Designing a spring mounted subframe system

Designing a suitable subframe system is paramount to having a successful platform to house a camper body. My intent is also to hang the batteries and water tanks from the subframe, along with some other stuff, so my subframe needs enough strength to do that too.
Unless you want to spend an exorbitant amount of money, the only real material choices you have are aluminium or steel. Each of these materials has their advantages and disadvantages. The obvious advantage to aluminium is that it does not rust, but in order to obtain equivalent strength to that of steel you need to use much larger sections. The major disadvantage to aluminium is that it does not deal as well with constant vibration as steel does. Aluminium welds can work harden and this can result in fatigue cracks or, in a worst case scenario, the welds fracturing.


If your intended use of the vehicle is exploring off the beaten track locations, as ours will be, then another issue could be what to do if the subframe system does develop one or more cracks. In my experience I have found that just about every farmer has a welder of some type for doing general repairs to their own equipment. Problem is that these are normally welders designed for steel. Finding someone that has an aluminium welder may be a bit of a challenge if you are not near a large town. Also, as a fitter I can assure you that welding steel is much simpler, and far less hassle, than welding aluminium. If you have an aluminium subframe and it develops a serious crack you could find yourself in a difficult situation. To that end, I have decided to go with a DuraGal steel subframe.


One of the issues you need to take into account on the FG Canter is that the chassis is designed to be part of the suspension; it will twist quite a bit as the wheels articulate. Any subframe design needs to take this into account and allow the chassis to have some movement. If you do not allow for this inevitable movement then, somewhere down the tack, something will have to give, and by give I mean break.
The upgrade to the five link coil suspension that I had done will definitely reduce the stresses on the chassis for the simple reason that the suspension actually moves, unlike the standard factory setup, but it definitely does not eliminate chassis twist.


Before starting the design of my own subframe system I looked at how others had gone about this. The main players in the Australian expedition camper arena all seem to use similar systems, that being spring mounts between the truck chassis and the subframe. These vary slightly in design between companies but they are all basically the same. My initial design incorporated a copy of the ATW system but as I progressed the design I was forced to re-evaluate how I needed to build my system.


The major difference between how I am building my subframe system and how others do it is that I am incorporating a “full contact” arrangement between the chassis and the subframe; my mounts do not carry the weight of the subframe or camper body.
The rationale behind this approach is that the load is distributed more evenly over the chassis and therefore any point loads should be reduced, or eliminated.
An unfortunate side effect of this design is that it is more costly to build. The polyurethane that I am using as a cushion between the chassis and the subframe costs more than all of the steel required to build the subframe!


I am still using a spring mount system but my mounts only have three functions, those being:

  • allow the chassis to flex independently of the subframe
  • provide lateral support
  • stop fore/aft movement


One of the side benefits of having the coil spring conversion is that the new rear suspension mounts protrude above the top of the chassis rails. The subframe sits snugly between the suspension mounts giving exceptional lateral stability in that area. I will also included a vertical polyurethane wear plate between the suspension mounts and the subframe to alleviate any metal to metal contact in that area.


More details, and pictures, will be included in my next build article; building a subframe system.