New Teardrop - Basic approach, weigh analysis OK?

[jmedclay]

Gang - looking for a reality check, advice, warnings, better ways.....

Instead of a cargo conversion as originally planned, we've decided to build a 5x10 TD, based on the JPod but with the profile of the 1952 teardrop ( http://www.davisengineering.net/52tear/52tear.html ). Dry kitchen, no built in cooking or washing equip, LCD-TV/DVD player. I was trying to hit the 500# dry, unequipped, weight mark. Doesn't look like that will happen without a kennyrayanderson, paradigm changing approach. I'm not that brave. Here is where I am with the basic approach.

Basic Trailer Frame:

* 2x2x0.125" steel tubing frame perimeter (~3.24 pounds/foot, 30' = 110#)
* Channel steel X members on 24" centers ( 1.5 # per foot, 20' = 30#)
* Trailer neck of 2x3x0.125" (3.88 pounds/foot, 5' = 20#)
* Suspension/Axle/Wheels (Dexter Torque Flex) = Dunno, budget 100# (wheels/Tires - same as on old Volvo wagon tow vehicle - use car spare)

Basic Box:

* Floor: 1/2" Baltic Birch. (4x8, or 5x5, shop scarfed) (85#).
* Walls: 1/4" Baltic sandwich/stringered walls with insulation (4x8, shop scarfed) (125#).
* Roof: 1/4" Same material as walls (4x8 or 5x5, shop scarfed) (120#).
* Bulkhead cabinetry: 1/4 Baltic Birch (budget 50#).
* Hatch: Budget 40#.
* AC only electric: Budget 20#.
* Very basic, dry kitchen.

Sum: 600#
Final Dry Weight Guestimate: 700# including finishes, windows, mattress, small construction items unaccounted for, maybe even the TV/DVD, empty ice chest, but otherwise unstocked and not loaded with trip gear.

To see how much lighter this basic design could be I reduced walls and roof to 1/8" ply (lost 80#), lightened the trailer as much as I dared (lost 25#) for a total of 105 fewer pounds, and got an estimated 600# weight . After additional reading, I don't want to reduce the metal trailer mass at all so with this basic approach it looks like 80# is about the limit for weight reduction. 700# down to 620# isn't doesn't seem to be any sort of paradigm changer so unless there are other reasons to go with thinner plywood I think it makes sense to use 1/4" throughout the walls/roof structure.

Does this seem like a reasonable approach, or do you see any big issues here, or lost opportunity for significantly lighter weight?

Thanks
John

[kennyrayandersen]

OK, even if you don't do the really light composite construction you could go half way there using conventional materials. Instead of using fiberglass, just substitute 1/8 in plywood skins on the interior and exterior and make your sandwich that way.

Also, even if you didn't want to do a sandwich construction you could still use Andrew's light-weight trailer A-frame design. The box itself is pretty darn strong and IMO not many people take advantage of that fact. Just make sure you joint the walls sides top and shelves etc. with biscuits, dowels, or a nail strip so that the joints are good.

OR even if you didn't want to do that you could still lighten up the gage of the steel -- especially around the perimeter of the frame -- the strength and stiffness comes from the vertical walls. 1/8 inch steel is quite robust.

Also, check the trailer tongue sizing directions in the design library and also see if there is a way to choke up on the tongue a bit -- it cuts down on the bending moment and can save a little weight there.

Keep in mind that the Dexter 9 is about 90 lbs, so if weight permits use the #8, it's only about 50 lbs. 12-13 inch wheels and tires are about 40 so it seems that's probably what you have in mind.

Anyway, something to think about.

[jmedclay]

Thanks much for the feedback Kenny.

I'm very interested in using 1/8" ply. My concern is more the survivability of the rough and tumble "undefined loads" (elbows pitched while sleeping, bumping into the trailer box, somebody jumps up on the roof, just general banging around). I've been checking the archives about this, and need to send a few PMs to "thin skin builders" to get some first-person field data on that. From the perspective of in-flight-loads, I'm completely on board with the strength of thin skinned foam sandwich panels and the utility of an overall monoque approach to the box.
I sketched out a couple of lighter trailer versions the other day - same basic design, thinner walls, different sections, I just couldn't know how close those were getting me to being under designed:

* Perimeter frame - 2 x 1-1/2 x 0.120" (2.7# per foot, ~75% as vertically stiff), 2-1/2 x 1-1/2 x 0.083" (2.2" per foot, I'm guessing vertically stiffer, but higher peak stresses, ...but the (trailer) box should be very stiff an contribute). The first saves about 12#, the second about 27# - but I don't know if 0.083" wall is getting thin for this sort of thing (welded joint stresses, tearout, &/or flex of long members).

I'll check out the A-frame. Suddenly, I wonder about a shorter version of the ladder trailer - for a 10' trailer box, make the trailer frame 6' or even 4' - that would distribute the loads into the monoque nicely, while letting the monoque support the static/dymamic loads of the overhang. Regardless of the trailer frame, I'll assemble every joint as a load carrying deal - glue/screws/small gussets/whatever - it would be nice to take advantage of that but I don't want to end up with something fragile, tempermental. I'm not a structures guy so I can't really evaluate this analytically. If using a shorter trailer or A-frame, the door hole shear would be ameloriated by the tongue. That's nice.

I'll mess with some of these ideas.
Thanks.

[jmedclay]

I just did another rough estimate to get an idea of the lowest weight that might be possible with a half-length trailer, conventional materials, while relying on the stressed skin sandwich design for self-support.

Floor: 5x5x3/4, qty 2 at 2.22 #/sqft (I'd build a sandwich, but unsure of details, so this for weight budgeting) 111#
Walls: 4x8x1/8, qty 5 at 0.3125 #/sqft, 50#
Roof: Same as walls 50#
Bulkhead: dunno, budget 40#
Perimeter rails: 2-1/2x1-1/4x0.083, qty 20' (5' square core) at 2.02#/ft 40.4#
X-members: 1-1/2x1x0.083, qty 10' at 1.32#/ft 13.2#
Tongue: 2x1-1/2x0.120, qty 11' (A-arrangement), 2.66#/ft 29.3#
Running gear est (Dexter 8 based): 100#

Total: ~425#

I haven't checked the tongue design for allowable stress, just guessing. To be consistant with yesterday, add a 100# budget for finishes, windows, mattress, small construction items unaccounted for, maybe even the TV/DVD, empty ice chest, but otherwise unstocked and not loaded with trip gear: Ready to trip stock: 525# (vs 700# est yesterday).

Have any of you used a similar approach and gotten similar finished weights (am I near the ballpark?), had good luck with trailer durability and so on?? Without loosing some running gear mass, I don't see how this general approach could get much lighter. If built with details to turn the whole box into a competent structural object, I want to think that this could be a fairly rugged trailer. It wouldn't stand any sort of fender bender though, but short of physical impact, maybe it'd hold up well?

What do you think?
Thanks,
John

[Esteban]

John,

What are you going to use to skin the teardrop? By my estimate typical aluminum sheets weigh about one half pound per square foot. In comparison 4 oz. fiberglass cloth and epoxy will weigh about 8 oz. (1/2 pound) per square yard. A big savings.

I used 1/4" plywood outside and it seems more than stiff enough. 1/8" ply outside would seem, to me, to be too flimsy. It's OK inside, especially when backed with up wood sandwich wall framing and foam insulation. Use heavier hardwoods in places like the door hinge side of the doors and where the galley hatch attaches. In most other places lighter weight pine seems fine.

I built a sandwich floor with 3/8" plywood on top, a frame of 3/4" wood inside filled with 3/4" insulation, and a bottom layer of 3/16" luan plywood (Home Depot luan). If I did it over I'd use 1/8" luan for the bottom layer of the sandwich floor - better luan from a hardwoods supplier.

Other things that add weight are nails, screws, bolts, glue, adhesive, hinges, latches, door knobs, windows, a roof vent, fenders, leveling jacks, a tongue jack, spare tire, wiring, lights, switches, a battery, battery charger, fuse box, electric inlet and outlets, an extension cord for campground power, drawer sides, drawers, cabinetry, misc. hardware, ice chest, stove, propane tank, sink, faucet, water tank, hoses, drains, a mattress, pillows, sheets, blankets, a tongue box if you want one, wheel chocks, an easy up or equivalent, on and on all add up. All together they all will weigh many 100s of pounds.

I erred on the side of caution and followed the Aussie Rules for tongue strength. A 48" long A-frame tongue made with 2" x 3" x 1/8" steel was the sweet spot for my 5'3"x11' design. It even allowed for some extra fudge factor capacity.

Good luck. Your thinking ahead will pay off.

[kennyrayandersen]

John,

What are you going to use to skin the teardrop? By my estimate typical aluminum sheets weigh about one half pound per square foot. In comparison 4 oz. fiberglass cloth and epoxy will weigh about 8 oz. (1/2 pound) per square yard. A big savings.

I used 1/4" plywood outside and it seems more than stiff enough. 1/8" ply outside would seem, to me, to be too flimsy. It's OK inside, especially when backed with up wood sandwich wall framing and foam insulation. Use heavier hardwoods in places like the door hinge side of the doors and where the galley hatch attaches. In most other places lighter weight pine seems fine.

I don’t disagree with 1/8 inch being a bit thin, but I think others have used it; however, what I would try and do with mine is hot foam the curved foam for the top as well so that I had the same sandwich going all of the way around the top as is on the sides.

Let me use an example of what I’m talking about on how to use the 1/8 in plywood effectively. Let’s say you want to build a shelf. Frame 1 inch foam out in 1x1 fir (almost as strong as spruce, but much cheaper) then cover the faces with 1/8 ply on both sides (you could do the same with ¾ x ¾). Now you have a shelf that can be joined by glue and screw, dowels, or biscuits. It’s very light and extremely strong. The vertical wall of the galley could be the same, but in addition to the frame, you run a 1x1 where the shelf(s) intersects (butts up); so, now you can join the shelf to the vertical wall with dowels via the solid wood in both parts. Very light, and very strong (you could use 1/16 if you could find it, or as mentioned earlier, fiberglass, which is much lighter than the plywood. The real weight savings is a result of allowing both the inside and outside plywood to be part of the structural sandwich.

The trick if you can do it (I’m sure about the fiberglass and 99 44/100 % sure on the 1/8 ply) is that you could still make the sandwich over the curved portion, and this is how you could do it: You know how they lay up barrel staves, right? OK what you would do is bond one side of the foam to the 1/8 ply. Cut up the strips on your table saw so the curve is segmented, like barrel staves with the 1/8 ply to the inside and the foam flush to proud on the outside. Make a radius and use a little fiberglass tape similar to the way (sorry I can’t remember his name) the guy did on the Puffin build (called glue and stitch – make a fillet, and the put the fiberglass tape and some epoxy over it). Put it between and glue the two up-standing walls to the roof portion and fillet and tape the joint between the wall and roof. Then use the top edge of the walls as a guide to hot wire the top foam. Apply adhesive (probably epoxy) to the foam and bend the outside 1/8 plywood over it clamp and let set up. Now you have a sandwich on the outside and only had to do a little fiber glassing.

Actually, I would want to put the fiberglass on the outside anyway, so you could skip the outside plywood and just put 1, or two if you are worried, plies of fiberglass cloth ((1) 6 oz or maybe (2) 4 oz plies). If you size the width correctly (58 inch wide) then the 60 inch wide fabric will drape over the sides by one inch and finish the outside joint. It will be slightly proud to the outside of the side, but you paint scheme can hide that easy enough.

I will run my wires on the floor along the side buried under a couple of plies of fiberglass fabric tape that are built kind of like a cove molding so that I don’t have to dig tunnels in the foam. Its low voltage and it will be under the fiberglass and under the mattress, so it should be OK.

[Miriam C.]

Is there a reason you want to get below #700? If you don't have a good reason you might build it stronger. I used 1/8" inside and on the roof and have no regrets. I did have a puncture in the galley roof but that would have cracked a 1/4" too. Just layered some filled epoxy on it.

Good luck!

[jmedclay]

[quote="Miriam C."]:o Is there a reason you want to get below #700? If you don't have a good reason you might build it stronger. I used 1/8" inside and on the roof and have no regrets. I did have a puncture in the galley roof but that would have cracked a 1/4" too. Just layered some filled epoxy on it. 8)

Good luck![/quote]

Good question, and thank all of you for the feedback. Lighter is just easier to handle, but on the other hand it won't change the tow vehicle (which can handle the range of weights under consideration). It might be more of a mental thing - 7 or 8 hundred pounds isn't an awful lot to tow. I was trying to bracket things with a sort of normal approach, followed by a how-light-could-it-be, with plywood and steel. I'm gonna have to mull it around a bit, check with upper mgmt. 1/4 - 1/4 sandwitch would give me no pause, 1/8 - 1/8, I know can be done but 20 years down the road I wonder...I'd like to think that someone else will be enjoying the trailer after we're dust. Philosophically I love light, but tend to err stout. Gotta check some more in the archives. Want to double check single layer construction too, see how competent the panel joints look, and whether or not the exposed stringers would be a problem (visual or real).

This I know - we are seriously stoked!
Thanks,
John

[jmedclay]

[quote="Miriam C."]:o Is there a reason you want to get below #700? If you don't have a good reason you might build it stronger. I used 1/8" inside and on the roof and have no regrets. I did have a puncture in the galley roof but that would have cracked a 1/4" too. Just layered some filled epoxy on it. 8)

Good luck![/quote]

Good question, and thank all of you for the feedback. Lighter is just easier to handle, but on the other hand it won't change the tow vehicle (which can handle the range of weights under consideration). It might be more of a mental thing - 7 or 8 hundred pounds isn't an awful lot to tow. I was trying to bracket things with a sort of normal approach, followed by a how-light-could-it-be, with plywood and steel. I'm gonna have to mull it around a bit, check with upper mgmt. 1/4 - 1/4 sandwitch would give me no pause, 1/8 - 1/8, I know can be done but 20 years down the road I wonder...I'd like to think that someone else will be enjoying the trailer after we're dust. Philosophically I love light, but tend to err stout. Gotta check some more in the archives. Want to double check single layer construction too, see how competent the panel joints look, and whether or not the exposed stringers would be a problem (visual or real).

This I know - we are seriously stoked!
Thanks,
John

[kennyrayandersen]

lighter is easier to tow, stops better, uses less fuel etc.

(see recent thread on stopping a 1400 lb trailer. If it weighs 500 lb there should be no need for brakes etc.)

[aggie79]

Even though I will probably reach my target weight (dry) of 1000-1100 pounds, I am far enough along in my build to see that it is way overbuilt. Initially, my reference was home/building construction, and what I was doing seemed to be on the underbuilt side of things. However, the deeper I get into my build, the more I see that things could have been done lighter.

One thing I learned from Steve Frederick's CD is to have a good assembly table for putting together your sandwich wall and floor assemblies. By this, he means flat and large. I built an oversized, Norm Abram's style, torsion box table with a surface area of 9' x 4.5'.

Although build #2 will be a standy with about 50% greater height and 10% larger footprint than #1, I plan to target the same dry weight as #1..

[McBrew]

John, since you're talking about keeping the towed weight down... I was just curious: What are you towing it with? A lot of people underestimate what their car can safely handle... especially since most foreign car manufacturers seriously lowball the towing capacity for the US market since we (Americans) tend to be lawsuit happy. For instance, my owners manual specifically says not to tow anything... but the European manual for the same car says I can tow up to 1,900 pounds... and that's with a slightly smaller engine! My previous car (VW Golf) was rated to tow 1,000 pounds in the US, but the same car was rated to tow 2,800 pounds in Europe!

[S. Heisley]

aggie79 wrote:

Even though I will probably reach my target weight (dry) of 1000-1100 pounds, I am far enough along in my build to see that it is way overbuilt. Initially, my reference was home/building construction, and what I was doing seemed to be on the underbuilt side of things. However, the deeper I get into my build, the more I see that things could have been done lighter.

Me too! I suspect it's very common to do that.

[del]

1/8 ply outer skin, with fiberglass covering.


Well over 10,000 miles. Would I build with 1/8 skin again, yes.

del

[GregB]

I've used 1/8" skins on 3/4" framing for my walls and 1/4" Aquatek for the roof on the Empress of the Asphalt (13' long x 5' 3" wide x 5' high) with 1/8" Italian poplar for the ceiling. I have not used any fiberglass. The only thing that concerns me is puncture resistance, and that would be an issue regardless of the skin thickness either 1/4" or 1/8". The key is the bond between foam and skin, it needs to be spot on or you've compromised the shear and bending resistance of the torsion box. I have a couple of areas along that 13' wall of mine that need more glue because the bond isn't what it could be. Still, it is very strong.

Also, 1/8" ply varies quite considerably. Baltic birch is denser with fewer voids and I have that on the outside. Meranti, or lauan, which I'm using on the inside seems to be stiffer, but lighter.

GB