Andrew/ engineers, what's going on here?

[Larry C]

Hi Larry C.

1x2?
That's a lil narrow for that length there partner.
Might consider another cross member than just that one, unless your planning on the Teardrop frame to be that extra cross member.

Stresses in a side wind or (heaven forbid) a bad sway may buckle or twist it.

Thanks for your comments. If you have engineering data that supports your concerns, please show me, as I believe my chassis to be properly engineered for my total loaded weight of 500-600#.

The picture in my previous post is my chassis upside down and the cross member is just a temporary brace that's tacked on till I get my Dexter 8 Axle which is the structural rear cross member of the chassis triangle. This design calls for no other cross member.

The The Teardrop body will be a torsion box that's constructed with sandwiched panels that are glued with epoxy and completely covered with fiberglass/epoxy. The torsion box (floor) will be the structural cross member.

My goal is a Teardrop that's 1/2 the weight of the typical 1200# build, hence my lighter chassis design.

Here's a pic of the frame right side up with the tacked on temporary cross member:


Here's a pic of the chassis design. I modified it form Stub axles to a full axle, but the tongue is still 1" X 2" rectangular steel tubing:

[fm-usa]

I only mentioned the frame size because I’ve seen 1x2 frames twist & buckle. My comment was strictly out of concern.
After seeing your drawing, looks good.

Every country has their minimum standards and sometimes they differ vastly and I admit, I know nothing of Australia’s.
When I work on customers buildings, I try to meet or “one upâ€

[Larry C]

[quote="fm-usa"]I only mentioned the frame size because I’ve seen 1x2 frames twist & buckle. My comment was strictly out of concern.
After seeing your drawing, looks good.

Every country has their minimum standards and sometimes they differ vastly and I admit, I know nothing of Australia’s.
When I work on customers buildings, I try to meet or “one upâ€

[fm-usa]

I want an Ultralight Teardrop, but I also want a safe one.........
BTW/ when you say you have seen 1x2 frames twist and buckle, can you give me any particulars?

Offhand the one I do remember distinctly was mmm-about 35 years ago.

Riding my bike through the Rockies I was behind a van pulling a mid-size, tarped trailer. It caught some gusty side-winds through the mountains and the van went into a quick hard sway. It looked like it was recovering nicely (van was heavier) but suddenly I seen the trailer lurch and went dog-tracking to the right. If that trailer went left the van might have road it out, but that trailer hit the guardrail hard and lost some of its load. As usual, I stop to help out.
I seen the trailers tongue, it buckled to the right. That tongue was very similar to yours but a little longer. That is why I responded otherwise I usually don’t.

This was very memorable cause just 2 days before another van pulling a boat had a rear tire explode. I heard the tire pop then seen the van swerve, then it was coming in my direction. Well that rig went into the center grass of the expressway hitting the ditch, nose first, really hard. That threw everyone out the windshield. The mother didn’t make it (looked instant) but I was able to help save the boy who’s throat was slashed nearly ear to ear. I was on him in 20 seconds with a plastic bag & pressure to his throat.
It’s seeing accidents like this that really hurt, . . . . . and wish to help others avoid them, IF and when I can.

I’m no expert, but I feel I’m experienced enough to offer suggestions.

Do take care. fm

[frank_a]

Just as a note, I rebuild old boats and have rebuilt a number of vintage boat trailers. Back in the day, a lot of boat trailers had steel rod welded to the frames as in the pic at the beginning of the thread. They are a pain to refinish!

Frank

[angib]

1x2? That's a lil narrow for that length there partner.
Stresses in a side wind or (heaven forbid) a bad sway may buckle or twist it.

To provide Larry with some conmfort, I'll chip in that I see no problem with a 1x2 A-frame design - but please note I said A-frame. The two sides of the A-frame form a triangle that is very strong sideways.

A 1x2 single tongue would be scary.

On the subject of strength, I think there are two practical requirements:

1) Enough vertical tongue strength. The only common way trailers break is by vertical collapse of the tongue, so it needs to be strong enough there. Some flatbed trailers may get broken near the axle when they are overloaded, but I've never, ever seen that in a travel trailer.

2) Enough horizontal tongue stiffness. I've never seen a trailer collapse sideways (except in a collison) so horizontal strength isn't a big problem but they can get a sideways shimmy and a couple of forum members have reported just this, with long single tongues - though others have had longer single tongues and no problems! The problem cases all got fixed by increasing the horizontal stiffness, usually by adding diagonal braces.

Andrew

[angib]

I like the word 'undertruss' - though I am worried it is the cure to an unpleasant medical condition.... I was thinking about these today while out cycling (and at my speed there's a lot of time to think).

The trick with undertrusses is that it's easy to get them wrong. With a length of square tube, you can't go wrong unless you stop the tube 6" short of the coupler. But with undertrusses, it's easy to get them wrong. It's unlikely that this will cause any problem but it may mean they add no strength at all.

Here are three examples:



The top one is an effective undertruss - it is centred on the back end of the tongue where the bending load is highest and it is a direct truss shape.

The middle one will still do lots of good, but it's not as good as the first one, despite its extra complexity.

The third one is uselss - it will add some strength to the middle of the tongue but it adds none at the back end of the tongue where the bending load are greatest, so it doesn't make the tongue any stronger.

So with undertrusses, it's no good just to make them look like they work - that's no guarantee they will do any good.

The other trick with undertrusses is to design them so that not only they work effectively, but that they can also be built. Those narrow angles can be a real problem to make effective joints and get access for welding.

So, for example, Reiner's design at the start of this thread puts a bend in his undertruss tube so that it meets the chassis rail at 90 degrees and a good joint can be made. The downside is that the bend behaves like a spring, reducing the effect of the undertruss.

I assumed because Reiner's undertruss was so long that it wasn't meant to be structural at all, but was actually a rock guard to stop the teardrop frame rails getting damaged if the trailer was grounded on a rock. More than half the undertruss could be left out without reducing its strength.

Andrew

[Larry C]

I like the word 'undertruss' - though I am worried it is the cure to an unpleasant medical condition.... I was thinking about these today while out cycling (and at my speed there's a lot of time to think).

The trick with undertrusses is that it's easy to get them wrong. With a length of square tube, you can't go wrong unless you stop the tube 6" short of the coupler. But with undertrusses, it's easy to get them wrong. It's unlikely that this will cause any problem but it may mean they add no strength at all.

Here are three examples:



The top one is an effective undertruss - it is centred on the back end of the tongue where the bending load is highest and it is a direct truss shape.

The middle one will still do lots of good, but it's not as good as the first one, despite its extra complexity.

The third one is uselss - it will add some strength to the middle of the tongue but it adds none at the back end of the tongue where the bending load are greatest, so it doesn't make the tongue any stronger.

So with undertrusses, it's no good just to make them look like they work - that's no guarantee they will do any good.

The other trick with undertrusses is to design them so that not only they work effectively, but that they can also be built. Those narrow angles can be a real problem to make effective joints and get access for welding.

So, for example, Reiner's design at the start of this thread puts a bend in his undertruss tube so that it meets the chassis rail at 90 degrees and a good joint can be made. The downside is that the bend behaves like a spring, reducing the effect of the undertruss.

I assumed because Reiner's undertruss was so long that it wasn't meant to be structural at all, but was actually a rock guard to stop the teardrop frame rails getting damaged if the trailer was grounded on a rock. More than half the undertruss could be left out without reducing its strength.

Andrew

Andrew,
Thank you for the great info. I still like the undertruss idea. I would like to incorporate the one you suggest as being effective. It makes sense.
Knowing the chassis design, which is yours, would you please suggest the length/depth dimensions of the undertruss? I know the welding will be difficult because of the angles. How about bench welding a short piece of flat stock to the ends, and just weld the flat stock to the tongue?

Thanks,
Larry

[bobhenry]

The barn had a flex problem so we trussed exactly as described by Andrew. It was flexing a good 2" and had crushed the sheeting on the barn.



The upright really is at the end of the frame. The barn is cantalievered forward 2' . To install the bow truss we welded the 4" mid suport 1 1/2" by 1 1/2" square tube first , then welded the 1 1/2' X 1/8" strap to it. We then welded front and rear. Now in order to pre tension it we used a monster "C" clamp and pulled the strap to the frame tube first front then rear and welded the edges to the tongue tube. After a couple stretches you could smack the strap with your fist or a hammer and get a nice base note. It was truely amazing how much strength was added !

[Larry C]

The barn had a flex problem so we trussed exactly as described by Andrew. It was flexing a good 2" and had crushed the sheeting on the barn.



The upright really is at the end of the frame. The barn is cantalievered forward 2' . To install the bow truss we welded the 4" mid suport 1 1/2" by 1 1/2" square tube first , then welded the 1 1/2' X 1/8" strap to it. We then welded front and rear. Now in order to pre tension it we used a monster "C" clamp and pulled the strap to the frame tube first front then rear and welded the edges to the tongue tube. After a couple stretches you could smack the strap with your fist or a hammer and get a nice base note. It was truely amazing how much strength was added !

bobhenry,
Thanks for the info. It looks like what I want to do. I see you used a strap. I was thinking of using round tube, don't know which one is better.

Also, do you think it was really necessary to pre-tension the strap. I am curious why you did that. I understand the principle of pre-stressing as it's done on flat bed trailers and bridge beams.

Maybe Andrew can chime in on the pros/cons of pre-stressing in this application.

I'm glad to see it worked for your application, I'm surprised trusses are not used more often on trailers.

Larry C

[Corwin C]

bobhenry,
Thanks for the info. It looks like what I want to do. I see you used a strap. I was thinking of using round tube, don't know which one is better.

Also, do you think it was really necessary to pre-tension the strap. I am curious why you did that. I understand the principle of pre-stressing as it's done on flat bed trailers and bridge beams.

Maybe Andrew can chime in on the pros/cons of pre-stressing in this application.

I'm glad to see it worked for your application, I'm surprised trusses are not used more often on trailers.

Larry C

The material forming the truss will always be under tension, the "post" always under compression (if designed properly). You could actually use steel cable and a turn buckle and have an adjustable truss.

Since it is under tension, the cross sectional shape of the material will make very little difference in the structure. In these cases (tension) the cross sectional area or higher tensile strength is what you need. Personally, I would use a solid cross section (rod/bar/strap) rather than tube because they are generally easier to get a good solid connection.

As far as pre-tensioning goes -- the truss will not add strength until it is under tension. If there is "slack" in the structure (i.e. arched truss rod rather than straight), that "slack" will have to be taken up before the truss will help support the load. I actually pulled a trailer with a truss that was "loose" and it made "twanging" noises and "hummed" as we traveled (it was a high school shop project ... good design, not so good execution). A taller post pounded into the structure while the load was reversed made it MUCH better. Because of this, I would definitely pre-tension if I were doing this myself.

[angib]

My first thoughts on a suitable design are:



The undertruss can work two ways:

1) only in tension when the load on the coupler is upwards;

2) in both directions, so with tension or compression in the undertruss.

(2) is better but in my heart of hearts I think tongues only fail with the coupler going upwards so (1) is nearly as good. This means the bottom of the undertruss can be made from something non-tubular. My choice would be flat bar, as wide as the frame rail so that it gets fixed at each end with a fillet weld to the underside of the rail - an easy weld to make if the frame is turned over.

The post can be anything sturdy - a bit of frame rail tube would be the obvious choice. Depth can be variable - the deeper, the stronger the truss is, but the more likely it is to get hit. Gut feel says 3-4".

I don't think a lot of "pre-stressing" is good but a little bit, just to take up any free play in the bottom truss member, would be good.

But.... I would like to check out the weight and strength gain before recommending the undertruss - you might find that you could get nearly as much strength just by welding a partial length of second tube under the frame rail.

That do ya?

[frank_a]

1x2? That's a lil narrow for that length there partner.
Stresses in a side wind or (heaven forbid) a bad sway may buckle or twist it.

To provide Larry with some conmfort, I'll chip in that I see no problem with a 1x2 A-frame design - but please note I said A-frame. The two sides of the A-frame form a triangle that is very strong sideways.

A 1x2 single tongue would be scary.

On the subject of strength, I think there are two practical requirements:

1) Enough vertical tongue strength. The only common way trailers break is by vertical collapse of the tongue, so it needs to be strong enough there. Some flatbed trailers may get broken near the axle when they are overloaded, but I've never, ever seen that in a travel trailer.

2) Enough horizontal tongue stiffness. I've never seen a trailer collapse sideways (except in a collison) so horizontal strength isn't a big problem but they can get a sideways shimmy and a couple of forum members have reported just this, with long single tongues - though others have had longer single tongues and no problems! The problem cases all got fixed by increasing the horizontal stiffness, usually by adding diagonal braces.

Andrew

Andrew, would a frame like Larry's be okay for a larger TD? Something maybe 6X10? We want it large enough for a queen sized bed (60" X 80"). How might you modify such a design to provide a safe and dependable chassis/tongue combination for a larger TD?

Thank you,

Frank

[angib]

I think that basic triangular chassis can be used for any body that is self-supporting - meaning with strong (glued) joints throughout.

To go up to 6x10 the size of the rails needs to be increased and the logical step is to 2"x2"x11ga(~1/8") tube. If you limit the tongue length to around 48", two of those allow a 1300lb trailer by the Australian rules - which I think means they are OK up to 2600lb for on-road use under a teardrop.

As you get bigger and heavier, the body design at the front attachment point (the 1/8" plate Body Mount above) needs to be thought about carefully. A good bit of framing in the front of the floor (2" deep framing would be good) and a good joint to the front roof panel which needs to be somewhere near vertical (ie, like a Benroy, not 'rolled under' like a Cub) are enough.

[frank_a]

Hmmmm, maybe I didn't ask the important question - does it make sense? Or would I be better off to go with a standard rectangular frame? I don't mind over-building much, just not a lot!

Thanks Andrew. I will no doubt have further questions as this all sorts out in my head.

Frank