If you want some heavy duty axles for possibly cheap, find a local scrapyard and see if they have the axles and wheels off of the big wood chipper trailers that the tree service companies use; these usually have very large, heavy axle tubes, bearings, and spindles. They sometimes are a bit short but a cut and a bit of schedule 40 steel tubing inserted and welded in the center will do nicely to extend them as needed.
One key issue with any flat trailer is to keep it rigidly flat; they are prone to bend/warp across either diagonal too easily if you use too light a material and don't have vertical stiffeners. So use perhaps 6" tall heavy gage channel along the sides and a couple of similarly sized boxes across the front and rear; 4" is too light for a large area flat trailer like this. Something to extend the sides up will help a lot more than you think; the side rails and the associated vertical supports on smaller flat trailer (that look like they are just there to hold stuff on the trailer) are actually the main stiffeners to make up for a light floor framework. Ditto for the otherwise flimsy sidewalls on enclosed trailers; they are stiff in their vertical plane and thus make the floor framework rigid. Car trailers are so heavy since you don't want anything on the side (so you can open doors once on the trailer) but you need much heavier framing to be sufficiently stiff.
If you mount the camper area in the front, this will make the trailer front heavy with no car, so you need a heavy tongue jack or maybe two. If you load the car on, then you still need to carefully look at the axle positions versus weight distribution and tongue weight, with the camper weight and the heaviest car you intend to carry in position. You're looking at a possibly high tongue weight combo here, with the large variations in weight and position of the camper and the car. Especially when you add in a need for directional stability.....
For directional stability, you typically want the axles a bit behind center with a long tongue and a bit further behind center with a short one. And the car needs to still be pretty centered right over the axles to prevent having a lot of weight behind the axles; otherwise it will be directionally unstable. So the axles will end up being pretty far back, much further in proportion to length as compared to the pix you posted. With all of this considered, you really need to keep the front section/camper short or otherwise you end up with a really long section in front of the axles and high tongue weights.
A platform with access ladder for parts and tires might be best placed above the car and approximately over the axles to avoid added length, tongue weight, and to minimally effect directional stability. It adds wind resistance however.
A triple axle arrangement comes in handy to relieve some of the weight distribution variation. It just comes at the expense of side force needed to turn it and the heavier tongue construction to manage that.
Sounds like fun; post pix when you are done so we can all tell you how you did it wrong!
Mark B.
Edited 1 time(s). Last edit at 09/16/2013 09:43AM by starion887.