ALU skin on galvanized STEEL frame?

[RoadForce]

Hi,

I would like to discuss using aluminum skin on a galvanized steel frame. In my case, it is a pop-up truck camper that I'm building, but it could be anything.

I'd choose ALU because of the weight vs strength, but my two top-most concerns are:

1) corrosion, because of using two types of metal of different potential,

2) expansion / buckling, because of using two types of metal with different expansion rate per degree Celsius (or, if you prefer: Fahrenheit).


As for the first, I was thinking the zinc of the galvanized steel would largely mitigate this problem, and as I plan to use 3M VHB tape, the two metals would not actually touch. But as the tape is 1mm thick, I am wondering if this will be enough to prevent the issue.

As for the latter, I remember reading a post somewhere on this forum about the ALU skin buckling in sunny and hot conditions because - given the same temperature difference - the ALU expands more than the steel. However, I don't recall if this was only theoretical rambling or an actual problem in real life.

So, do you guys have any experience with skinning a galvanized steel frame with aluminum sheeting, and how do you feel about it?

Thans a lot!

 

[Haf-E]

check out rob grays build - he did aluminum on steel in Australia.

www.robgray.com

 

[dlh62c]

Considering that aluminum easily dents, have you considered FRP panels instead?

 

[Victorian]

Considering that aluminum easily dents, have you considered FRP panels instead?

I like your thinking

 

[haven]

You'll definitely need to separate the two metals. One approach is to use plastic washers to separates the frame and panels, connected by stainless steel fasteners.

 

[Iain_U1250]

My camper is aluminium skin on a steel frame. I used 2mm aluminium, which is pretty strong, and whilst if may dent, a field repair is very easy and it won't crack like FPR, which would very difficult to repair in the field, or even the workshop properly.


I had two main reasons for not going with FRP for the camper, first was I plan to use the two seats in the rear to allow extra passengers, and according to the Australian Design Rules, in order to do so, I need the rear to be "substantial" and was directed by an Engineer to the ADR for buses - steel frames at various centres as a minimum. Given I need a steel frame, then aluminium became the easiest option for me.

On my build thread I have a lot of photos of how I isolated the skin from the frame, but I glued the aluminium on using Sikaflex 252, and to ensure I had a 3mm layer of Sikaflex to get maximum strength, I used 3mm rubber tabs as spacers, then as the Sikaflex started to harden, pulled the rubber out and squeezed in more Sikaflex. You need to design the frame to match the aluminium sheeting size, and I always had and extra layer of aluminium over each horizontal joint where water could pool to minimise the risk of water leaks.




My camper box (bare) including the OEM very heavy duty four point mounting frame only weighted about 440kg. I only bought 100kg of aluminium and 100kg of steel to build the box, and had a a fair amount of off-cuts. My camper walls are almost 100mm thick, with multiple layers of insulation and is full lined with a composite panel. The weight of the insulation and lining is around 100kg as well. My floor is a Aluminium /aerogel/plywood/wood composite, about 25mm thick and is also very light weight.


My camper shape was also fairly complex by comparison to the majority of square box camper people build, with a lot of angles.






The other main reason for not going with FPR panels has been the long term experience of a few owners with FRP skinned vehicles, that complain of water leaks into the FRP caused by the joints cracking on the roof caused by hitting tree branches etc, which to me is inevitable. DIY FRP is difficult, it takes a lot of experience and proper equipment, or you end up with a long term problem with the curing of the resin, if it is not 100%, then de-lamination will occur some time in the future, and I was not confident I could ensure proper curing etc. I doubt I would have been able to build

 

[evilfij]

Aluminium does not corrode with zinc (ie hot dipped galvanized steel). I have series one Land Rover doors with the galvanized frames and there was zero corrosion in 65 years.

 

[fog cutter]

... if this was only theoretical rambling or an actual problem ...

here? surely you jest

 

[dlh62c]

The aluminum vs FRP debate can go on forever. Both have their advantages and disadvantages. With either one, internal framing needs to be stout enough to prevent racking should you hit something.


Here in the USA, there's a whole industry built around truck cargo bodies, I'm sure Canada has them to. They come in many shapes, sizes and styles. Morgan cargo bodies are very well built. They have their own engineering team and will supply and ship bodies without a rear door and will supply a single sheet FRP panel to fill in the opening. You can request a build with FRP panels in either 3/8" of 5/8" thicknesses.

Some people think the front leading corners and edges on these bodies are a weakness, but their replaceable, Morgan has a parts catalog for their bodies.

 

[david506th]

Of note,

Morgan uses "FRP = Fiberglass Reinforced Plywood" meaning a thin layer of fiberglass over a sheet of plywood on both sides.
Supreme uses both FRP and "Fiberpanel HC = A Fiber Tech Fiberglass Honeycomb Panel"

 

[fog cutter]

wow. none of those bridge girders flexed a bit.

 

[toymaster]

I currently own a horse trailer that is galvanized steel frame and aluminum siding. Also, will be purchasing a large cargo trailer first of next year that will most likely be the same. Using the 3M tape is the best way, well maybe not for repair sake, the secret is to use thick enough siding. The thin aluminum sheets will warp and will not be appealing in short order. If you search for the current methods being used in cargo trailers you will find galvanized steel frame and aluminum siding is very common.

http://hawktrailers.com/

http://www.aluminumtrailer.com/

 

[Iain_U1250]

The main problem with using 3M VHB tape or similar, is that you cannot re-align a panel - and working with a large sheet and getting it aligned properly first time is not easy, and almost impossible to do by yourself, you will need an assistance. Using Sikaflex, you can re-align the panel quite easily and can remove the panel completley, but it is messy. The 3mm Sikaflex will also help with thermal bridging as well.

 

[RoadForce]

First of all, thank you all for sharing your thoughts.

I have done some thinking here are its results :Wow1:

As for FRP, I don't want to use it because of the thinkness of the sheets. For my design, I need sheeting of no more than 2mm (maybe 3mm) because otherwise I won't have enough clearance for parts that slide or fold into/over other parts. (I have factored in 5mm clearance with max 2mm skin, to 7mm in total)

As for the 3M VHB tape being unforgiving when fitting/moving the sheets, I don't really have many large sheets to handle and can call in some help if needed. I am aware of the "partly peel back the backing at both ends"-technique, so I hope I will be allright.




Which brings me back to ALU, and the inherent issues of corrosion and buckling. How do you feel about these two things:

1) I plan to use 1mm think 3M VHB tape to (attach and) separate the ALU skin to/from the galvanized steel frame. Will this be enough or should I keep the two materials more than 1mm apart? If more, how much more?

2) I plan to use no mechanical fasteners, only the VHB tape to fix the sheets in place and then some flexible caulking to seal up the edges, leaving some room for the sheets to expand. I have made a basic (not very true to scale) drawing to illustrate this, below. Will this work, as in: will it be watertight, and will it allow for some expansion?

 

[Victorian]

I have had conversations about steel/aluminum vers. FRP composite since I worked at Unicat many moons ago. I think it comes down to the idividual in what they are compfy working with and what's available to them. You will never convince a metal worker to tackle something with wood and vice versa. The same applies with building Expedition Campers. I personaly like the look of a composite box better than a wavy steel box. It's also easier to repair... Anyhow I'm not trying to praise our products but just the thought of the amount of time it would take to cut,weld,skin insulate a box is totally turning me off. Again, please don't see this as a sales pitch but have a look at our assembly video and tell me if that method isn't fast and simple. https://www.youtube.com/watch?v=B-2R8zTY82o