Camper Frame Material - Aluminium or Steel?

[93Canter]

I plan to build a lighter more streamlined version of my Canter Camper. But I am stuck on the question of whether to build the frame out of Aluminium or Steel.

I am keen to learn to TIG weld so it would be a good project to start off with. But if the weight saving is not significant it might not be worth going to the expense.

Any thoughts anyone?

 

[engineer]

Ours are Ally frames with ally sheet stiched to the frame on both sides. Stitches are 50mm with 75mm spacings, Bloody stong and durable, had one fall on it's side and haven't done anything to it.
You wont save much on weight, but provided you do it right, they will be stronger than most other bodies around. We've also had no issues with corrosion (internal or external) on ours. Most of the Tafe colleges will teach you enough to get a strong weld, but cosmetics are an artform.
Steel will be alot easier, but we steered toward aluminium because
1. less preventative maintenence required. (as long as there is no path for elecrolitic reacives.)
2. design was 3 times stronger than steel.
3. upgraded suspension reduced fatigue on body.
So to summarise, i feel that unless you soften the suspension considerably, do your homework and find a suitable design for your requirements, learn to do quality welding (which i think is fun anyway) then you are probably better off doing a steel frame out of Duragal.
Have fun with it all the same,
Engineer

 

[93Canter]

Thanks for the feedback Engineer. Weight is the critical factor, so if I am not saving significant weight I will stick with steel.

 

[richardq]

Although I stand to be corrected I think you will find a large weight advantage of aluminum over steel. The critical factor here is whether the structure will need to flex or not. If it does need to flex then 3x more aluminum is required and since it is a third the weight of steel it has no weight advantage BUT if the structure does not need to flex then you should achieve al least a saving of at least 35-45% in weight to get the same or comparable strength from steel. This is a massive saving in my opinion.

The marine industry make this comparison all the time Quote -
from this website -http://www.kastenmarine.com/alumVSsteel.htm

"Built to the Same Standard
An aluminum hull structure, built to the same standards, weighs roughly 35% to 45% less than the same hull in steel. As a result, if high strength is of the highest priority, the alloy boat can be built to the same structural weight as the steel vessel, and then be considerably stronger.

This is less of an issue for larger vessels which are able to carry the necessary displacement for whatever materials choice is made. For smaller vessels however, the weight of the hull structure is very much an issue. For a small cruising vessel, say under around 35 feet or so, steel becomes less optimum, as one must resort to a large water plane and a large displacement to carry the weight of the structure.

When alloy is designed to the same standards as steel (ABS, Lloyds or other similar classification society), it is made to be higher in overall strength. The reason for this is that aluminum reaches its "endurance limit" sooner than steel in terms of flexure. Therefore the rigidity of structure (deflection) becomes the limiting design criteria for an aluminum structure, and this forces a higher than necessary overall yield and tensile strength.

With steel, one designs to the yield point of the material instead, since for steel, flexure and rigidity are not ordinarily a limiting issue. For steel therefore, the yield point of an "equivalent" structure will be considerably less, as we will see.

One advantage of steel is that between the yield point of mild steel (around 36,000 psi) and the ultimate tensile failure point (around 60,000 psi) there is quite a large plastic range (around 24,000 psi or roughly 40% of the ultimate strength), permitting a steel vessel to endure deflection without failure, so permitting considerable ability to absorb energy. "

 

[mhiscox]

I plan to build a lighter more streamlined version of my Canter Camper. But I am stuck on the question of whether to build the frame out of Aluminium or Steel.

If lighter weight is a concern, might not having the exterior be a composite sheet such as NidaCore be a competitor? My understanding is that a properly chosen and finished composite exterior will vastly reduce the extent of the framework needed, yielding a lighter overall weight. Is this correct?

 

[mog]

I plan to build a lighter more streamlined version of my Canter Camper. But I am stuck on the question of whether to build the frame out of Aluminium or Steel.
Any thoughts anyone?

There is a lot of great information over here on the thread regarding Douglas Hackney's Fuso frame breaking.

http://expeditionportal.com/forum/showthread.php?t=24225

 

[ntsqd]

While in college I built a trailer for my bicycle. It is a single tired design with a hooked towing arm to reach up to the rear of the bicycle frame on only one side. I designed it to carry groceries since the market was a short ride away, but I looked at all of the likely loadings and forces on the design before selecting a material. What I found in that particular design was that by the time I increased the tube size and wall thickness of aluminum to meet the strength needs the trailer frame's weight was not significantly different than one made of steel, and the material would have cost roughly 3 times as much.

What that exercise taught me was that the design is more important to saving weight than is the material selected. If you design for aluminum to get the lower density then the geometry is far more important than it you design for steel. In my trailer's case I could not alter the design enough to take advantage of aluminum's lower density and still meet the strength needs.

 

[Robthebrit]

If lighter weight is a concern, might not having the exterior be a composite sheet such as NidaCore be a competitor? My understanding is that a properly chosen and finished composite exterior will vastly reduce the extent of the framework needed, yielding a lighter overall weight. Is this correct?

This is what I would do for a new shell. My camper is built this way, it has no frame at all. Its not built from nidacore because its 20 years old but its a similar thing. Its basically 1.5-2 inches of high density foam with fiber glass on both sides. Another bonus is the shell is automatically insulated and there is very little in the way of conduction to the outside.

Rob

 

[engineer]

Although I stand to be corrected I think you will find a large weight advantage of aluminum over steel. The critical factor here is whether the structure will need to flex or not. If it does need to flex then 3x more aluminum is required and since it is a third the weight of steel it has no weight advantage BUT if the structure does not need to flex then you should achieve al least a saving of at least 35-45% in weight to get the same or comparable strength from steel. This is a massive saving in my opinion.

The marine industry make this comparison all the time Quote -
from this website -http://www.kastenmarine.com/alumVSsteel.htm

"Built to the Same Standard
An aluminum hull structure, built to the same standards, weighs roughly 35% to 45% less than the same hull in steel. As a result, if high strength is of the highest priority, the alloy boat can be built to the same structural weight as the steel vessel, and then be considerably stronger.

This is less of an issue for larger vessels which are able to carry the necessary displacement for whatever materials choice is made. For smaller vessels however, the weight of the hull structure is very much an issue. For a small cruising vessel, say under around 35 feet or so, steel becomes less optimum, as one must resort to a large water plane and a large displacement to carry the weight of the structure.

When alloy is designed to the same standards as steel (ABS, Lloyds or other similar classification society), it is made to be higher in overall strength. The reason for this is that aluminum reaches its "endurance limit" sooner than steel in terms of flexure. Therefore the rigidity of structure (deflection) becomes the limiting design criteria for an aluminum structure, and this forces a higher than necessary overall yield and tensile strength.

With steel, one designs to the yield point of the material instead, since for steel, flexure and rigidity are not ordinarily a limiting issue. For steel therefore, the yield point of an "equivalent" structure will be considerably less, as we will see.

One advantage of steel is that between the yield point of mild steel (around 36,000 psi) and the ultimate tensile failure point (around 60,000 psi) there is quite a large plastic range (around 24,000 psi or roughly 40% of the ultimate strength), permitting a steel vessel to endure deflection without failure, so permitting considerable ability to absorb energy. "

The figures are impressive Richard, but as a marnie vessel, the stress loadings are quite different to a motor vehicle.
When i designed our trucks, i took into account the life of the vehicle and designed for 15-20 years. Through this they are to cope with repeated vibration and stress and not a one off tensile failure test.
We found that by factoring in a safety factor of 1, we were able to achive a durable frame that was strengthened by the inner and outer sheeting (1.6mm). Given that fatigue would be accountable for most damage in the later life of the vehicle, we also designed for stress points that were replaceable i.e. where the runners connect to the bearers and chassis mounting points.
The overall result was a SWB Canter that tares out at 4500, but suffering no fatigue damage since early2004.

 

[FusoFG]

My camper box is built with the same technique used here in America for enclosed trailers, car haulers, large truck / motorhome conversions, etc.

A welded steel frame built from 1" x 2" tubing on 16" centers covered with .040" aluminum panels glued on with Sikaflex adhesive.

Some builders use a thick tape adhesive made by 3M.

The spaces between the tubing are filled with pink / blue foam board insulation (not white styrofoam) and covered with Luan door skins.

The floor is framed with heavier gauge 2 x 3 tubing.

I think the small amount of material used in the frame would minimize any weight savings using aluminum. Especially considering the dimension increase required for equivalent strength.

 

[boblynch]

My camper box is built with the same technique used here in America for enclosed trailers, car haulers, large truck / motorhome conversions, etc.

A welded steel frame built from 1" x 2" tubing on 16" centers covered with .040" aluminum panels glued on with Sikaflex adhesive.

Some builders use a thick tape adhesive made by 3M.

The spaces between the tubing are filled with pink / blue foam board insulation (not white styrofoam) and covered with Luan door skins.

The floor is framed with heavier gauge 2 x 3 tubing.

I think the small amount of material used in the frame would minimize any weight savings using aluminum. Especially considering the dimension increase required for equivalent strength.

Would you mind sharing the dimensions and approximate weight of your camper box? I remember your build thread, but can't seem to locate it.

 

[FusoFG]

The box is 13.5' l x 7' w x 6.5' h and is mounted on a 3 point mount like the one Carl designed for his FG and Big Foot camper.

The truck, the camper, interior, 2 x 33 gallon tanks full of diesel, 3 deep cycle batteries, 18 gallons of water (now 30), spare tire, tools, spare parts, kayak, clothes, supplies, food, 2 adults and 2 dogs weighed 10,200 lbs when we left for our first 3 month trip out west.

Well under the 14,500 gvw.

So the camper fully loaded was about 10,200 - 5700 = 4500 lbs.

I have the weight of the truck with just the empty box somewhere but it will take me a day of 2 to locate it.

 

[haven]

Here's what FusoFG's current camper looks like

And here is his first camper on the FG chassis

While most of us are planning to build our first camper, FusoFG
has already built two, and traveled all over North America. So
I'd give his opinion more weight than most.

Chip Haven

 

[FusoFG]

I have the weight of the truck with just the empty box somewhere but it will take me a day of 2 to locate it.

The empty box weighed 1480 lbs including the rear storage door and the side entry door with screen but before the windows were cut out.

The 1 x 1 1/2 16 ga tubing for the side walls and roof weigh approx 500 lbs including the extra framing for the 2 doors, 5 windows, roof vent and the extra frame members where the exterior aluminum panels overlapped.

The 2 x 3 14 ga tubing for the floor weigh approx 400 lbs.

The aluminum side panels weigh approx 140 lbs

The aluminum covered 5/8" plywood roof (for walk on strength) weighs approx 200 lbs

The aluminum covered 3/4" plywood floor weighs approx 240 lbs

I think you might be able to save 300 lbs by building the frame out of aluminum but then you would have to deal with the electrolysis problem where the frame joined the truck frame.

300 lbs is a lot, but at less than 10,500 lbs I am still more than 3,500 lbs below gvw.

My biggest regret was the extra 200 lbs for the plywood roof because I haven't walked on the roof in 4 years.

 

[justduck]

Great info. Do you have pics of the build?