Light weight & off-road?

[M. Olsen]

Ok, here is the situation. I have a 1990 Geo Tracker that does pretty good off-road for what I have in to it. I need an off-road teardrop that would be light enough to not drain the little 4-cylinder of all of its life force.

I've been kicking around the idea of a tear based on the body dimensions of 5' wide, 4' tall, and 8' long. I'm looking to stay under 450 to 500 lbs total as a goal. Any chance of doing it? I was thinking about a nice solid built steel frame. Add an aluminum skeleton for the body. Either 1/4" or 1/8" birch or 1/4" luan plywood on the inside. Fully insulated. 0.040" aluminum sheeting for the outside.

I would use triangulated cross braces in the front and also at the rear bulkhead simular to what I did on the front of my first teardrop seen here.



Also the side walls would be tied together at the front and rear of the countertop by a tube of Aluminum running side to side. I imagine that this should give enough rigidity to keep the sides from racking when traveling down bumpy single tracks and washed out roads.

The aluminum body would bolt down to the steel frame. Either spring or torsion axle, not sure yet though.

If you use the figure of about 135' of tubing for an estimate I come up with the following weights:

1"x1" - 0.062" aluminum tube = 0.2729 lbs/ft = 36.84 lbs
1"x1" - 0.125" aluminum tube = 0.5132 lbs/ft = 69.28 lbs
1"x1" - 0.065" steel tube = 0.8264 lbs/ft = 111.56 lbs

I figure that I would use the heavier AL tube for the outside of the profile, the diagonal braces, the cross braces, and the door jambs. For all other horizontal stringers and verticle studs I would use the lighter AL tube. So if you split the difference then you end up with a body skeleton that would weigh in at 53.06 lbs.

The aluminum sheet weighs in at 0.58 lbs/square foot.
Figure 2 sheets 4'x8' would be another 37.12 lbs.
Figure a sheet 5'x15' to cover the front, roof, and hatch would be 43.50 lbs.

We're up to about 135 lbs.

I'm not sure what the weights for the plywood would be though. Any ideas on how to design the chassis? My thought would be to use 1.5" wide x 2.5" tall - 0.083" tube for the perimiter of the frame and maybe 1.5" x 1.5" - 0.083" for the crossmembers. 1/2" baltic birch floor. Triangulated tongue with a receiver tube for a removable tongue.

Well it's getting late and my ideas are starting to run together. I had better stop and let them collect again overnight.

Let me know what you all think.

Thanks!!

p.s. I have access to a local High School FFA group that puts out some really nice aluminum welding work. With AL I trust them to do the welding better than me.

[dacrazyrn]

You can start looking throuh here to help ya out with alot of that math geek stuff.
http://www.1stconnect.com/anozira/SiteTops/teardrop/intro.htm
The wood weight stuff can be found here (or "Wood weighs about 40 pounds per cubic foot or 40/12^3 = 0.023 pounds/cu in.")
http://www.1stconnect.com/anozira/SiteTops/teardrop/floor.htm
I have to go back a peruse this every once in awhile. Crazy to figure that much....but gets ya where ya wanna be in the end.

[mikeschn]

What will the axle weigh?

I think if you cover the alum frame with aluminum skin, you'd be pretty darn light. Don't even use an interior skin. Just glue some insulation in there. Mike Caddy used that bubble wrap stuff.

BTW, those old modernistics were all aluminum. They didn't have a framed body either. And their chassis was steel angle, pretty light stuff too. But then again, you couldn't take those off road.

Their design was such, that the front part of the body got stress fractures.

I think your headed in the right direction with your off road light weight aluminum teardrop.

Mike...

[angib]

Mike, here's a selection of thoughts in pretty random order:

I'm looking to stay under 450 to 500 lbs total as a goal.... I was thinking about a nice solid built steel frame.

You can have 'lightweight' or you can have 'nice solid built' - mixing the two will get you neither. 'Strong enough' needs to be your goal if you want to build light.

There are no racking forces on a trailer, because they sit on three points (two wheels + hitch) so they can't get twisted like a four-wheeler. There are some small inertia loads that will twist a trailer a little, but the lighter you build, the smaller those loads are.

A sheet of thin ply is a superb anti-racking structure - your diagonal struts won't do any harm, but they're just extra weight that's not needed. The critical thing is to make strong joints along the edges of the plywood sheets - the aluminum tubes will do that, though maybe not as well as wood would. Interestingly, your lightest aluminum tube is exactly the same weight as 1"x1" solid wood.

I think your aluminum tube body would probably work as well as an all-wood body, be as light as an all-wood body and be no more than twice the amount of work to make.

Your frame tube sounds a good choice but I would have to question what all the cross-members are for? Does 1/2" ply need any support if you're only going to lie, not dance, on it?

I'm not 100% sure what your tongue design looks like, but it sounds like the heaviest possible way to connect the trailer and the tow vehicle. The lightest/strongest is a simple A-frame - that's why all those trailer manufacturers use it.

Andrew

[jagular7]

I too am an avid wheeler and do take it back to the woods, even with the td I currently have. Though the road chosen could be driven by a minivan.

For what you are entailing, no one really knows the limits of any td structure as, AFAIK, no sides or the roof have imployed. So to state, these structures here are structurally sound. Some have lightened the overall weight quite a bit with a sandwhich side design, but they are still standing. But, there has been some tongue instances where the material cracked or broke near the front crossmember on a single tube tongue.

I like your ideas and plans, and I too have plans for a higher ground clearanced off-road capable td. I suggest that you start off with a square foundation, build your outer structure and then triangulate your inner substructure. Then test it's rigidity through push/pull on the axle location pivoting off the tongue. With string/cable and a fish scale, you can determine the amount of 'twist' in the structure you outlay. The more square it is, the more deflection it will have at the straight corners. So, I suggust a wedge roof design to a sloping front and back with minimal height straight sides.
Watch out for your unsprung weight?

As seen here, personal tastes intrigue others.

[M. Olsen]

'Strong enough' needs to be your goal if you want to build light.

Point taken. I guess I'll have to do some thinking and define what strong enough is. Remember that I'm not an engineer, so I have a tendency to overbuild things and this project will go against those instincts. I just don't want to go too light or not strong enough and have it fail at the most inopportune time.

There are no racking forces on a trailer....

If I can entice a Grizzly bear to place his paws on the top of the trailer and push would I then have a racking force?

A sheet of thin ply is a superb anti-racking structure

So what would be the best way to attach a thin sheet of plywood to an aluminum frame work? The Redneck Trailer Supply catalog lists a 3M tape called VHB Series 4611. I think that it would be outstanding in this application if you can get past the price of the tape. More information here.

Interestingly, your lightest aluminum tube is exactly the same weight as 1"x1" solid wood.

But which would be stronger?

...and be no more than twice the amount of work to make.

Ahh, that's where the skilled labor force comes in, the FFA students.

For my tongue design I want the flexability of an adjustable / removable tongue. I know that this will add a substantial amount of weight to the front, but that might be one of the compromises that I would have to live with. I agree that a simple A-frame would be light and strong which goes back to using geometry in stead of just sheer mass.

Andrew, thanks for the comments and insights. I like this type of discussion because it provokes mental gymnaastics on my part.

More to think about before I fire up the CAD program.

[M. Olsen]

Hey Mike I'll have to pick an axle and see what it's weight is. I'll also have to figure the weight of the wheels (aluminum), the tires (tall and narrow?), springs etc... Also thanks for the words of encouragement.

I like your ideas and plans, and I too have plans for a higher ground clearanced off-road capable td. I suggest that you start off with a square foundation, build your outer structure and then triangulate your inner substructure. Then test it's rigidity through push/pull on the axle location pivoting off the tongue. With string/cable and a fish scale, you can determine the amount of 'twist' in the structure you outlay. The more square it is, the more deflection it will have at the straight corners. So, I suggust a wedge roof design to a sloping front and back with minimal height straight sides.
Watch out for your unsprung weight?

jagular7 do you have a sketch of what you propose? I'm having a tough time converting it from words to picture in my mind. The unsprung weight could be reduced by using a torsion axle in place of a spring axle, using aluminum wheels in place of steel wheels and using a 32x9.50-15LT tire in place of 32x11.5-15LT's. (Tire size for illustration only.) I imagine the concern would be that of having a hi unsprung weight assembly working against the the light weight of the trailer body when going down bumpy, washboardy trails.

More to think about.

[Dave Nathanson]

Have you considered borrowing someone's flat trailer & piling various amounts of sandbags on it to see what is a good weight for your Geo to pull?

Sort of like building it in cardboard first...

[jagular7]

....snip....jagular7 do you have a sketch of what you propose? I'm having a tough time converting it from words to picture in my mind. The unsprung weight could be reduced by using a torsion axle in place of a spring axle, using aluminum wheels in place of steel wheels and using a 32x9.50-15LT tire in place of 32x11.5-15LT's. (Tire size for illustration only.) I imagine the concern would be that of having a hi unsprung weight assembly working against the the light weight of the trailer body when going down bumpy, washboardy trails.

More to think about.

I'm no arteest, but think of the Airstream or better yet, a Scamp all fiberglass camper. The sides are straight but then slope to a roof line. The curvature of the fiberglass gives it its strength. Square sides and top are easy to structure in a single plane, but add in forces from the other planes, then you have deflection. Triangulating between the planes ties the forces of compression and tension and displaces the forces over a wider area. You got the same understanding of that for the front tongue, just add another working plane to the mixture for the cabin structure.
You really can't do much with a square box, but you can round it out to a point of lightness. Finding the limits of rounding vs lightening the structure is a science in itself.
Others here have built many different tds. Some start with a full sheet side. Then the next build is something similar but lighter with a frame sandwhiched with inner/outer panels. Experience is the key.

[angib]

If I can entice a Grizzly bear to place his paws on the top of the trailer and push would I then have a racking force?

Sure, though it's likely the bear's paws will punch through the plywood long before the whole box collapses - it's often local strength/toughness that determines ply thickness, not overall strength.

Not enticing the bear would be another excellent design choice.....

So what would be the best way to attach a thin sheet of plywood to an aluminum frame work?

Epoxy glue would be very good and cheaper than VHB tape, though you would need to think of some way of clamping the ply against the tube until the glue sets. A pop rivet every 4" or so would do it, but they might pull through the ply.

However VHB tape is the absolute dog's bollocks (this is a technical term, not offensive swearing...) and very easy to use. You might ask a local adhesive supplier if they have a cheaper alternative - I used to buy tape for work at half the 3M price.

Andrew

[Gerdo]

After thinking about it I would think a TD gets more forces on it driving down the highway at 70+ mph than off-road. You are right, there probably isn't much racking force put on a TD because of 2 wheels and one hitch. Even with all the curves of a TD it is still punching a hole in the air going down the highway. Hold your hand out the window driving down the highway.

[Dave Nathanson]

I would think a TD gets more forces on it driving down the highway at 70+ mph than off-road.

Oh, I dunno! I was offroading today and hit a rock alongside the trail. It was a ramp shaped rock, hidden behind a shrubbery, at a trails corner. This trail had gotten so tranquil that I was mesmerized by it - until my reverie was jarred loose! The driver's side of the TD Trailer jumped at least 2 feet in the air!

As I hit the bump, I looked in my side view mirror & saw nothing - then saw the TD slam into view as it landed. Yikes! Luckily no apparent damage.

So I would have to say that in my viewpoint the TD is subjected to a lot more "forces" while offroading than merely parting the air down the super slab. Perhaps an engineer can confirm or deny.

[jagular7]

Dave remind me not to follow you too closely.

How did the Jeep handle the bump?
Think shocks and softer springs would have helped with the TD bouncing high?

My campers have always used torsion suspended axles, but my utility trailer uses 3500#s leafed axle. I do notice a vast difference in how they pull on dirt roads as well as paved. I think the utility trailer needs to be loaded to be comparable to the camper. I've varied tire pressures to help compensate an empty utility trailer.

[Dave Nathanson]

The Jeep did a similar hop over that rock.

The TD does have shocks, but maybe the leaf springs could be softer. I was aired down on all 6 tires to 16 psi or so.

What kills me is that I hit the very same rock the day before!

[M. Olsen]

Oh, I dunno! I was offroading today and hit a rock alongside the trail. It was a ramp shaped rock, hidden behind a shrubbery, at a trails corner. This trail had gotten so tranquil that I was mesmerized by it - until my reverie was jarred loose! The driver's side of the TD Trailer jumped at least 2 feet in the air!

As I hit the bump, I looked in my side view mirror & saw nothing - then saw the TD slam into view as it landed. Yikes! Luckily no apparent damage.

I think that in a case as Dave experienced above there would have to be some sort of racking forces exerted on the body of the trailer as it comes down and bounces from side to side. But then again I might be way off of my rocker too.

I would think that the suden directional change from side to side that the trailer experiences would lead to significant forces being exerted on the body, especially at the roof line.

My understanding is that the further away from your solid frame you go, the more leverage there is to be exerted on the joints that I have made in the trailer body. These forces would increase in proportion to the mass of the trailer. Decrease the mass and you decrease the momentum of the parts of the trailer body as they are subjected to the sudden changes in direction. Triangulated braces in the front of the trailer and at the rear bulk head would decrease the effect of these forces.

The other question is could you build a trailer with an aluminum frame work for the body that would be strong enough to negate the triangulating braces and still keep the weight down to the absolute minimum.

For the record my little tracker can tow my first teardrop which weighed about 1200 lbs but the engine and clutch didn't like it. It can tow my current teardrop but the engine still isn't happy. Hey I have about 75 to 80 HP to work with here so keeping the trailer weight to a minimum would be of highest priority. Any body have any really good, ground breaking, light weight construction ideas that need to be tried? Has anybody in recent forum history built a tear with an aluminum skeleton for the body?