Custom Frame Question

[Davagio]

I am needing some help deciding what tongue configuration would be best......The only thing different about these are the tongues. All measurements are the same on both.

By the way the gray area is the dropped floor, and thats why i would like to shorten the distance from the second crossmember to the the first crossmember from 18 to 14, so i can have more room in the drop floor. How much can u shorten this and still be safe?

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[angib]

The left one is better than the right, as at least the diagonals connect to the main side rails. The centre tongue member isn't adding much strength as either 18" or 14" is a very short distance compared to the tongue length. But there's no harm in it being there and it is an easy place to bolt the coupler to!

So, think that the two diagonals are what is really holding the trailer up, and the centre tube is just decoration, and choose the diagonals accordingly. There's certainly no point in making the centre member bigger than the diagonals. If you size the diagonals to provide all the strength, reducing the 18" to 14" is fine.

PS Sorry - I just have to write 'centre'.

[ctstaas]

Hi Davagio,
I agree with Angib about liking the drawing on the left. In addition, with a strong enough front cross member and diagonals, the interior cross brace could be eliminated completely.
Think about a triangle with the two tires being two points and the hitch being the third point. How much weight can you put in that triangle and when it bends, where will it bend? Even immaginging destructive testing is fun for me.
The part or the frame past the axle acts as a cantilever to the triangle edge between the tires.
The cabin can be used to strengthen the frame so plywood reinforced steel can be/ needs to be how strong? So many things to consider about so many things. Good thing there are so many experts from so many fields who are willing to add their two cents on this forum.
I didn't know of this site when I built mine and I only have a couple of things I need to fix, so starting here is the real expert move.
Enjoy this part and take lots of pictures, because the actually using it part is even better.
Chris

[Davagio]

So, think that the two diagonals are what is really holding the trailer up, and the centre tube is just decoration, and choose the diagonals accordingly. There's certainly no point in making the centre member bigger than the diagonals. If you size the diagonals to provide all the strength, reducing the 18" to 14" is fine.

Hey thank you very much for your input i think i will go ahead and take that advice while the trailer is being built. We actually just cut all the metal today and will hopefully have it all welded up this weekend. I did have one other question that i failed to ask in my post. I figured i had better ask prior to completion; If you look at my drawing you will see a thin red line. That red line is the axle and and the black bar in the middle is the cross member. Do you foresee any issues not having a cross member above that connecting the top of the wheel wells (of course that will cut into the drop floor which i dont want but i also dont want the frame to be weak there. Second thought let me just post a picture of what i mean exactly it will help!

[Davagio]

Hi Davagio,
I agree with Angib about liking the drawing on the left. In addition, with a strong enough front cross member and diagonals, the interior cross brace could be eliminated completely.

When you say interior cross brace i assume you mean the second one correct? I dont know how safe i would feel completely eliminating that second cross member?!?!?!?!? I would love the extra room for the drop floor but im not quite sure on that. Because the second cross member would essentially be in the middle of the wheel wells....about 4.5 foot away. I guess the drop floor would support a lot of the front half of the trailer. Interesting thought though and i appreciate it. I need some convincing on that because i just have no idea.

[Davagio]

This is what i meant about the cross member around the wheel well area, i am guessing that the one on the right will be stronger but hoping the one of the left will still work effeciently. The second eliminates at least a foot of room in the drop which i dont want...............any input?

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[angib]

Yeah, the one of the right is better, but the one on the left should work fine. It's hard to be dogmatic about this as you're into some really detail structural questions here.

Fundamentally, trailers break their tongues if the frame breaks anywhere. There are some travel trailer designs that have broken their frames near the axle but usually only in response to some nasty events, like jacking the whole trailer off the ground on its front and rear ends.

[KCStudly]

It's hard to be dogmatic about this as you're into some really detail structural questions here.

This +1.

Depends on many factors including the section shape and size of materials; the weight of the loaded camper; the structure of the cabin and how it is attached to the frame; the materials used to frame and build the drop floor, and how that contributes structurally to the chassis. This simple question that you ask is not so simple.

It can be done the way you want to, if done properly, but don't just assume that "it will be okay" because someone casually said it was okay. The thing to remember is that a soundly built cabin is more rigid than the frame it will be attached to. Build the cabin light and strong as a unit, attach it to the chassis well, and it will likely be good.

This is also why the tongue to main section joint is so critical. The cabin is rigid and the tongue can flex, so all of the bending happens like a hinge at the front xmbr. That's your main consideration. That and the axle attachment points.

I'm not a big fan of the interrupted main rails. What suspension style do you intend to use and how will it be attached? How will these loads be transferred from the inboard frame sections vertically into the walls of the cabin? Keep in mind that straight lines and triangles are stronger than interrupted paths.

[Breytie]

Please do not take what I am about to say as a personal attack or as being nasty or even being thankless. So, if you had a bad day so far, please do not read further.

I think herein lies the problem to us noobs, especially those that cannot use standard off the shelf trailers nor tank transporters as chassis: nowhere are there "tried and tested" or "known good" recipes for chassis, nor any basic guidelines as to what is good and what not. Everywhere we run into "depends on ", "properly", "light", "strong", "attach well" and "overbuilt". We do not know what all that means!

Please tell us what is good and what not and where necessary how to check, test or measure for it.
And do top it all off of with some disclaimer about cannot be guaranteed or whatever is deemed appropriate, but please give us some rules or guidelines.

We depend on the seasoned builders and experts out there to guide our foray into the dark world of TnTTT construction. Please understand we are blind people groping in the dark for solutions.

[KCStudly]

No ire at all. Just trying to help. You said it yourself; you are steering away from a cookie cutter build, so you will be relying heavily on the TLAR method.

Even if someone were to jump out and say, "I guarantee you 100/ct that you will get exactly what you are asking for if you do it the XYZ way", it is still going to be your responsibility in the end. You have to approve your own design, you will be the one building it, you will be the one doing quality control, testing and using it; so forget about shifting any of those responsibilities. Sorry if that is not what you wanted to hear.

GIGO. You are asking a complicated question and are hoping for a simple yes/no answer. It isn't going to happen without more detailed information and discussion.

What shape and thickness metal do you intend to use for the frame? If you are trying to build super light, you are already heading down the wrong path with that frame layout and foot well, but it does not have to be a tank if you use good techniques and don't overdo it on materials.

Do you plan to have cabinets or other internal structure over your wheel wells that will help triangulate suspension reactions to the side walls? Believe it or not the a trailer frame is a relatively 2D structure and can bend a lot (relatively) in the flat direction. The floor panel is similar. It is the side walls and vertical structure in the cabin that stiffens the whole thing up by adding rigidity in the vertical direction. By applying suspension loads in under the "open" area of the floor away from the walls you might get flex if not properly braced (can still be done relatively light weight using smart materials and good geometry).

What do you plan on supporting the foot well with (plywood, steel, hardwood corners, etc.)? This added depth in the vertical direction can be used to your advantage in transferring some of that flexing load outward to the side walls. Think of the rear of your foot well near the axle and infront of the wheel wells as mini bulkheads. Build and tie those in structurally and it will add a lot of strength across the frame w/o adding much weight at all. Conversely, if you waste the opportunity the foot well could add weight without adding much structural benefit, and you will end up with a heavier assembly and/or less strength.

It does not have to be over built. Use what you are planning to your advantage and the sum of the parts will create strength w/o adding more weight. Trying to build a frame that will support everything, then building a cabin that will support everything on top, then attaching everything to it is a heavy approach. Think of the whole thing as one big structure all tied together, then build it accordingly light, with tight strong joints and good structural geometry and you may achieve your goals.

<Sarcasm Alert> On the other hand, very few builds ever fail so catastrophically as to be a total disaster, so you will probably be okay no matter how you build it.<end alert> I'm just trying to suggest that it could be stronger and lighter if you consider beyond the question that was posed.

Peace.

[Davagio]

You are asking a complicated question and are hoping for a simple yes/no answer. It isn't going to happen without more detailed information and discussion.

Actually i was the original poster of the question, and i think he was merely saying it would be nice to have some guidelines and simple responses that way we can understand them.

What shape and thickness metal do you intend to use for the frame?

The metal i will be using is 2x2 tubing 1/8 inch thick for the entire frame and tongue. The drop floor will be made of 1x2 tubing also 1/8 inch thick.

Do you plan to have cabinets or other internal structure over your wheel wells that will help triangulate suspension reactions to the side walls?

the bed frame will be the entire length (from wheel well to wheel well) wall to wall, so i am hoping that will suffice.

What do you plan on supporting the foot well with

Plywood with hardwood over it (im guessing you mean the drop floor).

It does not have to be over built.

Actually if i had to re-buy the materials i would have gone a lot lighter especially with the cross-members as i said everything on it it is 1/8 (11guage). The frame, tongue, drop everything and it also weighs around the 500lb ball park with the wheels an axle.

[Davagio]

I think herein lies the problem to us noobs, especially those that cannot use standard off the shelf trailers nor tank transporters as chassis: nowhere are there "tried and tested" or "known good" recipes for chassis, nor any basic guidelines as to what is good and what not. Everywhere we run into "depends on ", "properly", "light", "strong", "attach well" and "overbuilt". We do not know what all that means!

I understand your pain brother. I think the frame construction seems like the most nightmarish part of this whole experience. I have refurbished an older trailer and the countless problems i encountered with that seem no where close to trying to figure out this frame and the many questions a person may have about it. Now that i have all the metal, the rims, tires, etc and its being built i would like to redo about 3 different things lol! Learning from my mistakes is what im figuring out now.