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Old 01-03-2009, 07:25 AM   #1
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What are everyones thoughts on constructing the coach frame using aluminum (floor cross members, body framing, etc) and then covering the outside with aluminum plate that will be welded to the framing and have all the seams welded?
I would use spray foam insulation for the walls/ceiling and either bond the interior panels to the frame with VHB/Sikaflex or bolt wood fir strips to the frame and attach the interior panels to this.
Welding all the plate seams would ensure no leaks and essentially make the outer covering "one piece". Since everything would be aluminum weight would not be an issue but the durability should be much better than sheet metal or FRP panels. Thoughts?
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Old 01-03-2009, 10:56 AM   #2
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DG,
I think one needs to do a cost benefit analysis on your theory. I am not the expert on building coaches but I have a solid, relative education and professional background manufacturing.

1.First, one would need to engineer the structure for steel and then re-engineer it again for aluminum. Structurally, you may need to increase the material thickness and size to make it work with aluminum. This will likely be the first rude awaking to fabricating large structures with aluminum. It can be very expensive to do when compared to steel. This is especially true in the ďover-building frame of mindĒ that we tend to work in when we donít have access to proper stress analysis engineering and design services to structurally optimize the design. We throw material at the concerns to insure there will be no failures. Otherwise things break and bend where we least expect them to do so.

2.The next reality is that structural aluminum is more difficult to weld than steel. Itís easy to stick together but much more difficult to control the twisting and warping that occurs when you apply too much heat or weld assemblies in the wrong sequence. It can be done but requires more skill and is therefore more expensive. And it is even more expensive if you donít have the skill since you ruin lots of good material. There are lots of manufacturers making good aluminum frames for vehicles but it is a different expertise and a more complicated process than what most of us are willing to pay for.

3.Another concern is welding aluminum plate for the outer skin. This is not really an option. The heat would destroy any reasonable thickness of aluminum skin and any skin thick enough to weld would be crazy expensive and getting pretty heavy. That is the reason they use bonding (tape and glue) or mechanical fastening (rivets or screws) to attach thinner sheets of aluminum.

I briefly looked into building my box top from aluminum and quickly realized that there was no benefit to the added cost of doing a steel floor structure and adding an aluminum box (donít forget that steel and aluminum ideally should not touch each other).

The weight savings just didnít pay off. On a 35Klb rig, saving 1,000lbs just doesnít buy you much at 10 to 20K miles per year. Now if your driving a 100K mi per year and you can get paid for an extra 1,000 lbs of freight every where you go, that is a different story all together. And if youíre building an airplane, itís really worth talking about. However, for a one-off coach with minimal engineering resources available I donít think I could make it work. If we could afford to engineer it and build it with aluminum, we probably wouldnít be building them ourselves and on used truck frames. But then the fuel savings might not be an issue either.

Good luck on what ever you decideÖ

Rad
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Old 01-03-2009, 09:30 PM   #3
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DG,

My truck conversion built by Transport Designs is all aluminum tubing frame. It was built in 2001 and I have 101,000 miles on her and no problems. Take a look at my blog to see how she was constructed. http://www.ustruckin.blogspot.com/
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Old 01-03-2009, 11:33 PM   #4
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......SPF is the best value for the money and it won't let go of any surface you spray it on...after all it is a glue with a foaming agent and a smoke and fire suppressant added....aluminum frame will not be cost effective or I would use it-fastening and welding is not a problem for me.......steel frame with insulated aluminum exterior and SPF with at least 3/8 inch plywood interior wall covering glued to frame will be quite enough to stand the test of time and road. Roof in 20 to 40 ga. aluminum one piece with NO openings will last forever if the edge is sealed properly....add a BAL air to air exchanger and your moisture problem will disappear all together-without keeping the windows open all the time you are in the rig.....people sweat/breath about a pint a day and it will mount up unless you exchange air-cooking-bathing and any "efforts" add to the air moisture rapidly.....SPF under the floor or a double floor with a layer of foam board between the sheets will keep the floor warm all the time....make it R-10 at least for comfort.....I was in a coach today with a wet tile floor and not only was it cold but slippery from the moisture.....$279,000 for it....another $2k to fix the issues like leaks and coldness-guess what company is out of business and the warranty is useless.....huummm....geofkaye
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Old 01-04-2009, 09:00 AM   #5
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ustruckin,
Looks like they do nice work and could also build an aluminum frame 5th wheel trailer if I decided to go that route instead. Thanks.
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