Let me know if this sounds like a good idea or if I've gone rubber-side-up and shiny-side-down.
So I've been warned off several times about making a DIY chassis out of aluminum. On this forum, people have stated that aluminum tongues tend to crack. A welder was just speaking to me the other day about watching an aluminum bar being TIG welded, and a crack forming right at the weld 6" behind the new weld area. Although there are aluminum trailers on the market, they are commercial products that have been through rigorous testing. Aluminum fails suddenly - unlike steel that fails slowly and usually bends before it breaks. I've seen steel farm equipment that was full of stress cracks that still held together. WWII airplanes made of steel would still fly with half a wing shot off. I'm confident I can build a steel trailer that won't quit.
On the other hand, there was also a statement here that "the material that holds up your rigid cab box can be anything stiffer than spaghetti noodles". An exaggeration of course, but the cab I'm building is a really sturdy box, built out of 2" foam/1/4" plywood SIPs. It needs to be strongly anchored to the trailer frame, and supported, but the thing it is supported by is under very little force.
Here's a plan for a hybrid steel/aluminum frame. The main parts of the frame - the tongue, the connection to the axle (I'm using a 1400 lb Flexride torison axle from Southwest Wheel Company) are all steel. Tongue is 3" wide by 2" deep 0.125" rectangular tubing. Main cross members are 2X2X0.125" steel tube. The main tongue goes underneath all the other framing so there is a big weld to make everything stiff and sturdy. 2X2 steel angle iron forms the diagonal struts. All of these sizes come from recommended designs seen on this forum.
But then the angle iron frame that holds up the cab is made of aluminum 2X2 angle. I'm not welding any of the aluminum - it is bolted together and bolted to the frame.
What about dissimilar metals? Stainless and aluminum don't get along at all. Aluminum and galvanized steel fasteners are OK, according to an article by Fastenal which I'll dig up again if anyone cares to read it. The steel frame will be painted before assembling it to the aluminum parts (maybe powder coated). Any fasteners connecting aluminum and steel will be galvanized fasteners on painted steel, with a nonconducting washer between the fastener and the aluminum and a layer of thin durable tape against the steel. This is, as I have read, sufficient to quit worrying about galvanic corrosion between aluminum and steel. My old trailer is built this way and has no problems after 5 years.
Why go to all this trouble? Light weight. I'm pulling this trailer with a seriously wimpy tow vehicle. My goal is to make a teardrop that weighs about 500 lbs. Calculations show the trailer frame coming in under 120 lbs, and the whole assembled trailer estimated at 565 lbs. Cab is already built, so some of this is no longer an estimate. Originally the all-steel frame I was considering, built all out of 2X2 tube steel, would have been 260 lbs by itself. I've shaved off 140 lbs by putting the steel where it needs to be for strength, and the aluminum where it just ties the trailer down to the frame.
Here are some plans:
Steel Plan
Aluminum Plan:
Overall plan with dimensions