Flex vs. stiffness?

[MatBirch]

Quick question.
I’m getting closer to settling in on a plan, but as it will a larger standy size, I’m curious about the flexibility of a foam box. I realize that the frame stiffness plays a huge roll in this. While I have no current plans for “off-roading”, many of our favorite spots are long miles on dirt roads. I don’t camp in “campgrounds”. Sometimes the roads in and out are in better condition than others. I have seen travel trailers “rack” and break apart from traveling distances on dirt roads. My brother-in-law’s steel horse trailer and the bed of his truck both cracked and came apart from extended time on the roads in to the trailheads. I wonder about the pros and cons of foam with this regard? Seems like the ultra light weight might allow the trailer frame to roll over irregularities in the road with the less torquing of the frame. Therefor less racking of the box?? Or... does the softer structure of the canvas allow the box to rack MORE, but without coming apart? Allowing it to “follow” the contours of the road?
My current plan is a sort of modified Canned Ham with a rear hatch galley. That would give me a good bulkhead near the rear, but the front will be a pretty open floor plan, just a dinette. Trailer frame will be 12’x 7’, box with the rounded profiles add about another 18”. Overall height about 78”.

Any thoughts or experiences?

[John61CT]

Floor frame, lateral / longitudinal load bearing dimensions

as stiff as possible within light weight goals.

The "living pod" IMO emphasis is on light weight, flexing is a Good Thing within reason, I like remove / disassembly for checking / maintenance.

If heavy loads like solar panels up top then 3D stiff heavier "cube frame" is needed, foam is then basically just infill / enclosure.

[KennethW]

I used a pickup fabric bed cover for my gallery hatch. It allows the body to flex. Plus no need for a heavyweight hinge mount. I also did not use any wood so the foam body can flex as far as the canvas will let it.

Sent from my Moto G (4) using Tapatalk

[GPW]

If it doesn’t flex , it will break !!! So you really have to build it for Off Road , only if you do it occasionally …. You’re Still going Off Road !!!

Like everybody else says .. “ Keep it Light” and overbuild for more durability … one simple way is to make the Foam THICKER , and use more reinforcing strips ( additional canvas strips on high stress areas ) in addition to the main skin ..
The foam is still the lightest component in the structure , and the lower weight lessens the Inertia that will tear a heavier rigid trailer apart … ( we’ve seen those off road wooden trailer videos … as soon as the door lock fails , the door swings open and tears the sides of the trailer off .. the rest collapses .. )
Added internal structure can help as long as they flex with the whole cabin in unison … ( see: Foam Arches )

Enter the Engineers !!!

Ps. and Trust Ken W … he’s been Everywhere in his !!!

[GPW]

… and more … A while back there was a British show on the TV about cars , and in one episode they took three different camper trailers Off Road in a "destruction test” , and it was amazing to see those trailers fly apart over rough roads , the all wood one came right apart , the others did only marginally better … Pretty scary !!! Especially in slow motion , as the trailers flexed wildly … It may have been a extreme test , but it showed what they wanted … The only thing I could think of is to SLOW Down on bad roads , but then the trailer should easily survive the occasional Bump , right ? …

Weight seems to be the real enemy , the more weight , the more weight that just wants to keep going in another direction from the trailer frame after a bump … and now maybe a reason not carry any extra weight in the trailer while towing it offroad …

Having tested foam wings by flying them directly ( but not intentionally) into the ground and trees for many years , we found the Foam is amazingly Durable , bends and flexes but rarely breaks .. The surface is soft and easily superficially damaged , but the structural aspects are predictable . The connections (floor to walls , walls to roof ,) are the important considerations for Flexibility … where a flexible connector ( like painted canvas ) can be used to reinforce all these areas …

[John61CT]

Well the military trailers buy a LOT of strength with their extra weight. I'm guessing survive the roughest road at get-out- of-Dodge-speeds.

Plenty build little camp trailers on top of those, all welded steel, RTT on top.

[GPW]

J ‘ … True , military eqpt. is ( or was ) made to survive most anything !!! Maybe why it costs so much and takes a Hummer to tow it … Dude, I just have a foam box and a Chevy … and no military budget ( retired on SS ) …. The worst conditions we've subjected the FS to was bouncing it over a high curb in a turn , it didn’t even bounce , just went up , and down … and it actually survived several trips up and down my long Dirt road , an offroad course in itself , as Ken W. can readily testify …

We all build what we need … and as in aircraft design , you build it , and if it breaks in use then you fix where it broke ( the weakest spot) and go again … I can’t image a trailer made of reinforced foam surviving a hostile roadway any better than another homebuilt camper . But being lighter and more flexible , you’d think it had a better chance … schmaybe ??? Kinda’ makes me think a Foamie trailer failure woudn’t be anything more than a “blowout" in a NIKE …. Remember Catherine’s too tall trailer got blown over in a Strong cross wind , bent the trailer frame and coupler , but the Foamie cabin did fine , just a little scrape on the wood floor at a corner ( likely from being dragged down the highway … )

Maybe we should take a lesson from Boatbuilders , whose craft are subject to enormous beating by water ??? … yet survive … ( sometimes)

[John61CT]

Yes, just speaking to

>> Weight seems to be the real enemy

in fact it's a balancing act.


Heavy

Durable

Cheap

pick two

[MatBirch]

… and more … A while back there was a British show on the TV about cars , and in one episode they took three different camper trailers Off Road in a "destruction test” , and it was amazing to see those trailers fly apart over rough roads , the all wood one came right apart , the others did only marginally better … Pretty scary !!! Especially in slow motion , as the trailers flexed wildly … It may have been a extreme test , but it showed what they wanted …

…

Top Gear! On a recent episode of their new show, Grand Tour, the bought old RVs. On a quick jaunt down a dirt road to the lake, one of them racked so badly the windshields popped out.

[tony.latham]

...they took three different camper trailers Off Road in a "destruction test” , and it was amazing to see those trailers fly apart over rough roads , the all wood one came right apart...

Can we all agree that 90% of factory-made camp trailers are good for the two-year warranty? (And thus junk and shouldn't be compared with the fine builds this forum is filled with?)

A well-built teardrop is a torsion box and using traditional methods is stiff as a bone. Barring water intrusion, they should last a generation if not longer. A poorly designed teardrop with a door opening that goes to the floor isn't a torsion box and will cause flexing and that's a problem with joinery.

I know this is about standies... but I'm using a teardrop as an example.



My tires flex and that's a good thing. But they're rubber (and don't have joints). The point being, it depends on the material and joinery, but I don't think you'd want a foam joint flexing.

Perhaps there's someone out there that's towed a foamie for hundreds of miles on washboard?

Just my thoughts.

Tony

[MtnDon]

Some things like the tires Tony mentions, as well as springs of all types are designed to flex. We have a couple of archery bows that are another perfect example of something that is designed to flex. Nothing will flex forever, some just longer than others. Other things like frames are not supposed to flex. If a metal frame flexes it will eventually fail.

A torsion box is wonderfully strong. Torsion boxes make light and strong structures. Hollow core doors are probably in most of the homes we live in. Amazingly rigid things and lighter than a solid wood door. The thin skinned walls teardrops use are another torsion box example.

Commercially made RV's are crap waiting to fall apart sometimes after the warranty runs out. They are not made for serious off road use. Totally unfair to even bring them up in a conversation about durability. If you are traveling over rough roads the suspension must be built to absorb the forces before they are transmitted to the frame and rest of the structure. Some of those impacts are absorbed by the tires; that is one reason why serious off roaders reduce tire pressures when we traverse rough roads and rocky terrain.

[GPW]

“Top Gear” …. that was it , Thanks Mat !!!

[Tomterrific]

My most destructive test was I70 last year to Oklahoma. 70mph and suddenly the roughest road you can imagine. My camper might just be the lightest of them all and came through okay. The trip did rip off two of the 4 little hatch latches.

An off road motorcycle is extremely light yet takes incredible abuse.

My point is, weight does not make strong, engineering does. A couple of facts.

The strongest shape is a triangle. Think of a homemade wood book shelf. The shelf will rack, move side to side, until the corners are triangulated by a board going from upper corner to opposite lower corner, a back is placed doing the same, or wires in an X do the triangulation.

Wood is very strong in compression, steel is very strong in tension. A square made of light wood members becomes quite strong when Xed with steel wire.

Tt

[GPW]

Just thinking about this as it would apply to an Off Road Foamie trailer …. I think you’d want to build it as Sturdy as you could , beefing up the foam sizes (still light) using more reinforcing strips of PMF along the stressed edges , and even using a trick Louella ( Eaglesdare) came up with originally … STRAPPING over the whole thing , just for “insurance” … Makes sense, keep it light , but use ALL the building tricks to make it Strong , yet flexible enough to bounce around … And also saying the trailer frame must be designed for this kind of off road abuse abuse with lots of “Throw” .. Adequate suspension, shocks and off road tires ( having also rode a little motocross in our Youth too ) … And then maybe NOT driving off road at 70 mph …

As an analogy , take a foam beer cooler ( a cheap one ) , and toss it out the back of the old pickup at 70 mph …. being light and strong enough , it will just lightly bounce and roll as far as the resistance to air and concrete will let it , but it will usually stay intact … unless it gets run over by the vehicle following you , then it will be reduced to many smaller particles … leaving little bits exploded all over the highway … which naturally gives some people who don’t know the idea that a Foamie trailer must have exploded due to driving faster than 30 mph …

[KCStudly]

I suspect that those TV shows "sabotage" their trailers before they torture them, just to be sure they do "explode"... so that they are sure to get good video. A couple of minutes with a recip saw and they wouldn't have to take much video to get to the good shots. Or maybe not.

What Tony said^, use radius corners in your doors and window openings and don't let them run too close to the floor or roof.

Also, I would think that a snug fitting galley hatch with draw latches would be better to stiffen the galley walls, than some other styles (like the T-handle ones with cross bolts).

Remember that over building the trailer frame and floor isn't really a good solution (especially on a single axle trailer). The frame and floor are only ever going to be a couple of inches tall, and therefore will still be relatively "wafer like", able to twist, bend and flex. Just like the example of putting a back panel on a book shelf for triangulation, it is the walls of the cabin that stiffen the trailer frame, not the other way around. And the roof supports the walls laterally so the walls stay rigid and don't buckle. With a 4+ foot tall "gusset" of a wall on each side that wafer like trailer frame gets really stiff. So good joinery between the cabin and trailer is a must.

Angib (who we have sadly not heard from in years... ) has some good explanations for why single axle trailers survive well; unlike a four wheeled vehicle, a single axle trailer only has 3 points that react (both hubs and the hitch), so if the TV or one wheel gets jostled, the other two points act as pivots, even if two points jostle, the third still pivots (where as with a four wheeled vehicle or dual axle trailer when one wheel meets an obstacle more of the suspension loading has to be transmitted through the frame).

I also plan on doing many miles of forest service road, so I will be running A/T tires and have built my suspension very stoutly, with long leaf springs, heavy spring plates and U-bolts, shocks, bump stops, and oversize axle/bearings, but I didn't really over build the frame (2x2x1/8 SQ tube mostly, 3/16 thk at tongue and front Xmbr, with relatively light L-shape intermediate xmbr's).