weight-saving techniques

[kennyrayandersen]

There have been several threads about light-weight construction and I think the number 2 thing everyone mentions about their second build is that they would build lighter (it’s so tempting just to through more material at the problem!). I’d like to post some of the lightweight concepts in one spot so that they are a bit more centrally located. There has been a lot of talk about building sandwich panels and weights for various styles of building tears. Some people have already built using some (or all) of these techniques, but there is always something to learn – especially as ‘rubber meets the road’. Obviously, there are ways to build that could be considered low-hanging fruit and some would require a great deal of effort to save that little bit extra. If all of those methods are laid out then builders could decide for themselves whether a particular technique was ‘worth’ the effort.

My personal motivation comes from the fact that I’d like to tow the tear with a microcar that has a VERY limited amount of horsepower (21 HP). It’s the little red car behind the hummer.



I thought it might be interesting to start by comparing some baseline weights with some alternatives to show, generally, what kind of weight-savings might be possible. Obviously, solid plywood is what would be considered the ‘easiest, but not necessarily the lightest
Solid plywood
¾ plywood ~75 lb/ sheet or 2.34 lb/sq ft
½ plywood ~50 lb/sheet or 1.56 lb/sq ft
3/8 plywood ~36 lb/sheet or 1.17 lb/sq ft
(Plywood, if you hadn’t picked up on that, weighs about 12 lb/1/8 inch of thickness for a 4’X8’ sheet)

Sandwich
1/8 plywood weighs about 12 lb/sheet
The Home Depot foam is around 2 lb/cu ft.
To make a sandwich requires 2 sheets of the 1/8 inch
There are 32 square ft/sheet per sheet and we’ll assume the sheet is 1 inch thick
Weight of the core is 32/12*2 = 5.33 Lb+
Total weight for a 4X8 sheet of sandwich = 2*12+5.33 = 29.33 Lb
Weight per sq ft is 29.33/32 = .92 Lb/sq ft (not including adhesive between the skins and core)

So far what I’ve shown is just a comparison to solid plywood and thin plywood with a foam core weight. Note that even using the 1/8 facesheets on a 1 inch core results in a structure that is lighter than building with 3/8 inch solid material and is 2.34 times as stiff as the ¾ inch solid plywood (which might be surprising to the non-engineer since it is only 40% of the weight). So, there is significant weight-saving using this technique and I think it is only mildly ‘high-tech’.

Fiberglass Sandwich
I’ll show some further possible weight-savings with an all composite panel.

With fiberglass, it’s a bit more subjective because it matters how many plies you use along with the ply ‘weight’. The rule is that the resin weighs the same as the fabric (final ply weight is 2 times the dry ply weight – when you are looking at buying fabric the dry weight is listed). To get a rough idea, I’m basing my initial estimate based on a helicopter cowling I worked on for the last couple of years. The cowling I worked on used .008 inch thick graphite material (fabric), which corresponds to 6 oz fabric. The 4 oz fabric is around .006 inch thick. As I’ve mentioned on a few posts generally the inner facesheet of the cowling was 1 ply and the outer 2 plies (and we are talking over 200 mph with high side-slip capability (i.e. big side loads!)). The one that preceded this one was made with a quartz fabric, that was much less stiff and the individual plies thinner (corresponding to the 4 oz, .006 fabric); so, I’m not going to get too hung up on the fact that the graphite was stiffer since the previous cowling was made from a softer, thinner fabric and there weren’t any problems with it despite only having one ply on the inner skin (the material change for reasons other than strength – graphite sounds SO high-tech!).

Ceding the fact that the home builder doesn’t have an autoclave and isn’t a trained professional, I’d say one should probably bump up the layup on the tear just a bit (how about 50% in the inside and a bit more on the outside (it was a bit over-sized for the outer skin)); plus, glass is a little less stiff than graphite (if we’re making a direct comparison). Now, this wouldn’t include local reinforcements at the frame to body attachment, or the area where the axles attach, but just the basic ‘field’ area.

For a typical small teardrop, if we put 2 plies of 4 oz fabric on the inside that would yield .012 inch thickness and 2 plies of 6 oz fabric on the outside that would be .016 thick. That sandwich could be used for the top and sides. For the bottom for the trailer maybe 2 plies of 6 oz on the inside with 2 plies of 10 oz on the outside (impact damage prone area) would be sufficient. I will verify (unless someone beats me to it) the ability of the (2) 6 oz plies to withstand the kneeling or step load that a tear floor will see, but I think 2 plies will be sufficient. We used (2) plies on one part of the cowling that opens up and acts as a maintenance platform, and so far haven’t had any problems. Considering that the teardrop floor gets covered with a mattress that further distributes the load and makes the loading even more benign. I’d consider that layup for the basic walls and roof on a small tear (if your tear is heavily loaded, or larger, you would likely need to go up a bit in ply thickness. For the walls and sides 1 inch thick foam will be fine; for the floor I would go with 2 inch thick foam to add a bit of robustness.

So, I’ll try to estimate the weight of a fully synthetic composite panel.

Most of the fiberglass specs and weight etc are given in yards, so I’ll show the calculations in yards with a conversion to get a comparable weight at the end.

Sides and top: 2 plies 4 oz inner, 2 plies 6 oz outer (sides and top) with 1 inch thick foam
4’X8’ = 32 sq ft = 3.56 sq yards
Ply Weight = 3.56*[2plies *(2*4 oz) + 2 plies*(2*6 oz)]/16 = 8.89 Lb
Core wt = 5.33 lb (ref above)
Total weight/sq ft = (8.89+5.33)/32 = .44 lb/sq ft


Floor: 2 plies 6 oz inner, 2 plies 10 oz outer with 2 inch thick foam
Foam weight will be double = 10.66 lb
Ply weight = 3.56*[2plies *(2*6 oz)+2plies*(2*10 oz)]/16 = 14.24 lb
Total weight/sq ft = (14.24 + 10.66)/32 = .78 Lb/sq ft

Looking at the wall we can see that there are some significant weight-savings. One could probably increase the skins a bit more to increase the robustness, but I think this can be handled by locally reinforcing the skins where it is actually required – most of the trailer will be fine with this thickness. Also, it should be noted that the floor, for instance, has less weight than even the 3/8’ thick plywood, but is many time stiffer.

I figure 32 sq feet for the floor and somewhere in the neighborhood of 112 sq ft of sides and roof (4’X8’ tear using 4 ½ sheets of plywood for the shell) would give you a total body weight (minus the local reinforcements and wood inserts) of:
32*.78 + 112*.44 = 74 Lb (outer shell only, but shelves and interior walls could be built with the same technique). Compare this to the shell weight using even the light-weight 1/8 skin on core technique. Using the same areas above the plywood sandwich shell would weigh about 141 lb.

Now, I think with the composite sandwich construction, there are some additional weights like extra plies at the rear axle attachment and possibly a few closeout members made from fir etc. so the total target weight could be somewhere near 100 Lbs for the body (I don’t think that this is unreasonable).

Additional ideas to save weight:
If you make composite walls and shelves for the galley etc, that weight would still stay quite low as well. When I get more time I will detail some methods for making light-weight walls and shelves, but basically it’s the same composite construction with a ½ X 1edge member that can be biscuited or screwed to the tear shell. For now I just wanted to get the thread rolling and I welcome any critiques as well as suggestions on how to save weight.

Kenny

[asianflava]

I think you neglected the stringers or studs in your walls. You need them to closeout the edges, and to provide some "meat" for the cabinets and shelves to screw into.

I built mine with 1/8in faces and 3/4 rigid foam glued together epoxy thickened with microfibers. I figure that mine was about 30% lighter than 3/4in ply. I also used these panels for the cabinets and shelving.

[kennyrayandersen]

I think you neglected the stringers or studs in your walls. You need them to closeout the edges, and to provide some "meat" for the cabinets and shelves to screw into.

I built mine with 1/8in faces and 3/4 rigid foam glued together epoxy thickened with microfibers. I figure that mine was about 30% lighter than 3/4in ply. I also used these panels for the cabinets and shelving.

I tried to state that, but it’s a pretty long-winded diatribe, so it could have gone unnoticed. At 74 lb, that doesn’t include what you are talking about, but I’ve got some ideas about that that will also save some weight with regard to that (well actually in this context won’t add so much weight). Initially I was thinking of putting in several ribs as you are suggesting, but now I’m leaning more toward kind of monolithic foam covered in fiberglass based on a few posts where the structure was quite minimal and still reported to be very strong. Actually the only thing the stringers help with appreciably is local load introduction. I think that can be accomplished with something less than a full stringer. So, there will be minimal reinforcement at the door for the hinge and latch, and a couple of small strips for the shelves and wall to attach to, and some local reinforcement at the [partial] frame attachment points but not much more.

Additionally, I started to think about how we construct and assemble composite panels in the [aerospace] biz and we generally use inserts rather than bury large stringers in the panel. I decided to “return to my rootsâ€

[afreegreek]

I think you neglected the stringers or studs in your walls. You need them to closeout the edges, and to provide some "meat" for the cabinets and shelves to screw into.

I built mine with 1/8in faces and 3/4 rigid foam glued together epoxy thickened with microfibers. I figure that mine was about 30% lighter than 3/4in ply. I also used these panels for the cabinets and shelving.

you don't need "studs" to screw stuff to. all you need are "pucks" to screw to.
(one per screw)

[kennyrayandersen]

I think you neglected the stringers or studs in your walls. You need them to closeout the edges, and to provide some "meat" for the cabinets and shelves to screw into.

I built mine with 1/8in faces and 3/4 rigid foam glued together epoxy thickened with microfibers. I figure that mine was about 30% lighter than 3/4in ply. I also used these panels for the cabinets and shelving.

you don't need "studs" to screw stuff to. all you need are "pucks" to screw to.
(one per screw)

I didn't think anyone would know what a puck was Though typically they are made from some type of potting compound, I think in this application a plug of plywood will suffice.

[afreegreek]

you forget to add the weight of the resin to your composite panels.. I figure you'll need 3 to 4 gallons to make those panels.. probably more

[kennyrayandersen]

you forget to add the weight of the resin to your composite panels.. I figure you'll need 3 to 4 gallons to make those panels.. probably more

I'm pretty sure I didn't. I doubled the weight of the dry ply weight (per the West System page) -- this gives you a bit of a resin-rich condition (50%), but this happens in a wet layup (as opposed to using prepregs) (please see calculations).

[afreegreek]

I think you'll have problems with the composite floor too. 2x 6 oz over foam won't last too long. on the boats I've built with glass/foam decks the schedule is 2 x 15 oz woven roving, a 6 oz cloth and a layer of 4 oz mat (for print through) and even at that there's delamination between the glass and the foam.. the better bet is balsa core. the problem is the foam. it's hard and doesn't have any spring to it. once you press into it, the weight/pressure crushes some bubbles and it stays down.. that's the beginning of your problem and it only gets worse from there.

it would be fine structurally but no good for the wear and tear of real life.

[kennyrayandersen]

I think you'll have problems with the composite floor too. 2x 6 oz over foam won't last too long. on the boats I've built with glass/foam decks the schedule is 2 x 15 oz woven roving, a 6 oz cloth and a layer of 4 oz mat (for print through) and even at that there's delamination between the glass and the foam.. the better bet is balsa core. the problem is the foam. it's hard and doesn't have any spring to it. once you press into it, the weight/pressure crushes some bubbles and it stays down.. that's the beginning of your problem and it only gets worse from there.

it would be fine structurally but no good for the wear and tear of real life.

I think you have a good point and verifying the core-crush/delamination due to kneeling/standing loads would be part of the test program. However, there is a pretty big difference between a boat deck and a teardrop interior. I think in the case of the teardrop that the floor is covered with a mattress 100% of the time. This means that the floor will be shielded from impact damage and will, even in the case of kneeling, stepping, or crawling, spread the load out so that it is less concentrated (impact loads can generate some really high stresses which essentially overload the laminate locally). I think if it were a high traffic area, or prone to impact damage (like a boat or truck bed) then there would be more cause for concern. As a matter of course I will do some impact tests (ASTM) and if there is an impact problem I can add more plies, or look at an alternate core.

The problem with balsa is that it is more expensive and heavier (generally) than the light-weight 2 lb/cu ft Home Depot foam. I think you do bring up a good point about abuse, and certainly depending on how a person is considering using their teardrop (there are some off-roaders who will take them some crazy places) adjustments to the basic layout might need to be considered.

[glassice]

This means that the floor will be shielded from impact damage what the botom with road cr*******
I think you should go buy a starter kit and do some real fiberglass some thing that you need to look at make a small box then go out and kick it around look at the finish put your nee in it you can go with less ply if your 100 lb then 300 leave it in the sun think about doing 60 mph and a rock coming off the back of a wheeler of a 18 wheeler and some cars just wont pull it one of the trailers we built has been pull by a restored 500 But in 2 grear and top sped of 35 mph even if you went as ligh as you can with a 36" wide and 7 ' and 30" high it might work but at any speed it the wind drag that going to kill you and then a truck pass you that will stop you. I sure start doing you will do great


In fiberglass you can spend a lot of cash real fast hell I will even send you some matal to play with

[oicu812]

Thanks Kenny ray,that's the infomation I needed for comparison. I was thinking about 2" blue styro sandwiched between 24 ga. stainless
[1.03 lbs sq. ft.]. 6' tall/13' long with tear profile, about 60sq. ft. 125 lbs. for one finished wall. That sounds comparable to plywood, maybe a little less than ply.

[GPW]

Here's an extreme idea born from my model plane days ... 2" blu foam can be laminated together and sanded into some really nice rounded shapes... I can imagine sidewalls 4" thick (2 sheets ), nicely rounded for streamlining properties... roof 2" thick , 4" floor/with an interior backing. Twice as thick is 8 times as strong .. So a light weight covering and paint would do it ... My current favorite material is Polyester Camo cloth(Wal Mart/sporting goods) ... Thin open weave , Very strong , and glues(Elmers/TB) and accepts paint well... Cheap too ... easily repaired/patched ... with some proper shielding in the rock area, it should be very durable and Light ... Nothing to rot except maybe a bit of thin ply on the floor ... no fiberglass weight/mess/exposure ... and since you'd probably cover and paint the inside too , no "interior" walls etc... Foam shelves/cabinets are entirely possible too with a little extra engineering... all Very Light ... Just an idea... Plausible !!!

[kennyrayandersen]

This means that the floor will be shielded from impact damage what the bottom with road cr*******
I think you should go buy a starter kit and do some real fiberglass by all your post you have never done fiberglass that OK some thing that you need to look at make a small box then go out and kick it around look at the finish put your nee in it you can go with less ply if your 100 lb then 300 you will never learn by just reading leave it in the sun think about doing 60 mph and a rock coming off the back of a wheeler of a 18 wheeler and some cars just wont pull it one of the trailers we built has been pull by a restored 500 But in 2 gear and top sped of 35 mph even if you went as light as you can with a 36" wide and 7 ' and 30" high it might work but at any speed it the wind drag that going to kill you and then a truck pass you that will stop you. I sure when you stop read and start doing you will do great


Please think there some that read the forms and think it fact. In fiberglass you can spend a lot of cash real fast hell I will even send you some material to play with

Glassice,
Again -- you bring up some good points.

The FIAT 500 is more of a show with the trailer than a go (it's mostly show anyway as it is severely under-powered). I'm about 10 miles from the park -- all on back country roads and have no delusions of speed with the 500. If I could get 35-40 mph I'd be more than satisfied. Heck, it'll only go 55 mph tops without the trailer. Air drag starts getting significant over 40 mph anyway, so there you have it -- no disagreement from me. The car is 52 inch wide and about 52 inches tall the trailer will be a little narrower at 48 1/2 and and just a hair taller. If in the end, it can't tow it with the original 500, I'll tow it with my new FIAT 500 Abarth (170 HP yada yada). I'm getting one as soon as they become available at a Chrysler dealer near you.

I've done a little hands on glass work as you know, but have been analyzing them as an engineer for 25 years including MRB (repair and manufacturing support) for both commercial wide-body and fighter aircraft as well as helicopters and tilt rotors. You act as though I know nothing about composites. I've admitted I have some practical stuff to learn but I've been picking the brains of some pretty smart people for several years with this very project in mind and one of my close friends is designing and building his own composite aircraft from scratch (I chipped in with some of the analytical work, which was verified by test!)(I think his 3rd or 4th) and he has pitched in as well. So I don't think I'm so short on expertise.

Rather than ignore you, I've actually taken some of your advice to heart and have nearly doubled the thicknesses that we are using on the helicopter that I was working on (to account for the road damage, durability, BVID etc. Additionally, I plan on doubling the number of holes in the foam surface that you recommended (after I read a foam manufacturers recommendation that a tighter hole spacing can be used (they also recommended a slightly larger hole at 2 mm)). So, I've tried to take what you've told me and mesh that with what I've been doing analytically in aerospace industry and come up with what I think is a reasonable compromise. If it turns out that I have to add a few plies, so be it. IMO, it better to do that and end up ultimately with a lighter structure than is it to start heavy and leave it there. If I used not very much more fiberglass, it would weigh the same as the plywood sandwich, which intuitively would suggest to me that it is a bit over-sized since it should weigh considerably less. If it doesn't, there isn't much motivation to make it out of fiberglass and it would be heaps easier to make the sandwich out of plywood (way less hassle). Well-sealed wooden trailers last 50 or more years -- I don't need it to last 100 -- I'll be dead and gone and could care less. My sole motivation for using composites is to save weight. If it ain't lighter, I'll do it the way that is.

What weight fabric do you generally use -- 4 oz, 6 oz, 10 oz?
I won't be back in the states (save a few days) until next Feb. but after that I plan on making some specimens and testing them. If you want to donate some material toward the test effort to establish some allowables that the teardrop community can use then I certainly wouldn't turn that offer down. Ultimately, I think if we all combine what we know and have experience with we should be able to come up with something pretty good in the end.

[kennyrayandersen]

Thanks Kenny ray,that's the infomation I needed for comparison. I was thinking about 2" blue styro sandwiched between 24 ga. stainless
[1.03 lbs sq. ft.]. 6' tall/13' long with tear profile, about 60sq. ft. 125 lbs. for one finished wall. That sounds comparable to plywood, maybe a little less than ply.

You would surely fail the core LONG before you failed the skins. Generally the skins and core are sized to fail not so far from each other. I have no memory -- remind me how thick the 24 gage sheet steel is? You may be able to use thinner material than that. Typically .02 inches is a pretty robust facesheet for an aluminum honeycomb panel -- steel has thee time the stiffness of aluminum...

[Hillmann]

I had though of building one really lightweight to pull behind my bike which when it was new, twenty seven years ago, only put out 17 horse power. To make it super lightweight I would have made the base 6 feet long and 18 inches wide and the top and sides would be made like a skin on frame boat is made with the sides pushing way out so it would be 3 feet wide just 2 or three inches off the the floor, for sleeping to save the weight of a mattress I would use a fabric hammock for the frame I would only have a piece of 2x2 1/8 in thick 8 feet long and only one wheel at the rear end, a single wheel like is common on motorcycle trailers so it can tilt not a single wheel like the swivel kind that is mounted in two places to the car. the galley could open from the side instead of from the back because the wheel would be in the way or make the shell of it 8 feet long so it can go over the tire then it could open from the rear like a regular teardrop. I think the entire thing could be built to weigh under 100 lbs. And for insulation you could sew up quilts to line the interior with. Of course sleeping two would be pretty tight.