Building my own trailer - need some advice

Most of the posts in this thread are giving good advice but not explaining the reasons for that advice. I know that telling me to do something just because I said so don't work, I need facts. I will try to explain some of the reasons for some of the issues that have been brought up.

A few thoughts from experience learned the hard way.

Electric brakes are great. I'm glad that you were on board with that from the beginning. When you get the brake controller for in the truck, make sure to get a progressive one. The progressive controller is able to sense how hard you are braking and modulate the power to the trailer brakes to keep everything in balance. The other type is just on/off. full brakes or no brakes, not real pleasant to use and hard on the both the truck and trailer with all of the yanking for and aft. Once the controller is properly adjusted I could actually stop faster with the RV hooked up than the truck by itself. Yet with it being progressive if I touched the brakes lightly just to slow down a bit I did not get the feeling the trailer was trying to pull the back of the truck off. A good controller will make the trailer feel like it is not even there as far as braking is concerned.

My first heavy trailer was a home-built car hauler. It came with the '69 Catalina that was on it. On the way home I learned real quick about trailer sway. At 35 MPH the trailer and my 1 ton full size 12 passenger van were going back and forth with the inside tires leaving the ground by about 18 inches, real close to going over. I knew better than to hit the brakes and I just let off the gas as and hung on tight until it slowed enough to get back under control. That trailer also had NO brakes, it was a challenge to bring it to a stop. The car was bound to the trailer by the tires, That meant the car was free to bounce around on its suspension. Every bump in the road was an adventure with the weight moving around even just a little. Water sloshing around will be a lot worse.

As for single vs dual axle. The laws of physics cannot be altered.
With a single axle when it encounters a say a 3 inch bump or hole it will follow the terrain. With a dual axle the fulcrum is located halfway between the axles. This means that when you go over that 3 inch bump or hole the front axle will move up 3 inches but the trailer will only move up 1.5 inches. This means that the load on the trailer and the welds and structure of the trailer will get exactly 1/2 of the abuse as they would have gotten from a single axle. Because of that same sharing effect weight distribution is a lot less critical on a 2 axle than it is on a single axle. You will also notice a big difference going up or down hill as with the high center of gravity will cause a significant weight shift to the front or back. With the load being water, this will be greatly amplified.

The load bard that have been referred to are more often called a weight distributing hitch. Many weight distributing hitches also come with an anti sway device. Since you will be hauling water that will be moving all over and messing with the weight and balance and center of gravity of the trailer I would absolutely have the weight distributing hitch with anti sway. When I towed my 6500 lb rv home after buying it, it came with a weight distributing hitch but my truck was not ready to accept it so I just used the 2" ball. It was not a pleasant ride home. that trailer fought me the whole way. Was way to much tung weight so the back of the truck was sagging which takes weight OFF of the front tires and I could feel that in the steering.
Depending on which hitch you have on your truck you need to look at its weight ratings. You want 10 to 15% of the trailer weight on the tung. 10% of 8000 lbs is 800 lbs, I'll bet that is more than the max tung weight of your hitch, That same hitch may have a 1000 lb tung rating with a weight distributing hitch.

Everything I have said is amplified on country roads and dirt roads that you have stated you will be traveling. Dirt roads are anything but smooth.

That water sloshing around is a big deal. my RV is 6500 lbs. has a 50-gallon fresh water tank. On the way home from my first trip I could feel that half full tank sloshing around until I stopped and opened the drain and left it on the side of the road. As you mentioned water is cheap. And that was just ~25 gallons in a 6500 lb trailer. Please do not under estimate wat 5000 lbs of shifting weight will do to an 8000 lb trailer connected to a 6000 lb truck. Yes the truck is rated to tow that but the trailer will still be heavier than the truck. Sometimes the tonnage rules saying is true.

I am not trying to tell you what to do. just trying to explain some of the things others knew were better but did not or could not explain why.

I'll shut up now. my fingers hurt from talking this much....
 
Last year I bought a 5x8 utility trailer and upgraded the heck out of it while teaching myself how to weld. Well now I enjoyed that so much I want to try something bigger.
I plan on building a pressure washing, hydroseeding, and fire fighting trailer unit, gonna have a skid mount hydroseeder, a big pressure washer, a water pump, and 2 ibc totes full of water.

The majority of the weight on the trailer will be about 5500lbs of water when its full (assuming 2 full 330 gallon ibc totes), and that plus all the equipment I figure to be safe that's about 7000lbs total, so I'm thinking an 8000 or 9000lb axle.

With leaf springs do I get them by the weight rating on the spring or on the axle? Would I get 2 8000lb springs for a trailer with a 8000lb axle, or would I get 2 4000lb springs for it? What do y'all suggest for the main frame / body of the trailer? Is C channel enough or should I go for square tubing? What thickness? I'm really new to metalworking in general and want to make sure to overbuild everything for safety since I'll be driving it on the road. If I need 3 1/4" angle iron braces I'll put 4 3/8" braces...

Is there some guide or source material I can read that will teach me how to calculate the weight a given profile needs to permanently deform? I want to learn how to pick/calculate what thickness material I need for a given project.
I have found that often a piece of equipment can be bought cheaper than shop built. IMHO, if you can find a factory trailer that meets you specs, this will be the better route to take. They are buying all the materials wholesale, building on a jig, and do it in a fraction of the time it will take to do a one time build.

If you do decide to build it yourself, I’d use box tubing and seal weld every joint and cap every end. No water or dirt can get inside or between metal, preventing rusting from the inside out.
 
Whenever I build a trailer I start with a subframe that holds the springs and the axles. Then I build the trailer frame with parallel sides so that the subframe can be located where ever it needs to be for a proper trailer CG. I use clamps to hold the subframe until I obtain proper balance and then bolt or weld the subframe in that spot. Don't be afraid to use heavier axles and tires that you think you need. They have to carry the trailer too and going too heavy does no harm. Always use disc brakes on every wheel. A lot of guys use box tubing but I use C-channel exclusively. Trailers rust from the inside out and C-channel has no inside. You can see every bit of it. You can paint every bit of it. The trailer in this picture is 20 years old and sees plenty of salt water. I paint it every 5 years. Good luck with your project. .
 

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i believe you will find that receipts are required to prove you paid tax to the state

if you were to build a custom car or bike you are required to present receipts to show tax was paid
Yes, taxes are probably the main reason for the requirement, I should have known better...
 
Someone who admits they don't know how to design a trailer, and who also admits their welding skills "aren't up to the task" has no business building a trailer that will ever see a public road. When that person also says they're not trying to make it perfectly safe in every situation I'm even more worried.

Aside from stability, load rating of the tires and the ability to not crash if a tire goes flat, there's another reason for dual axles on anything 7K on up....twice as many brakes. One set of trailer brakes isn't going to cut it for a trailer in the 8-9K range.

As someone else said, this is a disaster waiting to happen.
 
Dexter used to have a design manual online that helps with axle selection, mounting dimensions, etc. Usually you buy a certain style of axle with specific capacity and it comes with correct springs. For sure needs 2 axles. Besides capacity it will be more stable. It is fun and challenging to build your own trailer. But unless you have a good bit of material on hand it will be a lot cheaper to buy one. Trailer manufacturers get steel at truckload prices we can’t. And won’t have to be inspected.
 
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