing down on the chassis? I'm sure there would be.

Quote:

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Originally Posted by browndogrider1

Dougal,


I wish I had you engineering powers. This is good information. The only way I design is guesswork!


So before I lighten the design (which I agree is a bit heavy), I think your calcs might be centered around dead loads. If the trailers has a ton on board, and is travelling over humps and bumps at speed, could there be forces that equate to 3 or 5 times the weight of 1 ton pulling and pushing and forcing down on the chassis? I'm sure there would be.

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Correct those calcs are for dead weight. How much impact you get depends on your suspension and tyres. I think a 3:1 FOS on yield is conservative enough but also very realistic and won't result in too much over-building. This would mean even a 3G vertical impact (extreme) with max design weight won't result in anything bending.

What you have to do to work out the best floor design is work down from the decking material. First you need to work out the max span the decking can take with your maximum intended load. That sets the spacing of the support beams under and the spacing of the support beams determines the load on them which determines the necessary size.

If you think your trailer is heavy enough to not bounce you should see me towing our car trailer unladen... Even had the buggy trailer (loaded) off the ground.

Longer softer springs will make it ride better unladen and laden, as I said, use springs off the back of a ute.

This will minimize bouncing unladen and still carry the weight as well, if not better.

Cheers
Will

If this trailer is to be a multi use trailer also consider not using check plate on the load deck.

Check plate looks great, but if you ever put boxes etc in the trailer the plate will rip the bottoms apart...

A small, but significant factor if it will be used on trips...

Na, have towed similar heavy duty trailers with slippers. The more Substantial weight makes it ride a whole lot better. When empty, this trailer won't bounce around under normal conditions too much at all. If you want true weight carrying capacity, you cant get rid of all the bounce (unless you start getting elaborate with shocks, and air etc etc) - its just a trailer for firewood, sand etc


Though I do have a 750kg trailer that is half the weight ( probably like the one your speaking of) and it bounces around a lot.


Not interested in Ute Springs. Would be flat out trying to find a set that are rated at 1.5t. - need to be short travel and tough - ute springs have too much travel and are a compromise to give passenger comfort - and the travel is not short enough. Put a tonne of weight on a ute, and just watch how much it sags!!


You want your trailer, when fully laden to sag no more than 70mm.

Quote:

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Originally Posted by browndogrider1

Have designed, and am building this trailer.

Am interested in any comments regarding this design getting blue slipped in NSW. Hopefully this should not be difficult? Constructive design comments and advice appreciated.

45mm square axle, ford bearings, 10" elect brakes, 16"rims with 265 x 75 tyres, hand brake, 1.4t 70mm slipper springs, etc

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I just built the exact same trailer for a customer, however I used 14s not 16s. My one needed an extended drawbar for a mower box (yet to be completed) and a cage. Let me know if you have any questions & good luck with the build !!

www.jetcraftfabrication.com.au

My thoughts are:
No need for the ribs down the middle, they add nothing to the strength. Also running 50x50 into the wall of 75x50 will make the trailer crack as it flexes around.

Middle rib in drawbar is unneeded.

Relocate drawbar xmember to pass under the spare tyre and make the mount off that.

The plate for the spring hangers is not needed, just centralise the mounts over the main chassis rails and it works well. The mounts are 6mm thick and can be setup like that.

Floor plate sheet size is 1210x2400, but this can vary. Trailer floor work to 2420-2440 depending on what turns up. This also means you will need to run a cross member under the middle where the floor plate joins to get a good join. Don't just weld it together, weld it to a cross member. Xmember spacing at 400's approx.

If you do go to hot rolled steel floor, note that floor plate is a higher grade steel and the cheques increase rigidity, so you may find the hot rolled equivalent is 2.5mm.

Just terminate the drawbar in front of the spring hanger, no need to put the bend in it. Also don't terminate it forward, as it stiffens the trailer too.

Drawbar is very big. Maybe look at 75x50x4 and this might also work well with jockey wheel fitment.

Main chassis rails maybe run a 3.5 to prevent suspension mounts from ripping out. These rails take most of the punishment when bouncing around as all load goes through them, where the Xmembers have the load divided between them. Also the front rail due to drawbar forces.

Keep guard bottoms high and use mudflats so the guards don't get hooked up and bent.

Use a regular coupling mount on the drawbar for strength and fatigue life/cracking. Ensure that you've got nut clearance under the coupling plate

A little plate between the guard and tie down rail for extra rigidity and to minimise movement and cracking of the guards.

Fold the tops of the panel into a C section for panel rigidity and to produce a safe edge. Fold the bottom too to prevent leakage of you precious sand tonnes!

We do drawbar lengths at 1800-1900 from the front of the box.

For the side steps rather than using 30x30 RHS, just fold the outside edge down and that will do the same trick and be lighter.

For offroad, use shackles rather than slippers to prevent grinding and the spring from popping out of the mount if reversing. Spec shackle springs light, as the make the 4wd ones very stiff, way too stiff!

Run the corner uprights off the side of the trailer at the back and off the corner of the trailer at the front. The panels reinforce them into position. This lets them drain when water gets into them and prevents rust problems.

Run a 50mm clearance or more between the tyres and the chassis rails.

Run you wiring through the drawbar and then weld in conduit for the wiring to the back of the trailer and then across to the other light cluster. Make sure it's all debarred otherwise it will short the wires.

Axle offset, I can't recall if it's 1" per foot or 1/2" per foot, but check this.

Setup the axle as SUA or SOA depending ton the ride height you need it at to suit the tow vehicle.

Consider shocks for 4wd touring

I do not recommend Independant suspension because it point loads the main chassis rail and a Xmember which requires it to be a lot bigger/heavier, increases unsprung weight, and it doesn't really give you more usable ground clearance. It's a lot more complexity, things to break and increased forces in the trailer that don't give you any more than a properly setup leaf setup. Actually they are a bit Gucci though.

Heap of good tips "rrturboD".


Have read your info, and am processing, will include some of your suggestions.

Quote:

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Originally Posted by Tombie

If this trailer is to be a multi use trailer also consider not using check plate on the load deck.

Check plate looks great, but if you ever put boxes etc in the trailer the plate will rip the bottoms apart...

A small, but significant factor if it will be used on trips...

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x2... I just happened to see this thread... son bought an 8x5 single axle trailer for general household duties, it came with a "check plate" floor. He is normally pretty cluey about these things but in his haste forgot to order the plain floor. Unless the floor is firstly lined with a sheet of 8x4 3-ply when carrying anything of value, the result is a real mess ... as Tombie notes, cardboard boxes are history after even a short run...