Musings on structural integrity

[MickinOz]

Today being Tuesday, it was grandbaby sitting day.
We look after the little man while the DIL goes to work to earn his baby food and nappies.
His dad is #1 son, who drives a BAB quad, which I do believe I have mentioned on these hallowed pages before.
Pedaling a 74 wheel 4 trailer tri drive semi rig takes a lot out of you. Even when our son is home on a Tuesday he is usually coming off a 14 hour night shift, so the baby comes to our house regardless.
And so it was today, but it was our son who came to collect him.
Got a chance to talk for a while, while I was slapping a couple coats of the mix on the underside of the floor of #2.
Seems he was down around Port Lincoln way the other day and saw a few teardrops. It's a pretty popular tourist destination, especially since covid has kiboshed interstate and international travel.

He mentioned that the commercially made ones were quite small but appeared well made, but found himself scratching his head at one DIY 'drop in particular that he saw.
He reckoned the trailer itself was tracking smooth and straight down the road, but the cabin/bodywork was flexing all over the place like an old cardboard box.

Got me to thinking. My first one is double skinned with 7mm structural plywood, and built so the lower bulkhead between the cabin and galley is a structural brace. Teh upper section of bulkhead is removable, but even so it is screwed top and bottom so as to brace the 'drop.
The galley bench is glued and screwed between the two side walls to ensure rigidity.
The flat section of the roof is a double skinned torsion box that must surely contribute a lot of strength.
The front wall is in two sections, both double skinned. Torsion boxes too, I reckon.
Even the cupboards in the cabin contribute some rigidity due to the way I've constructed them.
Whether it be X Y or Z axis, doesn't matter which way my drop wants to flex, there is a structural member resisting it.
#2 will incorporate the same sorts of design principles,

In essence, a teardrop is a big wooden box sitting on a trailer.
That box has a framed floor, usually, and the other 5 sides of the "cube" are usually built strong.

I do wonder what sort of awful workmanship/design resulted in a teardrop that wobbles like an old cardboard box.

[edgeau]

It would have to be single skin with no galley divider surely. Perhaps unfinished? I can't see why else.

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[QueticoBill]

Flat roof section creates a hinge. Even 1/2 or 3/4" of curve in a foot makes it much stiffer.

[working on it]

...the trailer itself was tracking smooth and straight down the road, but the cabin/bodywork was flexing all over the place like an old cardboard box...
I do wonder what sort of awful workmanship/design resulted in a teardrop that wobbles like an old cardboard box.

* Looks like someone tried to build too light a trailer.

* Over the years on this forum, I've noticed the main focus is to build as lightweight as possible. There are many fine examples how to construct a teardrop/whatever properly, by using sandwiched walls, roof spars, or even structural foam with a firm outer skin to add rigidity, or even to go old-school and use solid/heavy plywood walls. The key to any "proper" build is not going to extremes.

* Just prior to starting my build in 2011, I was looking at junkyards, abandoned toppers, derelict trailers along my travelled roads, and spotted a teardrop-shaped structure in some high weeds along a back road. The trailer was really rusty, but might've been salvageable, but the cabin structure is what got my attention. It had a more-or-less teardrop shape (with front curve unstuck), but the walls must've been 1/4"-3/8" plywood, with only square blocks nailed-in at the four corners (top & bottom) to attach the walls/roof. No roof spars nor evidence of more the a half-dozen screws into the floor, either, to secure the top & bottom together. I'm sure that the builder gave up, not really having a good plan.

* After seeing that poor example, and never having seen the inner structure of any teardrop before, in person, I decided to go to the other extreme and build mine solidly. Indestructible and heavy, too. I think that somewhere in-between might be a better choice for most builders, but then, I'm not them....

[tony.latham]

Got me to thinking. My first one is double skinned with 7mm structural plywood, and built so the lower bulkhead between the cabin and galley is a structural brace. The upper section of bulkhead is removable, but even so it is screwed top and bottom so as to brace the 'drop.

Most people (I think) recognize how much shear strength the bulkhead adds to the cabin but few (I think) understand the importance of the countertop adding rigidity to the walls in the galley.

[img][Imgur](https://i.imgur.com/pyec88A.png)[/img]



Somebody way-back-when had his sh*t together.

Tony

[popper]

Didn't think about that but it makes perfect sense. I bet you that flimsy one he saw could be fixed up in a jiffy. Even without adding a galley bulkhead, you could probably just do a couple of internal front and rear shelves to fix it.

[MickinOz]

Flat roof section creates a hinge. Even 1/2 or 3/4" of curve in a foot makes it much stiffer.

I agree. Mine being a stretched Generic Benroy, I have a straight section of roof, then a foot or so of curved roof down to the main spar the hatch attaches to.
That piece of curved roof seriously stiffened the whole structure.

[twisted lines]

But the open space

[working on it]

...
Most people (I think) recognize how much shear strength the bulkhead adds to the cabin but few (I think) understand the importance of the countertop adding rigidity to the walls in the galley....

...Even without adding a galley bulkhead, you could probably just do a couple of internal front and rear shelves to fix it.

* I believe in strength and durability, so I braced and reinforced the basic structure as much as I could, by firmly attaching front wall & rear bulkhead with bolted-on steel corner & angle brackets (and all interfaces glued between, inside and outside with PL adhesive) before affixing the front slope, roof, & walls (in the same manner). In the same vein, I used full-width shelves (two of 3/4" plywood: galley and front overhead; one of 1" oak planking: the rear folding shelf inside the cabin), sitting on 1/2" oak side rails, steel brackets, or on both. And glued, too. All plywood used was 3/4", so no flexing, bowing, or bending is likely.

so much bracing before roof placed on top.jpg (218.75 KiB) Viewed 622 times

[popper]

I believe in strength and durability

I believe in making things as light as possible.. but maybe I will be converted to your way of thinking later. After trying to build something, which I have not yet. Of my worry I'm 99% worried about moisture, 1% worried about strength, based on what I've seen kill campers.

What I meant by "adding a couple shelves" is two L-shaped full width overhead shelves to reinforce the deal in the original post. I bet you that would fix it. Not actually having seen it, pure conjecture. Or even the one twisted lines posted with the totally open floorplan there. I bet a couple of overhead full cabin width L-shape shelves would strengthen that up considerably. Probably wouldn't even need to be that long or tall.

[twisted lines]

That opened up one is commercially made in the under 30K post, and is reinforced with angle's near the top

[popper]

Hey it wouldn't hurt adding even more strength.. then adding more. Then more. And more.

[working on it]

That opened up one is commercially made in the under 30K post, and is reinforced with angle's near the top

* Yeah, I see that brace now. But why not make the trailer a bit longer, and still use a bulkhead wall separating "toys/gear" from sleeping space. As much as I loved my project cars, I never slept with them.

[MickinOz]

Got me to thinking. My first one is double skinned with 7mm structural plywood, and built so the lower bulkhead between the cabin and galley is a structural brace. The upper section of bulkhead is removable, but even so it is screwed top and bottom so as to brace the 'drop.

Most people (I think) recognize how much shear strength the bulkhead adds to the cabin but few (I think) understand the importance of the countertop adding rigidity to the walls in the galley.

Yes, my galley bench top runs tight through and forms the bottom of the cupboards in the cabin. So I've got a full width piece of 15mm ply wood that runs about 800 or 900mm fore and aft, glued and screwed to the side walls. I believe the strength and stiffness contribution to be very significant. There is no drumming if you thump the sidewalls in that area.
It was deliberate, after reading how the first trailer you had (not your work, a purchased item) annoyed you because of flex in that area.

[rjgimp]

His dad...drives a BAB quad, which I do believe I have mentioned on these hallowed pages before.
Pedaling a 74 wheel 4 trailer tri drive semi rig takes a lot out of you...

Mate, I drove semis for 25 years and I think I only understood about 72.1% of that.

America and Australia... two nations separated by a common language.