frame strength, and how to predict need / results

[mdk]

I've spent a lot of time digging through the forums here and as of yet, I have found no real information on frame strength - as in, how to determine what you need - and then figure out the materials to meet that need.

There's a number of sites that have such things as calculators to determine how much "give" a specific specific piece of metal will have, given specific forces and points / measurements, but nothing that says how much 'give' is too much. Nor, how to take design and what you put on top of the frame and/or attach to it into consideration.

Weight is a huge concern for me, so not overbuilding the frame (not to mention cost) is important.

The first thing of importance is what you expect it to do - and that, for me, is to hold the "box" i'm building off the ground. The second is to make the floor strong, and the third is to add to it's strength in terms of stresses of being towed on rough roads - or least not be detrimental.

Has someone collected this stuff into a specific sticky and I haven't found it yet? Or (as I'm beginning to suspect) since most people are building teardrops of similar size and weight, they simply use previously proven designs? Nothing wrong with using what's proven rather than reinventing the wheel, but I'm not intending to build a teardrop, nor use it the average way.

Just wondering...

[KCStudly]

...I'm not intending to build a teardrop, nor use it the average way.

This last bit is the most crucial part to answering your question! We need more information. If not a TD, just what do you plan on building and how do you plan to use it?

Most all of the small campers built here rely on the composite strength of the complete structure, primarily the depth (height) of the side walls and cross bracing from the bulkhead and front walls for the majority of their integrity. The box provides the strength. Build a strong box and you don't even need a proper frame, just solid attachments for the tongue and axles (search ultralight).

For off road use or utility trailers things change. For my pseudo off road build the cabin sits on top of the full perimeter box tube frame. The frame is more there to protect the lower edges of the cabin from damage due to contact from rough terrain, rather than to strengthen the box. The box is already rigid as heck and it isn't on the trailer yet, and doesn't even have the roof on yet!! Don't believe it? See Aggie79's Silver Beatle build. He has a pic of his cabin as it is being built sitting on three jack stands located at the very front and rear. He was able to sit in the doorway with no perceptible sag! Think of the side walls as big tall I-beams that hold the trailer frame up, and the trailer frame as a secure place to attach the axles and tongue, rather than the frame holding the cabin.

The curvature in the roof adds a lot of structural stability, too, adding stiffness in multiple planes, so that is another plus for more traditional builds.

Utility trailers with no box, and cargo trailers with flat roofs, no rear bulkheads and large opening rear doors, not to mention higher load expectations, can't be built as lightly.

Tell us more about your plan and I'm sure we can help.

[mdk]

...I'm not intending to build a teardrop, nor use it the average way.

This last bit is the most crucial part to answering your question! We need more information. If not a TD, just what do you plan on building and how do you plan to use it?

A mostly frameless 'box' made of "SIP" construction - that is, foam bonded to ply - top, bottom, and sides. external dimensions of 78W X 80H X 172L, with a blunted V added to the front of 36in.

It won't be a just a hollow box with stuff sitting in it and the ends will be structural as well. All the cabinetry will be designed and installed to add structural strength and add to the vertical strength of the walls, to keep the top straight up, if you understand what I mean.

Most all of the small campers built here rely on the composite strength of the complete structure, primarily the depth (height) of the side walls and cross bracing from the bulkhead and front walls for the majority of their integrity. The box provides the strength. Build a strong box and you don't even need a proper frame, just solid attachments for the tongue and axles (search ultralight).

Yes, I understand that. And the fact that I'm not building a massively structural box, means that the frame has to have some strength of its own.

For off road use or utility trailers things change. For my pseudo off road build the cabin sits on top of the full perimeter box tube frame. The frame is more there to protect the lower edges of the cabin from damage due to contact from rough terrain, rather than to strengthen the box. The box is already rigid as heck and it isn't on the trailer yet, and doesn't even have the roof on yet!! Don't believe it? See Aggie79's Silver Beatle build. He has a pic of his cabin as it is being built sitting on three jack stands located at the very front and rear. He was able to sit in the doorway with no perceptible sag! Think of the side walls as big tall I-beams that hold the trailer frame up, and the trailer frame as a secure place to attach the axles and tongue, rather than the frame holding the cabin.

Well, I have the same idea, except that size and shape, along with the need to reduce weight (and keep center of gravity as absolutely as low as possible) means it won't be quite as strong as that - or at least I don't expect it to be.

The curvature in the roof adds a lot of structural stability, too, adding stiffness in multiple planes, so that is another plus for more traditional builds.

yeah, in this case, no curved roof - though I have thought about making the last 4-5 feet feet slope downwards some, as it's nothing but the bed and dinette. You'll never stand at all in the last 3 feet toward the back.

Utility trailers with no box, and cargo trailers with flat roofs, no rear bulkheads and large opening rear doors, not to mention higher load expectations, can't be built as lightly.

Think of this as shaped like a V-nose cargo trailer, but no big end door and with some partial bulkheads near the center.

My start to this is to use 1.5w X 2h x 16 ga perimeter tubing, plus 1x1.5 x 16ga laterals, except most of the front which will have 2x1.5 x 1/8 or heavier. two full length 3x3 x 1/8 beams 14 inches in from the edges, boxed in back and front, ( a 4x4 sheet metal box for storing sewer hose as a "bumper").

Then, 2W x 4H X 72 long X 3/16 members under the full length beams to which are attached the suspension. They, too, to be gusseted/boxed to prevent lateral deflection.

I calculate this, with tongue of 2x3 heavy tubes in a shallow V (tongue extends out 5 feet or a little more from the "box" of the trailer to add sharp turning ability) but no suspension attached, will weigh under 400 lbs. The whole thing was conceived to add strength to a 3/8 ply (top) X 1.5 foam (middle) x 1/4 ply (bottom) floor, by turning everything into a mostly cantilevered beam, none of which have more than 250 lbs at any point under any condition. The main beam is light, while the suspension load is spread across 6 feet of it by two extremely strong beams with adequate thickness to get really good welds for suspension mounts.

The gray and black water tanks will be boxed into the laterals and touch the floor itself, for maximum ground clearance.

By using 205/65-10 wheels/tires (tandem axle) the entire tire will go UNDER the floor with no wheel well, with a "bottom of the floor / top of the metal frame" height of 25.5 inches and with the tanks tucked up as far as possible, I the lowest point should be 9in and be the axle itself, the next lowest be the tanks at about 14 - 15 inches, with the biggest / lowest being mostly forward of the front axle. I want to be able to navigate logging roads, back roads in AZ and UT and NV without dragging or busting the plumbing. Even my truck won't have quite as much axle clearance at its lowest point, as the trailer will.

Tell us more about your plan and I'm sure we can help.

Hope this helps.

[bdosborn]

Good information in the design resources at the top of the page:

http://www.angib.pwp.blueyonder.co.uk/teardrop/tear84.htm

You want to avoid this:



Bruce

[KCStudly]

That all sounds very reasonable to me!

[mdk]

That all sounds very reasonable to me!

yeah, but "sounding reasonable" and being adequate... Well, those two things aren't necessarily the same. Nor is it obvious I'm not overbuilding. So... I'm looking for more resources.

[KCStudly]

Sure, you could do a basic structural analysis, even FEA if you have the right software, but those are only as good as the assumptions that you use for loading. Even if the static loads are fairly well defined, the dynamic loads will always be a variable thing. How big was that pothole? Did we really just drive over a large tree branch? Drag the tail thru that gully?

Don't underestimate the value of TLAR engineering. It is rooted in past experience and comparison to proven designs. $.02

Wish I could be more helpful to you.

[Shadow Catcher]

I have often said you are building an aircraft not a tank, But! CR has an all aluminum frame and is the seventh and last trailer made by a company. We owned the first one and traded it in for the seventh. They did not do the engineering well and #1 always had a rather bouncy bendy ride in the tongue. The second owner had a catastrophic failure, at low speed fortunately, and the tongue broke completely. #7 also disturbed me from day one with the amount of bending and bouncing. Aluminum has a finite modulus of elasticity and unless over built in critical areas will not survive. However if done right i.e. DC3 aircraft sill flying after 60+ years.
Steel is more forgiving but as you can see from the above picture, not enough of it can be fatal, reinforcing the critical areas of the tongue and extending it under the frame a bit if it is not sufficiently strong to begin with will prevent failures.

[angib]

Think of the side walls as big tall I-beams that hold the trailer frame up, and the trailer frame as a secure place to attach the axles and tongue, rather than the frame holding the cabin.

That is so true - I think most teardrop builders think their frame is stronger than their cabin when, with good building (joints), the reverse is true. Lots of folks are convinced that an item can only be supported by something underneath it, not something above it - I don't know how they pluck up the courage to cross suspension bridges.

mdk, I can't work out what you are proposing from your description - except that it sounds mighty complex, which is rarely good structural practice. A sketch showing the frame design (a photo of a hand-drawn sketch is plenty good enough) would enable better feedback - a sketch of the trailer 'box' design would also be useful, to see how self-supporting it is likely to be.

Here is an article about an all-plywood race car trailer, where the only steel isn't holding up the trailer! Race car trailer

Here are some photos of that trailer:

[mdk]

Here's a crude drawing of the frame...



The thicker lines are the heavier and stronger parts.

[KCStudly]

Cross members every foot are unnecessary.

[mdk]

Cross members every foot are unnecessary.

Changing them to a wider spread will, at most, save perhaps 20-25 lbs and 30 or 40 bucks and may result in a "squishy" floor. I want them under every plywood joint in the foam/plywood floor, which basically means I do 24 or 12 inch... Which would you do?

If memory serves, the stuff I'd be willing to use for the perimeter and crossmembers, event at 12 inches, totals about 110 - 130 lbs, depending on dimension choices.

[KCStudly]

Mine are 16 OC with 5 mm upper and lower skins on 2x cedar subframe and 64 inch wide. Granted, mine is not a standy, but I think your thicker skins will cover you there. When I dry fit to the trailer chassis it seemed more than adequate and I've had no issues walking on mine (with a loose piece of 1/2 ply down as a protector) during construction... but then again, I do have a temporary strong back cradle under there for now.

With 16 OC three bays gets you to 48 inch (remember to stagger/shorten the first space so that your seam lands on the center of the third).

As you probably already know, the key is to fully glue the inside faces of the panels to the foam core.

To put it into perspective, consider a hollow core door and imagine it with thicker skins. I bet you could lay one down, support it horizontally at the long ends and walk on that no problem.

Can't have it both ways. Either build light or don't. Every extra 100 pounds, or 35 pounds, or pound adds up by the time you get to the end. Just trying to help you achieve your stated goal.

[mdk]

Mine are 16 OC with 5 mm upper and lower skins on 2x cedar subframe and 64 inch wide. Granted, mine is not a standy, but I think your thicker skins will cover you there. When I dry fit to the trailer chassis it seemed more than adequate and I've had no issues walking on mine (with a loose piece of 1/2 ply down as a protector) during construction... but then again, I do have a temporary strong back cradle under there for now.

With 16 OC three bays gets you to 48 inch (remember to stagger/shorten the first space so that your seam lands on the center of the third).

As you probably already know, the key is to fully glue the inside faces of the panels to the foam core.

To put it into perspective, consider a hollow core door and imagine it with thicker skins. I bet you could lay one down, support it horizontally at the long ends and walk on that no problem.

Can't have it both ways. Either build light or don't. Every extra 100 pounds, or 35 pounds, or pound adds up by the time you get to the end. Just trying to help you achieve your stated goal.

Yeah, I know. This is lighter by far, than my original design. However, it does have somewhat conflicting goals... It's to be light AND very durable, so it can be towed on back roads and washboard, etc. It was my intention that the floor actually have no internal frame at all - just ply and foam, and cleating to which the walls will be fastened. This may result in perimeter hardwood (or other high durability wood, perhaps plywood strips glued up to be 'hard to strip' material to run screws into) 1.5X1.5 in functioning as a glue block and screw target. It would only be at top, bottom, and ends of the walls and floor, no actual internal framing (except for doors and windows) as the entire floor, entire roof, entire walls would be constructed of overlapping and fully glued together foam and plywood with no joint less than 12 inches from any other joint in the layers. It could be done on 16's, of course. I'll consider it. It would mean there's a seam in either the top ply, bottom ply, or foam at every crossmember.

I have not, however, worked out an elegant way to have the floor fastened to the frame. With only 1/4 in ply on the bottom, screws are not going to be all that strong. At the perimeter, there'll be excellent and deep material to screw to, but the frame's tube, meaning the screws would be crushing the tube... Again, not good. Perhaps it can be epoxied to the crossmembers and screwed to the perimeter frame.. or make the perimeter angle, not tube...

[KCStudly]

You may want to take a look at my build. I have solved many of these issues.

You will need to put blocking in where you want to attach stuff (cabinets, bed frame, interior lights, cook hooks, magazine racks, etc. etc.

It can be a tedious process and you have to make a bunch of decisions up front, but when you do you can avoid a lot of extra wood, especially in the walls.