Big foamie strength

Canvas covered foamies (Thrifty Alternatives...)

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Big foamie strength

Postby StandUpGuy » Mon Nov 21, 2011 12:57 pm

As we see the scope of foam builds increase in size I am left asking the question: Will these large foamies be strong enough to withstand the stresses of being pulled on the road?
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Postby starleen2 » Mon Nov 21, 2011 4:46 pm

I'm sure that they will do just fine - but I also wonder as the size increases and added rigidity to compensate for strength - will there be any advantage to building a large Foamie? Our Green lantern is very lightweight for it's size ( 6'4" wide X 5'3" h X 10'6"L ) and weighs 1,100 lbs fully loaded (and I'm sure i could have went lighter!). That's very light compared to smaller teardrops. With having to cover the foam with canvas - preferably on both sides to get maximum strength) or other materials and using 2 inch foam - I fear the weight savings is moot. Perhaps when Mike finishes his 10" foamie, we can collect some data on performance to answer the question.
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Postby StandUpGuy » Mon Nov 21, 2011 7:37 pm

Well what is the biggest foam trailer that has been built here and has a track record?
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Postby atahoekid » Tue Nov 22, 2011 2:01 am

I think Mike's Standie may be the tallest, mine may be the widest at 84". Neither trailers are done yet but we're both coming along nicely. I think what also has to be considered in the size vs. strength issue is aerodynamics. Personally I made sure I have a rounded front end so that the wind resistance is minimized as much as possible. I'm really pretty impressed with the strength of the form especially when you add a thin plywood skin for added rigidity. It will be interesting how our "larger" foamies perform, but I really think we haven't gotten close the point of diminishing returns. I think much bigger is possible without compromising weight and strength.
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Postby GPW » Tue Nov 22, 2011 7:38 am

Just keep thinking Unibody construction , where all the parts work together as one ... everything from inner bulkheads , cabinets ,closets, shelves , arches , all contributing to the overall strength... It’s the Design that makes it workable ... :thinking:

When in doubt , just make the foam thicker !!! ;)
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Postby StandUpGuy » Tue Nov 22, 2011 7:43 am

People here are in some cases way over building their trailers but I am wondering if the opposite is happening with the large foamies. I read of builders here attesting to standing on top of their wood built trailers but I am guessing large foamie builders would not want to do this.
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Postby starleen2 » Tue Nov 22, 2011 9:30 am

atahoekid wrote: I'm really pretty impressed with the strength of the form especially when you add a thin plywood skin for added rigidity. It will be interesting how our "larger" foamies perform, but I really think we haven't gotten close the point of diminishing returns. I think much bigger is possible without compromising weight and strength.

. . but see , this is the fallisey of the foamies - if you are adding to the foam then why not just encase the foam in plywood and forget about the canvas skin? I have looked at your build and it seems that you are already heavy with the frame (not that is bad - nice frame!) I think until a weight certificate is produced to prove otherwise - that large foamies will be an alternative way to build - but not necessarily the easiest, nor the lightest. as mentioned before, I have build very large and lightweight campers with traditional means (and the weight certificates to prove it) - so foamies just don't do it for me. Just my opinion and only that

:D :D
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Postby GPW » Tue Nov 22, 2011 12:29 pm

Thanks SS !!! Certain methods are good for some , not for others ... ;) Good we have so many choices !!! :thumbsup:
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Postby StandUpGuy » Wed Nov 23, 2011 11:25 am

If one builds a stress skin panel with 1/4" skins on both sides and 3/4" foam in the middle then you have a very strong panel. You can place the ends of a 4'x8' on blocks and stand on it in the middle. If you take that same 3/4" foam and canvas skin one side and 1/4" the other you will not want to place your toolbox on that panel, let alone stand on it.

The foam build is fine for a small trailer but at some point there is going to be inadequate strength as the build size increases.
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Postby GPW » Wed Nov 23, 2011 11:49 am

Quote "The foam build is fine for a small trailer but at some point there is going to be inadequate strength as the build size increases.â€
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Postby atahoekid » Wed Nov 23, 2011 11:51 am

Standup Guy...I'm not sure I necessarily agree with you. Your thinking applies well if you are talking about laying the panel flat and using it as a structural member, but if laid on edge, the panel does a good job of holding up weight. The only panel being laid flat is the roof and most of us have inserted spars to add support. I guess what you say about strength vs. size could apply to all building methods.

starleen2: I tend to overbuild everything. I know you were able to build lightweight with wood, considering the difference in weight per cubic foot, any wood structure I might build would be much heavier than any foam structure I would build. Even my frame was originally intended to build a standard teardrop that is much heavier than I really needed. AS GPW said different methods for different folks and the foam for me wasn't a weight issue but something new to play with, minimize wood rot and the foam allows for much more creativity than wood does.

To each his or her own I guess....
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Postby StandUpGuy » Wed Nov 23, 2011 1:00 pm

atahoekid wrote:Standup Guy...I'm not sure I necessarily agree with you. Your thinking applies well if you are talking about laying the panel flat and using it as a structural member, but if laid on edge, the panel does a good job of holding up weight. The only panel being laid flat is the roof and most of us have inserted spars to add support. I guess what you say about strength vs. size could apply to all building methods. ..


Exactly. I am using 1x3 flat framing with 1/4 ply in and out. That is very strong for my size walls. A larger build going on is Terry's TTT. He has beefier 2x3's it looks like. His bigger area walls require more substantial framing.
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Postby GPW » Wed Nov 23, 2011 9:13 pm

No Rules ... Build it like you think it should be built !!! :thumbsup:
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Postby GPW » Thu Nov 24, 2011 8:02 am

After more thinking on this subject :thinking: The use of judiciously placed Inside walls (or my favorite, arches) would add tremendous strength to an otherwise empty box... If you go BIGGER , you just have to design for it ... Much as in your home , not just one big empty room ... but inner “load bearingâ€
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RE: Large Foamie Strength

Postby mezmo » Sun Nov 27, 2011 2:41 am

Just some "ponderings and conjecture" on the question. So watch out any noses or toes
or knickers. 'Not intending to get anything out-of-joint, stepped on, or in a twist. This is
just discussion.

First off, I think the question is quite germain but not alarming. It is a serious consideration
for building any type of vehicle/enclosure. We need to consider all possible forces it has to
survive. More 'thinking like an engineer' in-so-many-words. This doesn't mean we amateurs can't
do this - build foamies of larger dimension - , just that we just need to think about what we're
doing when we do it - so we can eliminate or lessen the possibility of structural failures -
consider it getting good value for ourselves from our efforts.

I think the crux of the "problem/thing" thing is to consider just what is expected of the fabric
and glue - and to consider its origins. It is an old but practical techique, that to me seems to
have been used as a water-proofing measure or to provide a water-proof outer skin.
Our "sock-effect" phenomenon is a latter-day extension/discovery of/about it.

So, to me, the question we need to consider is: Is the fabric and glue 'matrix' really structural
or is it just a water-proof finish ? And, can it be both? I'd think it can be both - if only based
on our anecdotal/empirical evidence of the small trailers that have been built so far and used
successfully. BUT, I'd guess there is a diminishing strength type of phenomenon as the size
and spans of a unit grows.

"The strength is all in the skins" rule of thumb/statement needs to be determined for the fabrix/
glue matrix. How does it compare to the fiberglass & resin matrix in strength? I should think it'd
have to be similar to the fiberglass in that the more layers/plys used, the stronger that total 'skin'
is. [Of course this entails more glue a/o resin too.] And we also need to consider how rigid the
fiber and glue matrix is. Is it self supporting ? And is it really strong enough? I ask that because
fiberglass shells are when made correctly, and are usually @ 1/4 inch thick usually, from my observation.
Let me be clear that, that is a generalization, and that is for the shell only as the structure. That shell
strength can also be affected by factors such as the shape involved [thinking of boats and 'eggish style'
trailer bodies etc.] of course. We also need to consider how heat affects things. Will fabric and glue
hold up to heat as well as the usual fiberglass?

Now all that leads to the consideration that we are - as it has been pointed out many times - building
a composite structure - not a shell only. So we assume the 'skins/shells' held apart by the foam core
do not necessarily need to be as thick as single self supporting shells need to be - think of a single
thickness self-supporting shell as two shells with zero core/separation that thus 'appear' as a single shell.
Since we are trying to keep our two shells apart to take advantage of the strength benefit that gives,
I do think it's important to have what ever core that is separating the skins be kept attached to the skins,
especially as size/area increases. So far the tests done by forum members on the adhesion of various
glues and fabrics combinations show that they all vary in adhesion [no surprise] and usually end up
separating eventually at some time or stress level. The "sock-effect" is thought to contain any problems
with that as long as the skins/shells are still kept apart. But, we must be losing some strength and
possibly overall structural longevity when the skin separates from the core. Perhaps the solution would
be to attach the initial fabric layer to the core [foam] with the newly [to us] discovered 3M 30NF contact
cement [Or we should really find out what all the SIP manufacturers use to glue their foam core to their
shell layers.] and then coat the outside of that initial fabric layer with the TB II & III a/o resins and continue
on from there to as many layers as desired?

That being said, we also need to view the structure overall, and to consider ways to reinforce it. Sides and
roofs can be buttressed/strengthened by partitions and shelves and cabinets, as well as how they are
attatched to each other. Wall and roof shapes can also affect their strength. Trying to make it a unitized
structure/entity is an often used strategy for this and is really one of the unconscious ideas behind the
"sock-effect" too I think. Other approaches may be to use primeter framing of some sort around the foam
panels/sheets, or to build a space frame to attach our foam cored panels to. All sorts of 'hybrids' or kinds
of building techniques are possible I think. One thing I'm thinking about is doing a 'stitch-n-glue'
style [borrowed from plywood boat building] inner shell [i just love birchwoood interiors.] and applying the
foam core to that [I like the idea of good insulation too] and the fabric/glue or fiberglass/resin outer skin to
that. And we also need to keep in mind that curves impart strength. It's not as easy to build curves
compared to straight/flat but why not use all/any structural enhancements or methods for their beneits
if it fits our needs or design ideas?

Well it's getting late and my 'thinking cap' has fallen off so I'll trundle off now.

Cheers,
Norm/mezmo
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