Superlight Torsion Box

[TinKicker]

Has anybody ever used basswood for structural members? I was asking a friend in the cabinet business about different woods to use for my trailer framing, and he suggested basswood because people use it in airplane frames for lightness and strength.
I was thinking about how to make a torsion box with it that would be strong enough to mount the axle and tongue to, when I had kind of a flash of insight. What about using steel strapping ran down the length of the torsion box edges, all the way around, and secured through the box with bolts?
I'll try to describe this a little better because I don't have time to make a drawing...
Picture the torsion box lying on sawhorses and you're looking at the edge of it. Now take, say, 1" wide by 1/16" thick steel with holes drilled every foot or so. Kind of looks like plumber's strap, if you know what that is.
Run this steel the length of the box, on the top and bottom surfaces, along the edge. Run bolts through top to bottom so that you've got a tight sandwich of steel-box-steel.
Do you think the tension provided by the bolts would keep the box from bending and flexing (assuming the strap also ran around all edges front/back/sides)? If so, this would allow an extremely lightweight box with minimal use of heavy steel. The axle could be bolted right through the steel/box/steel with the same bolts.
If anybody has any opinion on this, let me know. I don't mind building a mockup for test purposes (me jumping up and down on it) but if it's not a good idea I don't want to waste the time.
Thanks!

[madjack]

...if your talking about building a plywood torsion box, I don't think the steel strapping you are describing and the attachment method will add any substantial strength to the box...as far as the axle and tongue, make sure you have a complete bolt-thru method for that attachment...possibly using some 1/8th to 1/4 steel plate for stress relief at the bolt and attqachment areas......
madjack

[dwgriff1]

My whole trailer is a series of torsion boxes. Small, rather thin webbing with insulation in the center and 1/8 plywood on each side.

My trailer is frameless, much like a modern car, with the torsion box that makes the "pan" supporting the tongue. It also has solid wood filler where the axle bolts to the "pan."

In 6000 miles it has worked perfectly.

Remember, the lighter you make it, the lighter it needs to be.

dave

[TinKicker]

Madjack, I was hoping someone had some experience with the lighter hardwoods, so all the steel strapping stuff would be relevant. The idea came to me as I was reviewing in my little brain about a website on steam bending wood I was checking out the other day. One method of bending involves fastening a steel strap to one end of the piece you're bending on the outside of the curve, and then keeping it tight along the wood as you bend. It really cuts the chances you'll break anything because the stress on the outside of the curve is carried by the steel instead of the wood. That made me wonder--which is why this post exists--could that strap carry the stress of the weight of my tear, taking the load off the torsion box?
Of course, the ultimate focus of all the stress is the steel's anchor point at the end of the box. Everything relies on that point not giving way. So that led me to wonder if several bolts passed through the box at intervals could "break up" the load and distribute it equally.
Which THEN led me to wonder if an additional similar band on the top side would maybe divide that stress in half.
I feel like I'm onto something, but nothing comes without a highly scientific experiment, does it?
My goal is to have the lightest frame possible without being unsafe, so I'm trying to find any way possible to cheat the system here (without going broke on high tech stuff like carbon fiber honeycombs and crap). I'm thinking on 1/8" luan type ply and the basswood for structure. If nobody's used that, then they may have used spruce. Thanks for the info on the stress relief plate sizes, though. That's something I'd been wondering about too.

Dave, I've always liked doing everything contrary to the crowd, which means I'm usually broken down on the road less travelled! HAHAHAHA!!!! So that, of course, means I LIKE your design philosophy. I guess I just like being a guinea pig for myself.
If you don't mind telling, approximately how much does your unloaded trailer weigh, how much load do you carry, and how big is the sucker? I'm plotting and planning on a 5x12 floorplan, 4 feet tall (as of this week). It's good to know someone's built a "survivor" like yours. Thanks!

[TinKicker]

Dave, I took a look at your album and I remembered your tear! I checked it out several weeks ago (in fact, checked out every picture in your album) and I don't think I'll forget it. VERY unique and well-engineered. That tongue bracing is COOL!
Now that I think about it, your tear is what originally got me thinking about torsion boxes. It's good when you can see something working in the real world that you just "know" will work, but which you can't quite figure out how to explain to someone else.
Great job!

[dwgriff1]

I used Mike's Ultra light design, but widened it to 5'. it is fully insulated and lined and weighs 580 dry.

With care I think I could get it down another 100 pounds and maintain the size, but it probably wouldn't be worth the trouble.

I used a lot of 1/8 plywood, fiberglass and house paint, birch and spar varnish. I got started bending oak and couldn't help myself.

dave

[TinKicker]

Dave, did you build your own steam box or purchase one? I've read quite a bit on the subject from a few different sources, and I think I can do it. I need a few more basic tools before I attempt bending, but it won't be long. I'm psyched!

[dwgriff1]

I own a steam box, but those bent pieces are actually done with thin strips of wood, about `1/8" that are glued to a form. That method has a few advantages, I used the edge of the tear to hold the center vertical piece, making it easier to get the right shape.

The others were bent against a form I made, often out of cheap particle board.

It could be done by steam bending, but those are fairly large pieces of oak and steam would have taken a while.

dave

[dwgriff1]

It seems the other part of this is bracing the tongue, which i did with those bent oak pieces.

If you look at early small trailer plans they often used a metal brace that was bolted near the end of the tongue to the mid section of the trailer on both sides.

I have thought of making a tongue box that basic shape, open from the inside only, that would brace the tongue without having to have exposed members.

If I were to build again, the top might well be covered in dacron fabric, like a piper cub airplane. That would make that triangle "tongue box" a lot easier to fabricate.

dave

[MrBuzz]

Dave;
I was looking at your album....What did you use for an axle? Are there springs on it or is it attached directly to the trailer floorpan?

[bobhenry]

If you click on the album picture it will size up then you can see that the tube is directly under the floor pan. It has to be a tortion tube because the center of the wheel is only a couple inche below the frame. This would not be possible with the arc of a set of springs.

[dwgriff1]

Yes it is a torsion axle and yes it is bolted right to the "pan."

dave

[angib]

Tinkicker, it sounds to me like you are thinking that a 'frame' has all the strength and the 'body' has none. As Dave's teardrop demonstrates, this isn't the case and the body, properly built, has all the strength needed for a 'frameless' construction - if you are very careful.

As has been said here more than once, building frameless is possible, but it is not an easy way out, as designing and building the axle connection and, most particularly, the tongue connection becomes critical.

Andrew

[Jiminsav]

don't look at my frameless construction...seems it wasn't good enough to stick in peoples minds.

[TinKicker]

Andrew, my goal isn't to build a strong frame to the exclusion of the rest of the unit. I've read enough of your excellently informative posts to understand that.
However, since the frame is the first component I'll be putting together, I guess it would seem to everyone that it is my only focus. I just HATE do-overs, so I'm more comfortable over-engineering and over-thinking the problem. If there is some method that will make the frame box markedly stronger, then I'd be crazy not to take advantage of it, even if the walls and all could hold up a tank.
There is ALWAYS a better way, stronger, faster, more efficient, whatever, and it's usually only limited by two factors: how much $$$ you can throw at it and/or how much you think through all the angles. I'm trying to let my brain save me some money here.
I think there's a factor involved that most of the builders out there will understand--sometimes a particular construction just "feels" right.
That methodology may be heresy to some, and mind you I LOVE to diagram and plan and calculate with the best of them, but I don't expect to build anything without some test time involved. Maybe a lot of it. Heck, that's half the fun!!!
So I think I'll make my plan, build my plan, observe my plan in action and fix what goes wrong. KISS in action. Screwups are very informative.

Jiminsav - I DID look at your album...but didn't know your tear was frameless. Has it performed well?