Overlander Camper Frame Material: Aluminium vs Mild Steel vs Stainless Steel?

[HAF]

In the metal fabrication world, many materials are "formed". For example, a channel shape can be made by extruding aluminum through a die. However, there is a lot of excess materials in an extruded channel which makes it very heavy. While an extruded channel may be well suited for structural uses, its may not be very well suited in the construction of a mobile product that needs to be light in weight. The solution is found in forming. I used a formed channel with 2" toes and 12" height-hence a formed 2 x 12 channel.
Cross sectional area is what produces strength and rigidity. Sketch shows how the walls and floor members are tied together. The frame is extremely rigid. The vertical area above floor members provides a massive area for glueing composites or ?

 

[john61ct]

Beautiful thanks, perfect level for this noob!

So standard profiles
1. are extruded?

2. might be stronger? say holding up a building or for the primary chassis framing on truck/trailer bodies

But those C-beams are neither standard profiles?
nor extruded?

because that would be overkill, too heavy?

Exactly how were they formed then, if I wanted a local fabber to recreate them?

Plate bent on a press brake?

What alloy, thickness?

Would a 20' length be a challenge for a low-end shop?

 

[HAF]

Its difficult to find extrusions this large in cross section. A shop will use a press brake to form the toes. The longest piece of sheet that aluminum comes in is 12 foot.
Go to a fabrication shop-not a welding shop.
The beauty of using formed pieces is the freedom of making any shape. Given the material would be laser cut prior to forming, one can have any holes later cut in prior to forming. You can also laser cut on holes with given shapes to lighten the members.
I used 3/16" thick for formed channels and 2 x 6 x 1/8" thick for corners members. Any internal member is not actually structural-those can go to 1/16" wall thickness.

 

[john61ct]

So for a 20' structural beam need to use a stock extruded profile, costs more weight, but could go to say 6 or 8" and still plenty strong. . .

Or could these thinner formed channels be joined in a way that is strong like you see girders on bridges, with riveted "joining plates"? dunno the right term

 

[HAF]

Extruded 8"-look at the lighter ones, would also be very strong. I would not splice with plates and rivets.
If you are building something 20 foot long, it seems extrusions would be better.

 

[JaSAn]

Those countersunk screws will transfer cold to interior. They can act as a bridge. Metal has no insulation value and will transfer cold. . . .

You guys are giving way too much weight to thermal bridging. The aluminum frame accounts for > 5% of the interior perimeter of my FWC Grandby. I can overheat it with a 5000 BTU heater in 0ºF temperatures.
Condensation becomes a issue only when it would be a problem anyway, like when drying wet clothing.

 

[john61ct]

Depends on the use case. Sitting for weeks / months at a time in arctic conditions

giving up living space for 3-4" of insulation

then you want to minimize your use of fuel per day.

Since I want 600-800lb up top with a lifting roof the framing needs to be outside anyway

meaning the slide-in foamie box does not need to be that structural, avoiding bridging is easy.

 

[HAF]

Minnesota guy here-frost will show up on inside of camper on every bolt. Minus 20 degrees F for a few days in a row will test cabin thermal integrity. If one can eliminate a path for cold to flow, its a good idea.

 

[JaSAn]

And how often do you camp in -20ºF temperatures?

My point is that thermal bridging should not be a disqualifier for consideration on a camper frame. It is one factor in many that should be taken into consideration. It is not that hard to work around. Compromise and cost/benefit need to be taken into account.

I grew up in Duluth and still spend time camping in Northern Minnesota in winter. It's been a long time since I have camped in > -20º temperatures.

 

[Alloy]

Thanks for the reply. Are you referring to the corners of the box where I have diagonal braces on all three of the perpendicular planes (in the image in my initial post)? Maybe it’s overkill but my intention here is not just structural but also to stave off distortion when welding by first tacking the diagonal braces before doing full joint welds. Do you mean this should be done differently?

3 pcs on each corner are not nessary. Corner gussets would be a better and increase the strength of the connections.

To be structural the connections should be made as close as possible to the (nodes) corners. Connections made part way along increase the risk of bending/cracking.

It should all be tacked before welding. In many places it only needs 2 sided welds.



Any localized distortion can removed with block of wood and a 5lb hammer.

Do you mind explaining what the importance behind that .

Aluminum welds are only 2/3 as strong as the parent metal. The shorter cross members will stay flatter and be stronger if you don't cut/ weld them.

I thought TIG causes less distortion than MIG due to a smaller heat-affectected zone.

Again, I’ve never used TIG before so I have no idea myself.

Reducing the heat affected zone reduces distortion. With the right settings MIG will travel 20X faster than TIG which reduces the heat affected zone.

To reduce distortion spending time to ensure a perfect fit is more important than the welding.

 

[Alloy]

@Alloy "intercostal" ?

I find a category of chest muscles attached to ribs

and of course intercoastal.

Could you link to an explanation or diagrams etc for your meaning in this context?

https://image.slidesharecdn.com/shipconstruction-shipdimensions-140823130214-phpapp02/95/ship-construction-ship-dimensions-13-638.jpg?cb=1418687592

 

[john61ct]

Amazingly rarely used outside of the medical "rib muscle" context.

99% of google's entries are just verbatim references to that 1918 McBride NA glossary entry

> Made in separate parts , between frames , beams , etc. , the opposite of continuous . ( Floors are continuous ; longitudinals , intercostal in a merchant ship . )

I love it!

 

[Alloy]

So for a 20' structural beam need to use a stock extruded profile, costs more weight, but could go to say 6 or 8" and still plenty strong. . .

Or could these thinner formed channels be joined in a way that is strong like you see girders on bridges, with riveted "joining plates"? dunno the right term

On a regular basis I've used 20'-25' aluminum sheet/plate and had it formed.

For the effort I'd stick with hollow extrusion. For something like 2"x12" two pcs of 2"x6" can be stitched together.

 

[john61ct]

It's been a long time since I have camped in > -20º temperatures.

I guess you mean < -20º as in "lower than" ?

Living all winter in ski area parking lots is not that rare a use case. . .

And I did not think we were talking about buying anything pre-built here, everything's from scratch right?

The principle is also sound running aircon in the noon Arizona heat. . .

You can add another 1/2 skin of insulation over those parts bridging right through your 3-4" layer, but it would be better to avoid them in the first place.

And of course the relative "importance" of the issue indeed depends on owner preference as well as milder use cases.

 

[john61ct]

On a regular basis I've used 20'-25' aluminum sheet/plate and had it formed.

For the effort I'd stick with hollow extrusion. For something like 2"x12" two pcs of 2"x6" can be stitched together.

Being main load-bearing beams, involved with say a cabover (what's the right term?) then extrusion's better right?

But I am curious as to "splicing", any link to help me visualize how?