Heavy load-bearing foamie

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Heavy load-bearing foamie

Postby John61CT » Mon Sep 18, 2017 10:56 am

I know bit of a contradiction 8-)

Looking for a design that will support 500+ lbs on a roof rack 8' x 7'.

Anyone familiar with the engineering calculations required to minimise the tare weight of the load-bearing elements?

Want to compare steel tubing vs aluminium, but probably can't afford the latter. . .
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Re: Heavy load-bearing foamie

Postby John61CT » Mon Sep 18, 2017 10:57 am

Rest of the build will be foamie to minimize weight.

Looking to end up at 3500lbs fully loaded. Would I need to go two-axle for that? Given that overall weight limit, and assuming lightest possible inside build, what approximate living space length do you reckon would be maximum?
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Re: Heavy load-bearing foamie

Postby John61CT » Mon Sep 18, 2017 10:57 am

Suggestions for a leak-proof roof design given that roof rack would also be most welcome.
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Re: Heavy load-bearing foamie

Postby GPW » Mon Sep 18, 2017 12:05 pm

John , I’m afraid you may have entered the wrong room here, we’re just the “Foamie guys ( and gals ) ' , The “ Hard Tent “ people … You ask a lot :frightened: … We get away with a lot because we build ultralight structures , with no roof racks , other than a solar panel . While not an impossibility , you need an Engineer … a “real” Engineer ... really !!!! Safety sake …
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Re: Heavy load-bearing foamie

Postby dancam » Tue Sep 19, 2017 9:53 am

My guess-
2inch thick foam.
2inch square steel tubing.
One piece widthwise on the roof at the front of the roof rack. One piece at the rear. Same 2x2 steel going down the walls to the floor from each onto solid points on the frame. Preferably welded. Aross the top where the roof meets the walls going front to back use 2x2 (actual size) wood.
It meets verticle 2x2 steel tubing at all 4 corners which are welded to the frame.
Front to back in the middle of the roof 2x2 metal welded to the widthwise pieces with verticle supports at each end IF your roof rack does not put the load near the walls.
If the rack puts the load near the walls you should be good then. If not make verticle beams inside the trailer that lock into place and support the widthwise metal the rack is bolted to for when travelling. Remove when stopped.

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Re: Heavy load-bearing foamie

Postby QueticoBill » Tue Sep 19, 2017 12:21 pm

I think skinning walls with ply - maybe 1/8 but more likely 1/4 - would be strong enough. Maybe block in walls and support roof loads off of "arms" from walls would be easiest. I suspect the ply skins would be less expensive, lighter, and easier than an internal frame just for roof loads but would have to draw both.
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Re: Heavy load-bearing foamie

Postby dancam » Tue Sep 19, 2017 12:28 pm

Thats true. I dont think 1/8th ply would be enough for 500 pounds on a 7ft span unless again the rack puts the load right to the outside.
1/4 would be good though with some wood framing. Hitting bumps or braking hard at/from high speed puts significantly more than 500 pounds of force on the roof, but plywood skins are better than my idea.
John- what does this rack look like? Most ones for cars say max 150 pounds so i presume this is something special.


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Re: Heavy load-bearing foamie

Postby Dan242 » Tue Sep 19, 2017 12:49 pm

My gut says you would need a steel frame to support that size and weight, roof rack ties to steel frame, foam would not be structural. Aluminum would be expensive and require larger beams/ studs. I would not want to put any kind of load directly on foam that can move up and down. As big and heavy as your trailer will be, you need to be 100% sure, with that much weight at 60 plus and bouncing, You have a lot of stress on that sucker . I'm not an engineer but been building stuff for 50 years.. ;)

I would fill in the gaps between structural members with foam then skin it with your material of choice, ply, canvas etc
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Re: Heavy load-bearing foamie

Postby linuxmanxxx » Fri Sep 22, 2017 2:59 pm

Build all foam and have the rack on outside welded to trailer frame. Be much easier to design and build and lighter as well.

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Re: Heavy load-bearing foamie

Postby John61CT » Fri Sep 22, 2017 6:43 pm

The design of the rack would be integrated to the design, not removed.

I'm thinking that last idea, no load bearing on foam at all.

The surrounding posts and beams could provide extra support for the foam shell, but from the outside, without being integrated.

I was hoping for reducing weight 8-(
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Re: Heavy load-bearing foamie

Postby KennethW » Fri Sep 22, 2017 7:11 pm

I would not want to pull a trailer that top heavy. :frightened: Any fast lane change could end up very bad. :shock:
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Re: Heavy load-bearing foamie

Postby GPW » Sat Sep 23, 2017 6:11 am

Ken’s right … we’ve discussed ( and seen the results of ) top heavy trailers in the past … :frightened:
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Re: Heavy load-bearing foamie

Postby John61CT » Sat Sep 23, 2017 8:34 am

GPW wrote:Ken’s right … we’ve discussed ( and seen the results of ) top heavy trailers in the past … :frightened:
You are assuming the center of gravity will be high?

Point taken, but the 500lb load bearing capacity is a conservative maximum, likely will be less.

There just isn't any other place to put solar panels.

And finally there are at least 1000 pounds of just batteries, propane and water/waste tanks, **under** the floor.

Not to mention the wheels suspension, framing etc. Plus inside "furniture" all sitting **on** the floor.

These are the very factors motivating me to "go foamie" on the shell and cabinets, trying to keep total weight down as much as possible, the rest weighs what it weighs.

I admit I may need to revise my overall weight goal upward, but I don't think the CoG will be any higher than the average cargo box conversion with panels up top.
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Re: Heavy load-bearing foamie

Postby ghcoe » Sat Sep 23, 2017 9:26 am

So I have thought about a roof rack on my #2 trailer. I have though about all that has already been mentioned and agree that some are impractical or just add too much weight.

In my thinking the foam has plenty of load bearing potential at a compressive strength of 25psi per sq. inch. The problem is how to attach to the foam and spread the weight across the load bearing walls?

So my solution? Well some people want roof spars anyway so why not incorporate them into the top portion of the roof. Imbed the spars so that they are flush with the top of the roof foam. Then you can glue and screw a piece of OSB/Plywood to the roof. In my build I envisioned incorporating the OSB/Spars into the roof canvasing process. This should give good attachment points in itself or also allow a point to attach roof rack side panels.

Just food for though.... :thinking:

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Re: Heavy load-bearing foamie

Postby John61CT » Sat Sep 23, 2017 11:37 am

ghcoe wrote:So I have thought about a roof rack on my #2 trailer. I have though about all that has already been mentioned and agree that some are impractical or just add too much weight.

In my thinking the foam has plenty of load bearing potential at a compressive strength of 25psi per sq. inch. The problem is how to attach to the foam and spread the weight across the load bearing walls?

So my solution? Well some people want roof spars anyway so why not incorporate them into the top portion of the roof. Imbed the spars so that they are flush with the top of the roof foam. Then you can glue and screw a piece of OSB/Plywood to the roof. In my build I envisioned incorporating the OSB/Spars into the roof canvasing process. This should give good attachment points in itself or also allow a point to attach roof rack side panels.

Just food for though.... :thinking:

drawing (2).png
I appreciate the constructive thinking! It is true that thick sidewalls have a lot of compression strength on edge.

But I think a heavy chunk of ply is overkill just as basis for an attachment point.

I know many racks are clamped out at the rain gutter, but on SUVs and minivans the rails are integral to the roof design, and in fact I've mounted a few to bare sheet metal with plusnuts very securely.

Where I'm having more doubt is that we're not talking about static loading here, and there will be *very* large lateral and also lifting forces at play.

I think having metal posts or some sort of "cage", well tied into the trailer frame, is required for stability in that context.

Perhaps simply four angle posts on the outside corners would be enough, strong support for the vertical dimension, and tightly tied into the foam walls and roof for bracing.

The addition of a flat piece spreading the load across the top of the sides would be fine as a mount point, and the rack itself is designed to not even touch the horizontal top surface.
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