Questions about deck overhang

[Zzyzx]

So I am hoping to get input from other foamie trailer designers/builders about deck overhang. My foamie is intended be very lightweight. It will never be used to haul a ton of gravel. The aluminum trailer kit I have in mind is 5 feet wide. Currently I am planning to use 3/4" plywood for the deck, mounted transversely. I plan to screw/glue 2x2s, inset 2", around the edge of the entire deck to provide a nice base corner for gluing the 2" thick foam sheets to the deck. How far can a deck overhang the frame, safely? Zero, 3", 6", a foot? If you could, please let me know if you base your estimate on gut feel, experience, and/or engineering analysis. Thanks.

[GPW]

Z3, from experience, the Foam Cabins don’t usually weigh much ( unless you make them mostly from wood ) so there’s little stress on the floor … However you can go too far … How far , we don’t know ? …. so we added 2X4s crosswise on top our frame to support the wider than the trailer floor and walls ( see pic ) … It’s been the tradition around here , when in doubt to OVERBUILD , which always seems to work , and the only negative aspect is towing More WEIGHT … and Foamies are usually lighter than other trailers so a little extra bracing likely won’t be noticed …
Without a real Engineer hanging around , and not having all the details , it’s difficult so make an exact assessment … So we’d personally recommend using good judgement , on the side of safety and you should be Fine …There’s many Foamies been built here now over the years and none have failed structurally , that we know of anyway …

EDIT: Measured , ours is a foot and a half inch on each side … Now with our new “Board Floor” and another added support cross member, it’s Very Strong …

[John61CT]

If the plywood is very strong good quality, and so well sealed does not get weaker with age, then no worries.

But personally I'd have the wall sitting over cross-members.

[GPW]

What we did … ( before trimming down to 79” on a 54” wide trailer frame ) … Pressure Treated 2X4 s

[greygoos]

I have used steel strut for support which is available at the big box stores. It is predrilled and easily attaches to the frame as well as the cabin. As a matter of fact I will be using strut to build the frame of my next trailer.

[Zzyzx]

Thanks everyone for the opinions and ideas. One of my main concerns is that I want to make the trailer deck as stiff as possible. My guess is that structural issues with the foamie itself are likely to arise primarily from twisting and flexing of the trailer frame. I once rode in the back of a pickup truck with a shell on it and I was amazed how much the truck bed/walls flexed over rough dirt roads. I believe that the deck serves to make the frame much more rigid. I do plan on driving rough dirt roads, but not hardcore offroad. I wonder if not attaching the deck directly to the frame but instead using transverse supports between the deck and frame, (as suggested by GPW's photo) would have an impact on rigidity. I have also toyed with the idea of making a plywood-foam-plywood sandwich for the deck. If I used 1/4" plywood for the top and bottom "skins" and foam in between, perhaps I could get the stiffness and strength I want. On the other hand, I feel like I may be over thinking this for hypothetical situations.

[pchast]

Perhaps if you use the deck detail like this.... look closely. I could not find a better view
tonight...

The bottom ply is a foam thickness wider than the foam and top ply. The sides are glued
to the bottom ply and foam and top ply of the base.

I did not also pin it together. That might improve rigidity?

[John61CT]

IMO, IANAE

the strength required of a flat deck providing rigidity to the overall frame would make it too heavy.

Just build out your chassis to be very rigid, then your sleep pod can be very light, just strong enough to handle the wind forces, side loading.

[Xanthoman]

It’s a relatively simple problem if you want to do the mathematics for it. (I am a graduated mechanical engineer but still considered an engineer in training...so take it for what it’s worth... haha). Find the approximate weight of the sidewalls and roof. (Simply calculate the weight of all panels and add them up and multiply by 1.1 to fudge high). Then calculate the area that they will bear on. (The width of the wall panels multiplied by the perimeter of bearing.) Then determine how much deflection you are comfortable with. If you want 1/16” sort of rigidity or a more playful 1/2”...Then go to engineerstoolbox.com or similar and search for a ‘cantilever beam deflection equation’. Find the E (modulus of elasticity) of the plywood (just find E of Douglas fir) as the material E in the equation. For the geometric inertia (I) just use I=(3/4^3)/12. For P (load applied) use the weight of your top that you calculated divided by the perimeter area you calculated. I think that should fill in the equation and solve for deflection. You can then choose different lengths of cantilevering out and see the deflection. (It’s nice do this set up in an electronic spreadsheet so you can run a bunch of different options all at once). Once you turn to composite sandwiches or extending purlins underneath the calculation gets a bit more involved, but the concept is the same. If you really want to run the numbers let me know and I can probably put something together for you later tonight or tomorrow if you can’t make this work. Matweb.com is a great site for material properties like E. Good luck!

[ghcoe]

One thing to keep in mind is that a pickup, as mentioned above, has 4 points of contact on the ground. A trailer only has 3 points of contact on the ground. Kida like if you have a 4 footed stool and a 3 footed stool. Which would work better on rough ground? A foam box is quite ridged and is similar to a unibody. With only 3 points of contact it is had for the trailer flex to overcome the unibody strength. Just my observation. You can watch my trailer in action here... https://youtu.be/PZSYVVIN5DM?t=350 .

Also for overhang, remember that the bulkheads will give great support to the floor on the overhang.

[Zzyzx]

Xanthoman thanks for the response and info. I am afraid that calculating the static deflection might not be of much use. I expect the dynamic forces experienced while bouncing down a washboarded road will be the most significant. Also, I have no idea how polyurethane soaked plywood fatigues over time.

Ghcoe, thanks for the video link. Your foamie certainly seems to be rock solid as it was towed down that rough road.

[GPW]

George’s video was really Wonderful allowing us to see how low mass trailers react … You can see it went up , then came back down with very little rebound… With less mass ( lighter) you have less Inertia , so the whole trailer can move and change direction quickly …
Now we need to slow the video down so we can see if the trailer and cabin are Flexing … Foam can flex and recover much better than wood … nice if we could observe it …

[KCStudly]

The floor is a diaphragm, not a beam. The walls act as beams in their height and length along the edge of the floor, but are diaphragms regarding sideways forces (perpendicular to the panels). The difference is that a diaphragm is supported all around its perimeter, and a beam is supported at points. So if you try to bend your cabin by lifting up on the front wall, say, the side walls act as beams that are attached to the trailer with girders (front wall and blkhds). The floor is a diaphragm that you are trying to fold up, but the walls stiffen the floor along its edges. It doesn't really matter what the I (moment of inertia) is of a composite foam wall, any beam of reasonable stiffness with a 4 ft section depth is going to resist bending pretty well (so long as it is rigidly supported along floor and roof to prevent buckling... need good joinery).

Don't think of the floor as stiffening the box; think of the box as stiffening the floor. Bulkheads/front walls/galleys with low knee walls at the back all act as athwart ship beams that help hold the walls up and rigid, and thus the edges of cantilevered floors are well supported. In a TD you aren't going to be walking on the floor right next to the wall anyway.

We have seen many successful builds on this forum with simple plywood floors that extend up to 12 inches outboard of/in front of/behind their frames. Even in standys.

A good unitized cabin design, and good solid glued joinery are key factors to success without having to overbuild.

That being said, I built up my floor (64 w x 9'-8" long) using a perimeter frame of 2x2 nominal cedar, 1-1/2 thk blue foam, and 5mm Okoume skins top and bottom fully glued (and the skins weren't even full sheets; they were smaller pieces butted together over 1x cedar stringers and xmbrs... trying to increase yield in the overall scheme instead of having a bunch of wasted off cuts from other parts of my build). Weight was comparable to 3/4 cabinet grade ply, but the rigidity was much greater. I could lift (with some effort) one corner with only the far diagonal corner touching and the sag was minimal, perhaps 1/8 inch across the 11 ft diagonal. Very stiff compared to straight plywood. I used this method more for the sound and thermal insulating properties; the rigidity was a bonus.