Trailer frame strength

[treebucker]

The answer is that it is a bad idea all the way around. But I want to tackle this one problem at a time.

Someone I know is in the process of converting a house trailer into a general-purpose trailer. He has it stripped down to the frame. The bed frame rectangle measures ~ 5' 5" x 12' 6". The plan is to add 2 more cross members so that there is one every 2'. Then add 5/4 decking. The end deck will measure ~ 6' 10" x 12'.

It has dual drop-down type axles w/single tires. He has turned them over to raise the bed height.  The main beams are 2" x 3" steel tubing. There were no holes in it so I couldn't measure the wall thickness. These beams seem to me to be the hardest to verify but most the likely, and easiest to overlook, failure point. I tried to point out that the walls of the house trailer box served to stiffen the trailer and both the walls and frame shared to load. I'm outnumbered on this. The other side is saying the trailer frame will hold more than I think and that the deck will make up for the removal of the walls. I know they are wrong but would like to get some numbers to verify my position.

The other issues with the axles, hitch, tires, wheels, springs, etc. will be tackled later.

[footer]

I'm with you here, although I have never seen a house trailer frame made of box tubing. Just real light duty I beam. The I beam frames I have seen would have to be boxed in with plate steel to make them strong enough to hold any significant weight. He might be OK as long as he doesn't haul much weight. Another issue I susspect is that if he turned the axle over, his wheel alighnment will probably be out of wack.

[metalspinner]

Here is a lengthy discussion about this very topic...

https://forestryforum.com/board/index.php?topic=14452.0

[treebucker]

I used the wrong name when I typed house trailer...it's a little travel trailer (early '70s vintage) like this: http://www.mantelli.com/rv/inv_details.asp?MtsStockNo=04%2D18109

[Tom]

 :D :D  Different.  :D

[Murf]

You are very correct about the frame rails. The superstructure adds a lot to the overall stiffness of the trailer.

Years ago I owned a small fabricating business and we made a lot of frames for house and office trailers, and I've seen lots of old frames made into trailers like this one.

Every single one of them flexed and wagged and bounced like a puppy with a new bone.

The only way to stiffen them up is to both add more cross-members and cross-bracing, and to increase the effective height of the main beams.

Another word of caution here also, despite the fact they look like it, a lot of small travel trailers do NOT have 3,500 pound capacity axles under them. Some the size of what you're talking about had just 2,000 pound capacity axles under it.

:

[mike_van]

The axle's should have an aluminum tag wrapped around them giving the specs for it. All the tube ones i've seen look like they're bent, thats the camber so when it's loaded, the tires ride flat. Upside down, it's been cambered X 2, but the WRONG way. Only the outside edges of the tires will be carrying the weight.  If it's just to haul a lawn tractor or atv around it's probably o.k., a really good deck of 1 1/2" lumber will stiffen it up too.  You load it up with lumber, a pallet of bricks, or  2 of those big  zero turn mowers,  i'd be pretty leery of it.

[Murf]

...it's a little travel trailer (early '70s vintage) like this: ....

I'm not sure if they tagged axles back then, but even if they did, I doubt the tag would still be there and readable some 35'ish years later.

Good point about the camber also, but again, I'm not sure if they did that back then.

[treebucker]

Excellent input. Someone said they found 2000# on the axles.  The front axle now looks as if the drop is pointed slightly forward. I think the springs need to be turned front to back as I suspect they measure differently from their center alignment pin to the shackle. My idea was to leave the axles alone and add some tubing to the top of the existing to both raise and strengthen the frame. But my idea got shot down.

I need to look see if there is any camber. And yes it started w/atv and lawn mower hauling and then graduated to logs/lumber, etc. That's when I started getting uneasy. smiley_anxious Ok, down right fearful. I've pushed my point as far as I dare now I need some concrete numbers so I can prove it. Anybody got any idea on that 12' 6", 2" x 3" tubing frame?

[low_48]

Look at the way they add strength to all those cheap 16' trailers. They add vertical angle iron and a rail. It gives it strength like the previous structure did. Angle those verticals and you have a truss. Well you don't have a truss, the trailer does. If you had a truss tha would be something entirely different........ :D

[Murf]

Treebucker, just tell them the big problem with flexing is it lets the binders go slack and fall off.

Tell them to go ahead and build it, but you're not towing it with your vehicle, or riding in the one hauling it, or even driving behind it for that matter.  :

Hopefully they will get the idea.  ;)

BTW, there's lots of complicating factors to calculating the strength of that frame, including crossmembers, welds, etc., it's not as simple as merely saying it will carry X number of pounds.

[Don_Papenburg]

If your retangular tube has square corners it is a very light gauge metal . If it has a large radius it will be thick.   Bend a piece of sheet metal at a 90 you will get a tight bend , take a 1/4 inch plate and bend a 90 in it and you will have a rounded corner .
Beef up the frame with a 2x2 tube under the main with a 2to4inch gap and add ladder or diagnale truss between.

[bitternut]

Here is a picture of the 6 x 12 trailer I made that uses 2" x 3" x 1/8" tubing for the main frame and for the cross members. The single axle is 3" schedule 80 pipe. Yes the trailer is grossly overloaded with this New Holland skid steer on it but the frame itself is plenty strong enough and did not appear to flex or bend when loaded. This machine weighs some where around 6500#. I have also hauled scaled loads of stone between 5500# and 5800#. The stone hauled fine but the skid steer was a little uncomfortable. As suggested you could add sides to the trailer which would stiffen the frame quite a bit and would be an easy fix.

[rebocardo]

> The main beams are 2" x 3" steel tubing.

Even small trucks such as a Ford Ranger have a min of a 6h x 2w x 1/8+ wall frame under the cab and engine. Boxed or not. The sectional strength to resist bending is in the height of the channel or tube, not so much the width of the tubing/flange or wall thickness.

www.pirate4x4.com has good articles on trailer design and steel structural strength when applied to trailer design. Plus calculators.

If you search the trailer section on E-Bay most if not all the car trailers advertise a min. of 6" x 2" x 1/4" for the frame rails on a 16 foot trailer.

My little home made 4x9 utility trailer that I wouldn't put more then 1200 or so pounds in has a bigger frame then you listed and I see it flex off road when loaded with wood and it is spaced every 18 inches or so for cross members.

If you go here you will find that for

> into a general-purpose trailer

http://www.championtrailers.com/UTILITY_TRAILER_KITS.HTM

2x3 or 2x4 or 3x4 angle is used. Which is good enough for general purpose -> on road. Once you start loading cars, wood, etc. on a trailer and start taking it off road, that sizing becomes 1/2 to 1/4 of what you really need without excessive trailer flex.

Also, there is a big difference between a 12 foot trailer and 16 foot trailer when it comes to sectional height needs. For a 12 foot lightly loaded trailer that sticks to pavement, you do not really need much so 2x3 x 1/8 is enough.

> that the deck will make up for the removal of the walls

I think trailer makers add that rail at least in the front to prevent trailer flex, it acts sort of like a wire spar because most mild ASM 500 (?) steel has a yield strength of at least 35,000 psi so a piece of 1/8 2x3 angle needs a lot of pressure to rip it apart end to end so it holds the sides together.

Unless you run the wood, right to left, it does nothing. Certainly not 5/4.

The theory is easy enough to prove just with a small cardboard box on a 1:12 scale. Put in on dowels or ballisters, put a plastic bag filled with water that touches all sides in between the dowels, such as a garbage bag, and start cutting sides off.

If the trailer needs to be stiffened cheap, use bed frame rails. I do not know exactly what they use to make them, I think maybe spring steel, but, besides being almost impossible to drill it is very hard to bend and it is usually free.

[Quartlow]

They don't weld worth a hoot either  :D

I bet it ends being stronger than you think, I used one from a 24 ft Holiday rambler once to build a bump trailer.
The dump, hoist and cylinder came of a 1950's era dump truck. I don't remember what size it was but it had 20 inch tires on it and the box was 10 feet long.

The first load I put on it was 617 at the limestone quarry. The loader operator just kept looking at me funny because I kept telling him to keep dumping. Sad part was, I couldn't pull it. Not enough traction. Ended up dumping about 1500# in the bed of the pickup. Went out across the scales I was 22,000. I didn't load it that heavy after that  :D Not because I was worried about the trailer, the poor old 460 was crying for forgiveness by the time I went 3 miles to the inlaws.

Of course I was young and dumb then too

Want interesting, take the floor out of a semi trailer and drag it down the road. Did that once, talk about a hula dance. That thing was all over the place.

[Raider Bill]

They don't weld worth a hoot either   

I heat them up with the torch then weld, seems to work that way. I grab any I see on the side of the road. 8)

[scgargoyle]

I'm getting ready to make a light utility trailer, so I stopped by the big box store to see how their's are made. Up to about 2K#, they are just 2" angle iron! They do weld sides on them, but they flex torsionally like crazy. The last trailer I built was a single axle 6X12, made from 4" channel, and decked w/ 2X12 PT. That thing was solid. I make my own drop axles, using AMC front spindles, which are bolt on. I weld 1/2" plate on the end of heavy 2" square tubing, and brace it with a piece of plate. I hauled a measured 3500# with it, not including the trailer itself, and it handled it easily. I want to make my next trailer a lot lighter, but I don't like the way that 2" angle flexes. I think a lot of this depends on how the trailer will be loaded. I used mine for moving machine tools, which are a lot of weight concentrated in a small area, and it's not always possible using shade tree methods to center the load. I think an angle iron trailer would just fold up. But if the weight is distributed evenly, such as lumber, I think you can get away with a much lighter trailer.

[sawmilllawyer]

 8) Don't know all the math, but I built my first trailer in 1989 (still have it and use it often) from 2" box tubing with 1 1/2 angle iron cross braces every 12". 3" x 3" sq tubing hitch. Purchased a new 3,500# axle and leaf springs from a trailer supply. Have pulled 4,000#s weighted gravel and/or sand in it too many times.  Compared to the one's at the big box mine seems pretty substantial. Still running...