SIPs and foam and aluminum, oh my!

[Utility Beef]

After writing this whole thing, I thought I'd go back and preface it with an apology for the brain dump. If you decide to hit "back" now, I won't blame you one bit.

I'm in the dreaming-about-construction techniques phase of my TD build. AKA, I already have too many irons in the fire, and don't even have room in the garage until I finish siding the house. Anyway, I posted a thread earlier about my main design constraints, that it be very lightweight (<500 pounds loaded) and very low profile, about the same height as a Miata (48").

Also, I have an interest in building it completely out of non-organic materials, primarily aluminum. I was enamored with aluminum honeycomb panels for a while until I sat down and figured what they cost. I realize that wood is used for good reason- it's relatively light and strong, but above all cheap. My problem is that it rots and otherwise has a tendency to deteriorate. Besides, I just like to be different.

When thinking about how to build the frame, I kind of settled on Andrew's ultralight a-frame as the ideal combination of weight savings and strength for my comfort level. Figuring out the lower platform was my next task, and I came across this thread where captainsam did some really interesting testing with laminating foam and luan into relatively lightweight panels (SIPs) that ended up being VERY strong. I'm really curious to see how 2" of Foamular 250 XPS (AKA pink foam board) laminated with .04 aluminum in a vacuum press would perform.

So I had a couple construction techniques that pushed all the right buttons with me, and I wondered if there was a way to combine them. While driving home this evening, I thought about building the frame (out of 2" square aluminum tube) and axle pretty much as usual, but sandwiching it inside the SIP with a 2" foam core. To retain the strength of the panel, I know I'd have to cut the foam around the frame to very close tolerances. Then I got to thinking about building the side panels similarly, most likely out of thinner foam, and embedding tubing where it would be necessary for fastening bulkheads and walls.

So what's wrong with this plan? I know it would take longer and be more expensive than more traditional methods. It would also require very careful design and construction. There are details yet to be worked out, such as how to join the roof and floor to the walls. And it might not even save any weight in the end. But to me, going camping in the thing is almost secondary to building a neat project. There are lots of big brains around here, I'd be very interested to hear what flaws these techniques might have that I haven't foreseen. Thanks in advance for the input.

Josh

[kennyrayandersen]

Josh – thought about an illustrious career in aerospace engineering? The pay is decent and so far the jobs are holding steady…:lol: You wrote a lot, so I give you the same fair warning that you gave – go forward from here at you our peril (it’s a killer rabbit! – look at the teeth!)

Also, I have an interest in building it completely out of non-organic materials, primarily aluminum. I was enamored with aluminum honeycomb panels for a while until I sat down and figured what they cost. I realize that wood is used for good reason- it's relatively light and strong, but above all cheap. My problem is that it rots and otherwise has a tendency to deteriorate. Besides, I just like to be different.

There is always a way to do nearly everything; however, you show note that not all are painless, and some require a LOT of time, money, equipment, and tooling that for a one-off, are just not feasible. Now, there is a LOT you can do, so I’m not trying to discourage you, just point out that some of these problems have vexed better minds than you and I will ever meet. What you are talking about is pretty much construction that follows aircraft building techniques. I’ve been working in the aero-structures field for almost 25 years so I feel pretty comfortable telling you what would and would not be feasible. You can believe be or not, but I’ll try and give you some useful tips and ideas. One thing we try to do is keep an open mind. Depending on conditions, we use aluminum, steel, titanium, graphite, or even fiberglass. We generally bond or mechanically fasten things together and the fasteners can be blind (pop) rivets, to 220 ksi hardened steel bolts (and everything in-between). You might be shooting yourself in the foot a bit by insisting on only inorganic materials. If you don’t get water infiltration wood will last 100s of years – steel might not last so long if improperly cared for! So, think about that aspect.

When thinking about how to build the frame, I kind of settled on Andrew's ultralight a-frame as the ideal combination of weight savings and strength for my comfort level.

I think Andrew is a pretty smart guy. Both his A-frame, and the single tube type tongues are the way to go in my estimation with the single tube type the lightest of the two (the A-frame offers a little more robustness and comfort, but really I think the single would work just fine and I will probably use the latter myself – A-frames are for sissies )

Figuring out the lower platform was my next task, and I came across this thread where captainsam did some really interesting testing with laminating foam and luan into relatively lightweight panels (SIPs) that ended up being VERY strong. I'm really curious to see how 2" of Foamular 250 XPS (AKA pink foam board) laminated with .04 aluminum in a vacuum press would perform.

Pound for pound there is nothing better than a composite panel for strength to weight. They do suffer from one perennial problem and that is that they are notoriously difficult to join together. First off, a .04 in facesheet is huge, and by huge I mean ginormous. I was actually thinking of using a single ply of fiberglas which is .01 inches (I may have to use 2 plies in some areas). Aluminum is stronger than fiberglass (once the glass gets woven into the epoxy matrix etc.). The E (Youngs modulus) for fiberglass is about 2.5E6 and for aluminum it’s about 10.4 E6. Since the failure will likely be stability in a thin facesheet, and that a function of E, the aluminum will be much more robust than the glass; however, the aluminum is not as easy to join together as the glass is (with fiberglass you can just use a stitch and glue method, whereas the aluminum is more problematic. So, it can be done, but it’s very difficult to go ‘only’ that way.

So I had a couple construction techniques that pushed all the right buttons with me, and I wondered if there was a way to combine them. While driving home this evening, I thought about building the frame (out of 2" square aluminum tube) and axle pretty much as usual, but sandwiching it inside the SIP with a 2" foam core. To retain the strength of the panel, I know I'd have to cut the foam around the frame to very close tolerances. Then I got to thinking about building the side panels similarly, most likely out of thinner foam, and embedding tubing where it would be necessary for fastening bulkheads and walls.

I’m pretty sure you read through the Ultra Uber thread and this is basically the same method. The difference is by burying a 2”x2 (or 1x2) wood with the facesheets on the top and bottom of the wood, two things happen. The core is bonded to the wood so that you have shear continuity, and the facesheet ties into the wood so that there is continuity in the facesheet as well. The bonus is that the wood can now be bolted or screwed or even stitched and glued to the walls – sweet. With the aluminum it’s not so simple. The first problem is the difficulty in bonding aluminum. Yes we do bond aluminum in the business, but the adhesives are first-rate (that means there is a very tightly controlled and repeatable process that covers the bonding procedure), the personnel trained, and the best bonds are carried out at elevated temperatures. You can get good bond at room temperature, but all of the above issues still hold for room temperature bonds. The 2x2 aluminum tubing is difficult to join in the corners (are you going to weld it? – you need special equipment, and a bit of practice and talent) – tubes are difficult to bolt together – wood is not. Once it is welded, it would still have to be ground flat so that the facesheet could be bonded to the top and bottom of the tube. Wood can be glued, screwed, biscuited, doweled, etc. – it’s a much more forgiving and easier to work with medium. I can conceive of some ways of joining the Aluminum, but none of them simple or easy – thus I wasn’t going in that direction. You might use your aluminum facesheet with a foam core and a wood closeout (framed in wood) (especially on the outside for looks and durability (I’ve even considered that myself). I’m not saying it’s a bad idea, it’s just difficult and has to be done pretty carefully if it’s going to work.

So what's wrong with this plan? I know it would take longer and be more expensive than more traditional methods. It would also require very careful design and construction. There are details yet to be worked out, such as how to join the roof and floor to the walls. And it might not even save any weight in the end. But to me, going camping in the thing is almost secondary to building a neat project. There are lots of big brains around here, I'd be very interested to hear what flaws these techniques might have that I haven't foreseen. Thanks in advance for the input. Josh

I built some round speakers once. Laid up some MDF like barrel staves machined them round, hand laid a walnut veneer on them – tops had book matched walnut burls. Drivers were recessed. I made the pedestals and bases wrapped in veneers and hardwood rounded circular corners. They are weighted internally and externally with lead shot and sand and internally braced for vibration reduction and the satellites weigh in at about 110 lb each. They are stunning really, and only appreciated by those who woodwork! Everyone else goes there nice. So, I think though they please me and all, I might have gone a little overboard spending 300 hours on danged speakers and since then have tried not to spend too much life-time on a ‘vision’ FWIW.

[Utility Beef]

Kenny, thanks for the thoughtful reply.

Josh – thought about an illustrious career in aerospace engineering? The pay is decent and so far the jobs are holding steady…:lol:

In another life, I thought about it... But the math just never clicked with me. I loved the sciences and engineering, so that was a frustrating problem to have for sure. I find I can intuitively "get" and apply most mechanical things, but calculating them out (as you so ably did in the Uber Ultra thread) is beyond me. C'est la vie.

There is always a way to do nearly everything; however, you show note that not all are painless, and some require a LOT of time, money, equipment, and tooling that for a one-off, are just not feasible. Now, there is a LOT you can do, so I’m not trying to discourage you, just point out that some of these problems have vexed better minds than you and I will ever meet. What you are talking about is pretty much construction that follows aircraft building techniques.

I appreciate the insight of somebody "in the biz". The notion of "best material and method for the situation" is well taken. After all, if they still use balsa wood in the newest Corvettes, it can't be that bad. I suppose it's no coincidence that I'm thinking in aircraft terms. My ultimate dream project is to build a Van's RV8, and I've spent some time lurking around their forums too. :D Spent a lot of time around planes as a youngster as well.

I think Andrew is a pretty smart guy. Both his A-frame, and the single tube type tongues are the way to go in my estimation with the single tube type the lightest of the two (the A-frame offers a little more robustness and comfort, but really I think the single would work just fine and I will probably use the latter myself – A-frames are for sissies :lol: )

I prefer the term cautious to sissy, thankyouverymuch! ;)

I’m pretty sure you read through the Ultra Uber thread and this is basically the same method. The difference is by burying a 2”x2 (or 1x2) wood with the facesheets on the top and bottom of the wood, two things happen. The core is bonded to the wood so that you have shear continuity, and the facesheet ties into the wood so that there is continuity in the facesheet as well. The bonus is that the wood can now be bolted or screwed or even stitched and glued to the walls – sweet. With the aluminum it’s not so simple. The first problem is the difficulty in bonding aluminum. Yes we do bond aluminum in the business, but the adhesives are first-rate (that means there is a very tightly controlled and repeatable process that covers the bonding procedure), the personnel trained, and the best bonds are carried out at elevated temperatures. You can get good bond at room temperature, but all of the above issues still hold for room temperature bonds. The 2x2 aluminum tubing is difficult to join in the corners (are you going to weld it? – you need special equipment, and a bit of practice and talent) – tubes are difficult to bolt together – wood is not. Once it is welded, it would still have to be ground flat so that the facesheet could be bonded to the top and bottom of the tube. Wood can be glued, screwed, biscuited, doweled, etc. – it’s a much more forgiving and easier to work with medium. I can conceive of some ways of joining the Aluminum, but none of them simple or easy – thus I wasn’t going in that direction. You might use your aluminum facesheet with a foam core and a wood closeout (framed in wood) (especially on the outside for looks and durability (I’ve even considered that myself). I’m not saying it’s a bad idea, it’s just difficult and has to be done pretty carefully if it’s going to work.

Working with unconventional materials in my garage presents problems to be sure, but I haven't seen anything that's insurmountable yet. After all, I'm building a camper, not a space shuttle. I will also build several test panels before beginning the actual build. Try several different adhesives and clamping techniques. I had planned on welding the aluminum as you assumed. I don't have the knowledge or equipment to do it, but I'm acquainted with a gentleman who has many years practical experience and was a welding instructor at a local community college for a number of years. He owes me a favor anyway. ;) Through my employer, I also have easy access to aluminum sheet, tube, channel, etc..., all at wholesale prices. No exotic metals or anything, but the basics are easy to come by.

As you say, wood is indeed easy to work with and make changes with as you go, but that's part of the reason I'm having so much fun planning this now. Careful planning and precise construction can largely obviate the need for running changes, at least in something as simple as a TD... Though maybe not in an airplane. I have some experience with Autodesk Inventor, and would probably model the whole thing there first.

I built some round speakers once. Laid up some MDF like barrel staves machined them round, hand laid a walnut veneer on them – tops had book matched walnut burls. Drivers were recessed. I made the pedestals and bases wrapped in veneers and hardwood rounded circular corners. They are weighted internally and externally with lead shot and sand and internally braced for vibration reduction and the satellites weigh in at about 110 lb each. They are stunning really, and only appreciated by those who woodwork! Everyone else goes there nice. So, I think though they please me and all, I might have gone a little overboard spending 300 hours on danged speakers and since then have tried not to spend too much life-time on a ‘vision’ FWIW.

They sound beautiful. Do you have pictures? That last bolded part is deep, man. :D My wife would probably agree with you.

Thanks again for all the input, it gives me a lot more to think about... You've almost got me talked out of it. ;)

[kennyrayandersen]

My ultimate dream project is to build a Van's RV8, and I've spent some time lurking around their forums too. Spent a lot of time around planes as a youngster as well.

Note, that in the case of Van’s build techniques they use a lot of rivets and stiffeners and not much if any in the form of sandwich construction. I think that this is due to the fact that their kits are homebuilt. Van’s could pre-build the panels, but it’s a lot of labor and the tooling is expensive. You can almost get the same benefit by using thin gages of aluminum which are stiffened by riveting a stiffener (panel breaker) to.

Now, there is one other way of doing this, which would be a bit more similar to airplane construction: if your buddy can weld aluminum, you could make a frame from 1”x1” square aluminum tubing (6061 is relatively cheap and can be welded (not all aluminums can be)) and then rivet the skins to the outside. You could make you aluminum panels by closing them out with a ‘z’ or ‘c’ extrusion and the fastening them to the frame. You skins could then be stiffened as required with stringers across the short dimension. 1 inch foam could be used to fill the void in the 1”x1” frame. It would be a little tricky on the outside corners, but if you used countersunk the rivets used to join the skin to the frame you could come over the top of that with a dead-soft aluminum angle (so that it will form to the contour easily), which would cover the rivets and seal the two panels together.

As you say, wood is indeed easy to work with and make changes with as you go, but that's part of the reason I'm having so much fun planning this now. Careful planning and precise construction can largely obviate the need for running changes, at least in something as simple as a TD... Though maybe not in an airplane. I have some experience with Autodesk Inventor, and would probably model the whole thing there first.

Computers can help, but the level of detail that has to be pre-modeled is very high if you don’t want to paint yourself into a corner on this one.

Do you have pictures?

I have some but sometimes, for whatever reason I sometimes can’t load them from work to the teardrop website, so if I can’t load them now, I’ll try and load them later from home – I’m 180 degrees out of phase with most everyone else on the board since I’m in Asia (BTW – COOL SOLAR ECLIPSE!! Yesterday! – My second (I was in Israel for the first!)).

Thanks again for all the input, it gives me a lot more to think about... You've almost got me talked out of it.

I’m not saying don’t do it exactly, I’m saying the more realistic you expectations are, and the greater understanding you have of the challenges, the more likely you will be successful in the end. Speaking as one who has ‘several’ uncompleted projects, I can say with some authority that you really have to plan for success and the easier to execute your plan the higher the chances of success. You CAN sink a basket from half court, it’s just not likely; so, the simpler that you can keep it (even using new or different techniques) the better off you’ll be.

[Big Dave]

You could buy your composite panels pre-made?

http://www.signboards.com/products/alumalite.asp

We use this stuff all the time and it is extremely rigid. A 4 x 8 attached at the to two post can withstand 120 mph winds. Our cost is around $100 a sheet.

You could also do fiberglass, build your frame then stretch cloth over it and apply resin. You could reinforce with additional layers on the interior to keep the outside smooth. Similar to this, but with aluminum instead of the mdf:

http://volvospeed.com/Mods/fiberglass_speakerbox.html

[kennyrayandersen]

You could buy your composite panels pre-made?

http://www.signboards.com/products/alumalite.asp

We use this stuff all the time and it is extremely rigid. A 4 x 8 attached at the to two post can withstand 120 mph winds. Our cost is around $100 a sheet.

You could also do fiberglass, build your frame then stretch cloth over it and apply resin. You could reinforce with additional layers on the interior to keep the outside smooth. Similar to this, but with aluminum instead of the mdf:

http://volvospeed.com/Mods/fiberglass_speakerbox.html

These types of panels come up from time to time, and one of these days they make one that will work really good and be cheap etc.
Generally, though not always, the plastic that is used in the center is a polypropylene. Polypropylene is used because it can be thermally formed and it quite cheap. Polypropylene does suffer from one thing though and that is flexibility. Most of the panels are kind of rigid, but they maintain some flexibility so they don’t snap off in the wind. The second problem is that about the only glue you can use on them is a type of contact cement – most glues just won’t stick to it (I’d be surprised if epoxy did, though I think they make some special ones now [that you can buy in small quantities] that might work. The factory probably used some time of hot-bonding process.

With fiberglass facesheets, you can glue and stitch with fiberglass tape an epoxy at the seams and the extruded foam is pretty rigid (so it will transfer load between the facesheets well). Additionally, if you are making your own panel, you can laminate wood closeouts at the edges, which allows you to assemble the panels together using standard, proven joining techniques.

You could probably use them to do the sides, but at $100/sheet, my guess is that it won’t change the way people are building. If they made them with fiberglass facesheets of 1-2 plies and an ESP foam core and got it to the market for $50 they could rule the world.

The second method used, is basically using a solid laminate to make the ‘wall’ which will be neither light, nor light-weight. His ‘calibrated’ structural test was amusing!

[Big Dave]

What I envisioned was 1" aluminum tube framing with these panels attached with screws or even glue or d/s tape. You could put 3/4" -1" pink insulation in the voids between the framing spars. The build would have to be pretty angular and every joint covered with trim. But if the idea is to have it ultra light and made from non-organic materials, this is what I would do.
As far as adhesives there are many that will stick as the surface is painted aluminum, I mostly use liquid nails or auto body panel bonding adhesive, but I have even used a hot glue gun and silicone in the past. (non-structural)
As far as flexing, I don't know what 1/4" material would be less flexible than this stuff, especially at this weight. I can carry a 4x8 sheet up a ladder with one hand. I have piles of scrap pieces if you would like to see it in person?

[kennyrayandersen]

What I envisioned was 1" aluminum tube framing with these panels attached with screws or even glue or d/s tape. You could put 3/4" -1" pink insulation in the voids between the framing spars. The build would have to be pretty angular and every joint covered with trim. But if the idea is to have it ultra light and made from non-organic materials, this is what I would do.
As far as adhesives there are many that will stick as the surface is painted aluminum, I mostly use liquid nails or auto body panel bonding adhesive, but I have even used a hot glue gun and silicone in the past. (non-structural)
As far as flexing, I don't know what 1/4" material would be less flexible than this stuff, especially at this weight. I can carry a 4x8 sheet up a ladder with one hand. I have piles of scrap pieces if you would like to see it in person?

I'm not saying it wouldn't work, just that there are some unique challenges, and if you weren't going to make your trailer curved then you could probably make it happen. Also, maybe they're using something other than a poly core so the stiffness of the core may be higher than some of the other products that we've seen in the past. If you attach with screws you will surely get dimples at every screw location, because that core will crush no?, but bonding them on with Sikaflex or other car panel bonding stuff (if you do bare metal to bare metal and do a good surface prep) would probably work.

It may even carry the shear loads of the body ( else you will have to triangulate at least the larger sections of the frame that are 'open' (large bay sizes)). For the same reason you said it was pretty stiff though, it wont make the curved part, and I would think it's not structural enough to walk or kneel on, eh? Also, you would have to frame out the door with Al so that you could mount the hinge. It might make an interesting build -- if you do decide to go that way I, for one, would be interested in the results.

[SlyTerry]

With composites you don't need frames of any kind.. When we started four years ago we were using luan /foam/ luan contruction with great success....Like KennyRay I can't help myself and have progressed to vacuum bagging PP honeycomb cores between E-glass and using propriortary techniques have solved the bonding issue of the Polypropylene materials. At the end of the day the body of our largest model the Queen weights less than 200 lbs and doesn't need any chassis at all. The reality of this is that no customer in their right mind would buy a trailer with no frame and until one of the big guys in Elkhart falls on their a**es and actually improves the product this is where we have to remain. A 4x8 sheet of 1/8" luan/1" foam/1/8" luan weights 21 pounds...Five sheets to a trailer you have a maximum weight of 105 lbs. before extras....

[kennyrayandersen]

With composites you don't need frames of any kind.. When we started four years ago we were using luan /foam/ luan contruction with great success....Like KennyRay I can't help myself and have progressed to vacuum bagging PP honeycomb cores between E-glass and using propriortary techniques have solved the bonding issue of the Polypropylene materials. At the end of the day the body of our largest model the Queen weights less than 200 lbs and doesn't need any chassis at all. The reality of this is that no customer in their right mind would buy a trailer with no frame and until one of the big guys in Elkhart falls on their a**es and actually improves the product this is where we have to remain. A 4x8 sheet of 1/8" luan/1" foam/1/8" luan weights 21 pounds...Five sheets to a trailer you have a maximum weight of 105 lbs. before extras....

Terry and I have had a couple of interesting off-line discussions. Terry has the ‘advantage’ of being a manufacturer. As such, a lot of things change – everything from buying in bulk to using techniques that aren’t generally available for the home-builder. He can buy sheets of adhesive, whole rolls of fiberglass ginormous buckets o’ resin etc. When you buy 1 T-nut, it’s $.25, when you buy a box of 100 it’s $7 bucks etc. The biggest advantage, IMO, is that he can spend a significant amount of time on tooling and other manufacturing equipment because he spreads that effort and cost (time IS money) over many trailer bodies. I’ll never have that luxury, because this will never be a career for me (it would be difficult to compete with my day job!).

What I tried to do over on the Ultra Uber thread was lay out a way that someone at home could do, without so much initial cost and effort (Terry’s finishing will be quicker because he has the tools, but the home builder should save the time, or most of it, that building tooling would take) reasonably duplicate a ‘high tech’ composite-type body. It is certainly not the only way to do it and materials are constantly changing (for instance Terry is able to use the PP core, which is a relatively new development in structural panels). So, nothing is fixed in stone, but I was trying to suggest in this thread that using the aluminum sheet, stringer (like an aircraft) was possible, as was making it out of various panels, though those that are pre-made pose some special challenges. Lastly, for the home builder (as Terry pointed out with his luan example – and luan is pretty cheap BTW), there are a LOT of situations where it’s just pretty darn hard to beat the cost, workability, and structural goodness of wood. UB (utilitybeef) was trying to narrow his options and I was trying to head him off at the pass.

[Billy Onions]

I know it's not exactly what your after but for fastening the roof here is something I was thinking of for my next build.




I was thinking of a steel frame with the profile rolled from inverted 1.1/2" angle. The panels would sit inside the angle fastened from inside. The roof spars I have since changed my mind from box section to C section. Based on my current build I decided 1.1/2" SHS would be way way over the top.

The roof I plan on using 2 sheets sandwiching fibreglass wool. The outer sheet could then fasten to the top of the angle which would be sat over the side panels.



Please excuse the metric dimensions the drawing was already saved to my pc.

[kennyrayandersen]

I know it's not exactly what your after but for fastening the roof here is something I was thinking of for my next build.




I was thinking of a steel frame with the profile rolled from inverted 1.1/2" angle. The panels would sit inside the angle fastened from inside. The roof spars I have since changed my mind from box section to C section. Based on my current build I decided 1.1/2" SHS would be way way over the top.

The roof I plan on using 2 sheets sandwiching fibreglass wool. The outer sheet could then fasten to the top of the angle which would be sat over the side panels.



Please excuse the metric dimensions the drawing was already saved to my pc.

I wish your pics were a bit bigger – it’s the devil to read them. Can you give us a bigger shot?

My initial take is to suggest that when you make a steel frame to bolt or glue the skins to it ends up being quite a stout affair, and by stout I mean heavy. In the end most of the strength is derived from the large skins and not the frame. Therefore, when you have a fairly robust (heavy) frame, the net result is redundant structure, and thus more weight than is required to do the job. Now, I’m not peaux-peaux-ing your concept just making a structural observation. If you take a look at Roly’s quarter nelson there is 1/8 inch outer skin and that’s pretty much it structurally – I think the sides are ¼ inch – sure he can’t drive a truck over his teardrop, or use it for a bridge, but I think if it’s finished properly it will last for years.

Now, if you do it that way because you have the material on hand, or you just really like working in steel, then it’s all good, but it’s certainly not needed for strength FWIW.

[Billy Onions]

Kenny.

Not going to disagree with you, I work in steel and therefore that was the first design I came up with. I know very little if anything about working with fibreglass

There is a guy near me who builds massive fifth wheel RVs similar to the ones you have over there

http://fifthwheelco.com/about/us

He reckons no steel or wood is used in the structural manufacture of his bodies, after laughing when I told him how I was building my tiny camper

Larger pictures below for the short sighted

[kennyrayandersen]

OK – makes sense if you’re a metals guy. Just a couple of quick comments: I would use no more than 20 mm SHS for most of the construction with exception of the tongue (and frankly even that is oversized, by a significant amount – I’d use the thinnest gage I could safely, and consistently weld.).


If you are gluing the panels together yourself, then I’d at least use 3 mm instead of the 6 mm on the interior. If you aren’t bonding the foam to the exterior ply (just stuffing it in) then maybe the 6mm would be OK. Bonding the GRP to the outside is mandatory; it REALLY is best if the whole sandwich is bonded – it lighter and much stronger.


The tricky part is the roof. You won’t be able to bond the panel and then bend; it will be far too stiff. You could lay up the inside sheet, score the foam the short direction ¾ of the way through and then use the trailer as a form to easily bend the scored upper panel around the curved portions of the center section of the tear. After that was glued down you could follow it up with the 3 mm over the top, gluing it to the foam. The problem there is that if you put the glass on the outside, it won’t bridge the gaps that the scored foam makes when it is wrapped around the curves. So, you can fill the gaps, or procure the outer sheet flat and the wrap and bond on the outside. Or, you could use the 3 mm material on the outside, which would bridge the gaps, save some time and just put one glass ply on that (probably easier).

[kennyrayandersen]

OK – makes sense if you’re a metals guy. Just a couple of quick comments: I would use no more than 20 mm SHS for most of the construction with exception of the tongue (and frankly even that is oversized, by a significant amount – I’d use the thinnest gage I could safely, and consistently weld.).


If you are gluing the panels together yourself, then I’d at least use 3 mm instead of the 6 mm on the interior. If you aren’t bonding the foam to the exterior ply (just stuffing it in) then maybe the 6mm would be OK. Bonding the GRP to the outside is mandatory; it REALLY is best if the whole sandwich is bonded – it lighter and much stronger.


The tricky part is the roof. You won’t be able to bond the panel and then bend; it will be far too stiff. You could lay up the inside sheet, score the foam the short direction ¾ of the way through and then use the trailer as a form to easily bend the scored upper panel around the curved portions of the center section of the tear. After that was glued down you could follow it up with the 3 mm over the top, gluing it to the foam. The problem there is that if you put the glass on the outside, it won’t bridge the gaps that the scored foam makes when it is wrapped around the curves. So, you can fill the gaps, or procure the outer sheet flat and the wrap and bond on the outside. Or, you could use the 3 mm material on the outside, which would bridge the gaps, save some time and just put one glass ply on that (probably easier).



BTW, you friends trailers are BIG money!