my two bits about structural strength

[fatehunter1972]

Hello all,

I just wanted to put in my two bits about structural strength. I see a lot of posts refering to 'beefing' up an area, making something stronger, reinforcing this or that. With all of this one overiding principle must be remembered. 'The structure is only as strong as its' weakest part'.

any additional strength away from that weakest area is just extra weight.

This problem crops up all the time in boat-building when one area is beefed up only to result is stresses being transferred and something else, (usually more difficult to fix), breaks.

Keep this in mind when building a teardrop.

One way to go is to have a strong frame carrying all the load and the 'pod' simply sitting on top of the trailer.

If the teardrop is one composite unit, many stresses/loads can be caried by the skin itself, just look at a modern airplane. this is called monocoque construction. Of course you will need 'furring' strips (nailing strips) to attach things too but it is amazing what stresses the skin will hold when you put a little shape into it.

O.k. don't mean to lecture. I am just staring at the pile of work in front of me but dreaming of building a teardrop!

[Ira]

I guess you got the whole point:

Most of us build these damn things with itsy bitsy 1 by 2s when framing and very thin ply. Or NO framing, and 1 by 2 spars, and the ply is STILL not that thick.

And they're still rock solid.

You'll hear the word "overbuild" here a lot, but like you said, who are any of us to lecture? But when someone is building their first, they're scared to death they're going to kill themselves and others if they don't build it the way "other" things are conventionally built. So we all overbuild.

Mine survived Hurricane Wilma where huge boats in the boatyard where I keep it were blown across the yard and severely damaged. So in the case of TDs, when we say "beef up," we're really talking minimal beefing.

We don't build these these things with heavy 2 by 4s.

Mine is 3/8 ply sides, 1 by 2 spars doubled up and this is NOT even necessary, 1 by 2 framing, 1/4" roof skin and cabin ceiling, and 1/8" cabin wall skins on the framing, which is basically just thin wood paneling.

Not what you would call "structural" material at all.

[angib]

Yep, one of the things folks forget when they 'build strong' is that it often means 'build heavy'. And if it's heavier, some or all of the extra strength will be used up just in holding up the extra weight.

So far I think we've had three documented strength issues:

- John Blewett of Tiny Tears had the axle ripped off his foam composite chassis-less teardrop. I'd call it a design/build error rather than a strength issue - from what I understand John forgot about peel strength.

- Dave Nathanson kindly tested his teardrop tongue to destruction. His tongue was medium length, light and had a bolt hole at the point of maximum load. Also Dave seems to believe that a trailer is happiest when it's bouncing in mid-air off washboard roads......

- Chip had a wobble issue with a tongue that he solved by adding long angled braces. This was a stiffness issue, not a strength issue, so we need a tongue to be stiff enough as well as strong enough.

Anyone got something I've forgotten?

One message I get from all of this, and the reports of frame cracking I've read about from the folks at FiberglassRV is that it is really only the tongue that can be a strength problem - for everything else, it doesn't seem possible to build it weak enough for it to fail.

Andrew

[brian_bp]

...One message I get from all of this, and the reports of frame cracking I've read about from the folks at FiberglassRV is that it is really only the tongue that can be a strength problem - for everything else, it doesn't seem possible to build it weak enough for it to fail.

Andrew

The tongue is certainly by far the most popular failure point, but the worst frame designs under those moulded fiberglass eggs have shown cracks all over the place. I think the floor and body shell are holding some of them together; when the tongue fails, you know it!

[asianflava]

Don't forget about the Lil Guys that had cracking and/or breaking tongues.

[fatehunter1972]

This is great stuff. With lots of TT out there problem areas can be identified.