The decision about the chassis material was not an easy one. I made a design with steel, aluminium and also everything in FRP. The latter looked very interesting, but it would have required a lot more research and testing with a chassis of that size and weight, so I canned it. Still, I would love to give it a try, but maybe in another life.
There is a never ending discussion between followers of either camp. Arguments pro and contra one or the other are plenty. There is the C350 and C450 steel, which made stronger steel lighter, but I am not sure which manufacturers actually use it or not.
Aluminium is definitely harder to work with. You need greater wall thickness with the same profile size than steel to achieve similar strength and this does not make it attractive under a certain total length. Steel is cheaper, easier to weld, stronger and more flexible, BUT it is heavy. Another advantage of the steel rails is that they can use suspensions with only one mount point each side, if the rails are strong enough to minimise deflection. However I personally would never use such a concept. Better force distribution and lighter rails would benefit the design. Heavy built vans are not smart built vans and some do it with a sledge hammer rather than a bit of engineering. We have examples in Australia from both camps, making very heavy and also very light vans. For me both extremes are not really desirable for several reasons. Anyway, I was talking about chassis material.
Once we had our final floor plane and the decision was made to build a low height goose neck with storage over the neck, the length was defined and with it the chassis material was a no-brainer. Steel would have been too heavy, unless I go really light weight and thin but that was not the idea. We wanted a chassis which withstands heavy corrugations and tracks, not really “off road”, but what the industry calls “off road vans”. We own a 4.5t goose neck horse float, which is 30′ long and built from 100x50x3 galvanised steel RHS. Very heavy interior of living quarters and all steel sidewalls and fibre glass roof. This is ok for the road after we have thrown out the rubber suspension it now tows much better and smoother for the horses.
Nevertheless, total weight for the 34′ would have been much higher because I would not have used the same profile and sidewall thickness. I decided to use aluminium and I made the chassis design using 6082 T6 structural aluminium. There is a common profile of 152x76x6.4, which is used for caravans however the usual length is only 6m or 6.5m and nobody actually stocks it in 6082. So I had to find a supplier who is willing to extrude the profile for me with an acceptable minimum quantity. Some wanted 500kg minimum, but that was way too much for my project. I managed to order a bit over 250kg at a length of 9.90m which allows me to use the 8.1m rails in one piece and have minimal cut-offs, because I optimised the cutting and 9.9m turned out to be the most efficient length, price wise and length wise. I do not need all the length of this profile in one piece, but I also needed smaller profiles as cross members and braces. Some of the lengths I will cut horizontal and some vertical, which returns channel profiles of 152x37x6.4 and 75x76x6.4 roughly. The channel is of advantage where one needs to put bolts through the aluminium to avoid compressing the RHS.
I had no other choice because I could not order all the different profiles I wanted in 6082, because this would have been way too much material. Cutting the profile was the most cost effective way.
Under the main rails I use another profile (76x76x6.4 also in 6082 T6) bonded with the main rail (staggered welds), which results in a total height of the chassis rail of around 9″. This will carry the desired load easy, even on very bad roads without too much deflection.
The chassis will have a storage system all around under the floor and angle profiles are used to provide the rest points for the side walls.
Using aluminium as a chassis has a few more consequences, such as precautions against galvanic corrosion, connecting steel components with the aluminium and the welding technique. I have seen chassis with welds across the top of aluminium main rails, or bolts through aluminium and steel without steel plate on both sides. While this may work and even withstand the test of time, I personally do not feel comfortable with that.
We will use MIG welding for the main structure because it creates less heat than TIG welding, it is faster and creates stronger connections. I use some custom parts (own design) to connect my suspension trailing arm with the chassis and use a steel aluminium sandwich with the aluminium plate between the steel plates. Mylar is used to separate the two and the steel parts are powder coated. All bolts have nylon sleeves and nylon washers and will not touch the aluminium or the steel. The custom design is as such that there is no sheering force on the bolts, which avoids enlarging the holes through the aluminium under vibration. The forces on the connections are only collinear with the axis of the bolts, not perpendicular.
There are concepts available of building a truss system instead of a RHS main rail to further reduce weight but it has a number of other drawbacks and designing it with minimal deflection is also not a very light weight affair. It is ok for a street van with very light running gear and interior, but I was not confident that it would have been a better solution, since I made a few other assumptions and decisions which would have resulted in design conflicts. With a light truss system one would have to find other structural components (like tanks and even the body) to give the whole van enough rigidity. This is very difficult if one adheres to the requirement of 5 times the safety factor for off road use and wants to avoid cracks or delamination in the body panels.
The material is ordered and paid for and expected some time early February 2015.