Suspension for an offroad Tandem Trailer

Ok, I'll start by referring you to some documents that you should be aware of when it comes to building modifying a trailer in oz.

That said, if you are looking to uprate the load capacity of the trailer you could retain the leaf spring system. Utilise the rear spring from a Landcruiser Ute H/FJ 45/75 which are as long if not longer than a XA/B/C falcon wagon spring but with twice the capacity. You will need to replace the axle to one which is 65mm and ensure that it is turned to suit Falcon bearings. That's important, as holden bearings are only good for 3/4 tonne per side the falcon is good for 1 1/2 tonne a side.

You then get hubs to suit the rims you want to use, I suspect as you have Range Rover bits scattered around you'll be using those rims. That way you can retain the single axle setup and still carry the weight.

My $6million dollar box trailer has that setup, it has a 8'*4' box with a longish A frame draw bar made from 50mm*50mm*6mm RHS with 50mm*50mm*6mm Angle stitched underneath. I had 100mm*50mm*5mm single draw bar on it previously but wasn't happy with the flexing when fully loaded. I've carted some fearsome loads with that trailer over some very ordinary roads, I've lent my trailer to friends knowing that they won't break it. The current incarnation of the trailer has been in service for 6 years, on the same set of bearings and the second set of tyres..

Now if you want to do the tandem axles and you were thinking of air bags on the front, for a little more aggro do the airbags on both the front and rear sets. Then let the pnuematics deal with the load sharing. That would be kosher on the rego front.

I had thought of doing that for a car trailer project, with the benefit of dumping the air out of the bags to lower the trailer for loading/unloading. You will need a suitable (electric) compressor and reservior(tank) which are all easy off the shelf stuff.

One thing though is the brakes if you are carrying any sort of weight (and it looks like you are) then you'll need to consider the options. My preference is for overide hydraulic brakes much better in practice than electric brake IMHO. If you are using your Rover front end bits with Disks then hydraulics is easier. Do both axle sets with brakes.

Hopefully that will give you somewhere to start from.