What is the best beam design for my aluminum tandem axle trailer?

[erobz]

If I add the 1/2 x 3 1/2 to the top and bottom flanges I get a 91.5 MoI.
View attachment 324318

If that doesn't work I'll have a useless pair of $1500 beams and ..... well .... let's just say my neighbors kids are going to get a lesson in some very loud cussing.

If the entire beam had a M.o.I of 90 in^4, the deflection comes to about 0.7 in. Is there some reason you aren't placing the 1/2" plate on the exterior faces ( top and bottom ) of the channel? It's going to get you more bang for your buck in M.o.I. increase.

Also, keep in mind that your frame should be more rigid than that beam once constructed. So the apparent deflection of the beam may not be as big an issue as it currently appears.

 

[IdBdan]

If the entire beam had a M.o.I of 90 in^4, the deflection comes to about 0.7 in. Is there some reason you aren't placing the 1/2" plate on the exterior faces ( top and bottom ) of the channel? It's going to get you more bang for your buck in M.o.I. increase.

View attachment 324336
Also, keep in mind that your frame should be more rigid than that beam once constructed. So the apparent deflection of the beam may not be as big an issue as it currently appears.

Another quagmire in this trailer!

I have a max deck height restriction and that 8" member just squeaks by. The Trike has a very low 'break over' angle. (3.5" road clearance) Long story short, every inch up gives me an exponentiation add to ramp length and the need for over 25' of load space to get in on and off the trailer. Trailer + Trike + Ramp Length.

Also, a 27' trailer (@ current height) going down a standard 14 degree boat launch needs 22' of ramp to get the tail height parallel with the ramp. The higher I go with the deck the more distance I need to hit water that will float the boat off. At low tides I'll run out of hard ramp area and chance getting the trailer wheels stuck on the drop off with a 32' motor home attached to it.

I think your correct on the complete frame with cross members being much stiffer than the beam by itself. That's why I'm going to do the build and mod method. When I first started designing the trailer a builder at the coast told me a 2x5x.25 tube would do the job? Just goes to show what the "professionals??" are putting on the road today?

I've added another cross member to shorten the spacing's between them under the boat area. I had cut drops in the 2x4 material anyway. I'm also doubling the tube at the rear and adding a diagonal tube to that for bracing to the beam. The boat transom tie downs connect to the 8"beam at the rear. I want that stiff.

Thanks for all your help! I owe you one. Let me know if you ever need CAD work done.

 

[ChemAir]

The only reason I'm building this myself is because it has been impossible to find a manufacturer to build it. Tried for 3 months. Searched the entire east coast from New jersey to Florida. Even the custom trailer builders won't build it because they have so much standard work they don't need it. The big manufacturers all have a standard line of trailers and they don't do custom. And if they will do it, it's a 6 month wait. Right now torsion axles are a 24 week wait. There's one manufacturer that will give you 6 weeks but at twice the cost. And reason #2 - Aluminum builders are scarce.

The other thing I found is that most don't even have an engineer on staff? Their working off 30 year old designs in some cases. And when they fit bunks for boat trailers they use the 60/40 rule for axle location & tongue weight ... and that's it. The most important part of trailer safety is the tongue weight ratio!

Any trailer under 26000# is virtually unregulated. They don't fall under the Title 49 Code of Federal Regulation, Parts 500-599. They have a handbook with 100's of pages and 2 paragraphs on trailers. If you call, you're told to contact the National Association of Trailer Manufacturers and buy a membership for help. A non-Government entity.

And all the important parts of a trailer (axles, brakes, hoses, lights, receiver, etc.) are the part manufacturers safety responsibility. The only responsibility for safety in a trailer build is engineering and the fabrication processes.

So in reality, I believe I'm doing a better job of 'regulatory safety' than they are.

While I commend your enthusiasm, I am a metal fabricator that does build in aluminum, and engineer that, after doing our regular HVAC design, and custom ductwork fabrication, among other things, does operate and repair DOT inspected trailers, so....I think I may have some liability perspective, here. I've actually talked to insurance people about trailer manufacture and repair (and performed repairs). This is a large area for potential litigation. I have that risk when I work on trailers, even though I have welders certified against my AWS and ASMEIX welding procedures for processing carbon steel, aluminum, and a variety of stainless and Nickel alloys.

I know, for instance, that if you attempt to weld an aluminum trailer without knowing what you are doing, you can have fatigue failure problems at the welds. These can be caused by lack of heat treatment, poor welding, bad electrode selection, poor metal selection, and just bad practice. Most aluminum trailers are bolted after forming, and that is because the welding is very difficult. If you would like, talk to a couple of aerospace guys about welding aluminum wings. I expect they will tell you that is a bad idea.

If you build a trailer, and it is properly ID'd and tagged, when there is a deficiency in an accident, the manufacturer WILL be involved in any lawsuit. If you are the manufacturer AND the driver/hauler, you will be sued in spades if you made bad decisions, or at least decisions that aren't best manufacturing practice. Blaming the guy that sold you brakes will not absolve you.

As I said before, I'd suggest talking to your insurance representative and tell them that 1) you will be building a trailer for over the road service, and 2) it will be a welded aluminum trailer. If you deliberately mislead the insurer for short-term approval, you may have trouble 'down the road' so to speak.

If you choose to not take my advice, that's fine. But I have had experience with the legal system in the US, and I think that you are optimistic.

There's enough potential liability that I am uncomfortable telling you how to do it right, because, as a professional with expertise in this area, I could inherit some liability, simply by doing so.

If you want a deep dive into this, I can help you navigate, but this is an area I have had no desire to accept any further liability.

 

[berkeman]

And with the very good post by @ChemAir it is probably a good time to close the thread. It has covered a lot of good ground, but the liability aspect is something that we cannot assume here at PF. Thanks all for your contributions.