I like the idea of loading the quad on the front of the trailer in sideways. You can make enough room for this in front of the trailer fenders. Then load the other quad on the back end of the trailer with the front tires even with the trailer tires. I had a 12 foot trailer set up like this, and could get two 4x4 quads with 26" tires on it no problem. Just make it 12x7. Just another idea before you start welding. That 3500# axle will be plenty, but that trailer will be a bouncy S.O.B. if you're ever hauling it empty or with a light load. I'd see if you can find something in the 2000-2500# range. I swiped the axle off of a wrecked boat trailer (boat caught on fire and melted). It's a 2500# axle, and seems to be just right. As far as steel goes for the frame, use channel and not tubular. That will be a big weight savings. The only place you need tubing is for the tongue unless you make the tongue an "a-frame" with channel from up under the decking. Either way works. I've built 3 trailers in the past few years.

 

I used 1 1/2x3x.120 rectangular tubing for the main fran & tongue, 1.5x.120 square for the rail above the deck. As far as it bouncing with the 3500# axle with springs to match it dosen't. I would have perferred to use a 3500# TorFlex axle like I used on my 18.5 ft tandem and one I built for a friend, 6000# tandem, 20 ft double deck. He hauled 12 quads, 8 were 1991 Polaris 250 Trail 2 2x4, the other 2 were 250 4x4 the same vintage, total weight loaded was 9600#'s. The axle and spring we used on the 12 ft was what was laying around at the time. The overall width is the same as a stock 2000 Kodiak, we used wood for the deck this time the replacement will be deck plate.
Aluminum would be nice however the price is about 3 times the price for steel. Setting up to weld it will also run the cost up. Plain argon cost more than a mix of argon-co2 or just co2 for the Mig welder.
Pullerguy Les

 

PICS >>> CLICK HERE FOR PICTURE!!!!!!
I made my trailor. The front one loads from the side, the back one loads from the rear. Its an awesome setup. I use the space on each side of the back quad to hold gas tanks. Works out perfect. The rails on my trailor come off and attach to the rear and sides to provide ramps to load the quads. Its an awesome setup.

When I say I made the trailor, i mean I found one that was all mangled and beat to crap, then completely reconstructed it. Torched off all the mangled peices, welded on new steal, all new paint, new wood, complete new design for the deck and rails. Notice the wheels are inside.. this is to give me more deckspace without making the trailor wider. I have wheel wells on the trailor, but they dont get in the way at all. Its perfect.

Here is one pic of it behind my 800.00 POS Blazer i bought. its got a 2.8 V6, and its an auto... its a horrible truck, and sucks.. but has no problem towing (except the trans is starting to slip, needs a rebuild).