Unibody style frame?

Indoorwindmill

New member
I've been thinking about putting together a compact trailer with the usual RTT and similar equipment setup but had a bit of an unusual idea for the frame. Has anybody build a trailer with a unibody-ish style frame before? I was thinking of possibly using a sheet of 1/4" steel plate with the sides bend up to form a 48x72" or 72x72" pan with a 3x3" square tube running the length on the inside of it forming both the tongue and a keel of sorts. Possibly a pair of reinforcement channels tying in where the (most likely) Timbren suspension will attach to the centre tube.

With some basic internet poking around the 72x72" 1/4" pan is 450lbs, 10' tube 95lbs, Timbren 110lbs, matching wheels and tires (235/85R16 x2) 140lbs for a total of roughly 800lbs. A few utility trailers in the 5x8' range seem to be around 900lbs ready to go so 800lbs for a rolling skidplate base doesn't seem unreasonable to me.

Thoughts? Is there some obvious issue that I'm overlooking or is it simply the equipment for bending wide sheets of steel not available to most people?

Thanks either way for the input.
 

high-and-dry

Active member
It wont be as strong as you think, it will oil can, aka be a drum. If you could press some ribs into it, it would probably be okay then.

I can build a very solid 5x10 frame including the tongue but no axle for less than 300 lbs. Even if I seriously over built it under 400 lbs, and then it could carry 3k lbs with out an issue. add a 100 lbs for the axle, and another 150 for 31 inch tires and its only 550 lbs and will carry 3k.

Dont try to reinvent the wheel
 

ottsville

Observer
Bending 1/4" is pretty hard and 1/4" is overkill. Think of car bodies - they are somewhere around .7 mm. But yeah, the ability to bend sheet well and appropriately for the task is hard.

There's a trailer design floating around on TNTTT - a basic A frame with a torsion axle as the third side. That's probably one of the simplest designs out there.

Dont try to reinvent the wheel
 

NatersXJ6

Explorer
Seems far overkill and as others have noted will be more floppy than you think.

If you have a 6-10’ press brake you could work with 14 - 18 gauge and get something strong by welding on hat sections and rolling in some profiles, but hard to say it will be “better” than traditional body on frame trailer construction.
 

old_CWO

Well-known member
I did something very similar with a Chevy LUV step-side box which is just a small sheet metal tub with tailgate and a few ribs underneath. I think it was 18 gauge but don't remember for sure now. It "oil canned" at first even with the factory ribs. In the end it worked fine with a little more reinforcing than originally planned. Would I do it again? No, probably not with the possible exception of a military trailer. Have often thought you could de-chassis an M105 and weld tongue and suspension attachments directly to the box. It would probably still be a heavy beast even after losing the frame!
 

john61ct

Adventurer
See rally cars built from round tubing.

Then add sheet and bigger beams only where needed.

Probably can get an engineer on the gig-work outsourcing platform to help for a few hundred, just don't shoot for super light the first prototype.
 

billiebob

Well-known member
This is how a lot of commercial trailers are built, including my Mirage 5x10 deck. But it uses 1/8" steel.
Sides are 12" high, inside is 8" tall, 1.5" decking, 2" x 1/8" wall crossbeams, plus a a 1/2" return on the sides, top and bottom.
IMG_0699.jpeg

Trailer weighs 700# empty including wheels, tires, axles

IMG_0700.jpeg

10 years old and still as good as new.

IMG_0701.jpeg

This was a typical work day, 3500# of brick chimney, heading 40 miles to the landfill.

DSC_0001.jpeg
 
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high-and-dry

Active member
Your is a wood deck, he is talking about a 1/4 steel plate deck, not wood. Yours is built the normal wall with an actual frame.

This is how a lot of commercial trailers are built, including my Mirage 5x10 deck. But it uses 1/8" steel.
Sides are 12" high, inside is 8" tall, 1.5" decking, 2" x 1/8" wall crossbeams, plus a a 1/2" return on the sides, top and bottom.
View attachment 659038

Trailer weighs 700# empty including wheels, tires, axles

View attachment 659039

10 years old and still as good as new.

View attachment 659040

This was a typical work day, 3500# of brick chimney, heading 40 miles to the landfill.

View attachment 659041
 

Indoorwindmill

New member
Dont try to reinvent the wheel
That is good, direct and probably very accurate advise. It's also not very much fun. ;) From James May: "It's an ingenious solution to a problem that should have never existed in the first place."


Bending 1/4" is pretty hard and 1/4" is overkill. Think of car bodies - they are somewhere around .7 mm. But yeah, the ability to bend sheet well and appropriately for the task is hard.
I work at a fabrication/machine shop and have access to pretty much all the equipment which includes a 200 ton brake press. We regularly bend both wider and thicker plate which is the only reason I'm even considering this. At least I can do my own goofy project and see what happens, sending this out would be way too much time and money. I'm not too worried about the work needed to build it, it's more of a what/should I build. That said yes the 1/4" is overkill, I've been intentionally overestimating as reasonably possible as it should be simpler to scale back down than trying to patch in reinforcements afterwards. As strange as it sounds I might prefer a heavy solid base to help keep the centre of gravity lower and allow for a lighter structure build on it.


This is how a lot of commercial trailers are built, including my Mirage 5x10 deck. But it uses 1/8" steel.
Sides are 12" high, inside is 8" tall, 1.5" decking, 2" x 1/8" wall crossbeams, plus a a 1/2" return on the sides, top and bottom.

Trailer weighs 700# empty including wheels, tires, axles
While it's not exactly what I was thinking of it's a good comparison to see a trailer frame that isn't made out of box tubing.
 

Indoorwindmill

New member
As mentioned in a few replies yes 1/4" is overkill and I'd most likely reduce it to 3/16" or possibly 1/8" but I'd rather overbuild all the load bearing items than tear a wheel off in the middle of nowhere. The rest of the box would be 1x1" square tubing possibly changing the wall thickness depending on location (ie tent supports or just keep the cladding on). The bend up edges are 4" with a pair of 45* bends at the front and rear to allow more welding area with the tongue plus hopefully to act as a ramp if it gets hug up on something. That said with 2 feet of ground clearance if it's getting stuck my truck won't be going anywhere.

This is the part where I know I'm forgetting some specific that's important. Have a few ideas for stabilizer jacks as well as a parallelogram mount for the tent/annex/awning but those aren't exactly critical currently.

After partially relearning the basics of SketchUp I've come up with these monsters. The side profile is the same for both, only change is between a 5' or 6' box with the same wheel spacing. It's a bit of a mess with the tent as I've drawn it in both lowered and folded in plus raised and folded out with an annex attached. I'm trying to keep it as compact as possible while still being as useful as possible. All the dimensions aren't going to be exact but should be pretty close for planning purposes. Screen Shot 2021-05-07 at 8.05.27 PM.pngScreen Shot 2021-05-07 at 8.04.00 PM.pngScreen Shot 2021-05-04 at 8.49.40 PM.png
 

highwest

Active member
Apache Eagle trailers had an “X brace” that was made from formed sheet metal - this part was the only frame-like object in their whole build. The tongue and leaf springs were attached to the bottom of the X. 3/4” plywood was attached to the top of the X. The trailer’s tub/body was attached to the plywood only.

Here’s a poor picture of my past Apache project. The X brace is upside down, the trailer tongue has been removed, and the leaf springs have been replaced with a new tube subframe. But you can see the formed shape of the “X brace”. It was very strong.
8D79CFCE-58CC-4F55-90A7-F0833380CC5A.jpeg
 
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