The Subframe. What for? How?

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Beforehand, I would like to thank Doug H. for posting on a forum his views about the need for a subframe on an expedition truck. This is what originally drew my attention to the subject. Here is an attempt to summarize my take on the issue.
A 4x4 vehicle going off roads needs maximum traction. One obvious way is to keep as much as possible all wheels in contact with the ground.
With this in mind, the engineers who designed this type of truck managed to keep some flexibility in the chassis, i.e. allowing some twist between the main chassis beams which run from front to rear. Imagine a ladder warping.
When a living unit (box, camper, etc.) is bolted to the chassis, it is likely to break unless it can cope with more than 100 mm (4") twist on one side and then just as much on the other side, and again and again...
Here there are two options I considered to prevent the living unit from breaking apart:
a- build a very strong and rigid platform on the truck or build a living unit with a very strong and rigid bottom... and when crossing axles off road, compensate the loss of traction of any wheel spinning in the air by the use of differential locks. This means having the budget for it while assuming it is technically possible to fit diff locks on a particular truck...
b- build a rigid subframe that will keep the bottom of the living unit flat while letting the chassis flex as the terrain requires it. Here again there are at least two options:
b1- Three point pivot. The subframe is connected to the chassis through three points (equilateral triangle). Two points are located on the main chassis beams while the third is on the center of a cross member. Geometry teaches us that it is always a possible to have a plane surface sitting on three points. This explains why the relative movement of three points (on a truck chassis) is compatible with the subframe (plane). More details here.
b2 - Something else? We did not like the idea of concentrating the unit weight on only three points, two of which are right on the chassis. Why? Our experience of a stretch of 1,000 km (600 miles) of corrugations through the Tanami Desert made us curious to research other options. The mechanical stress was intense and I would not have liked to be stranded with a broken unit, chassis or attachment part in such a beautiful but remote place...
What else could we do? It was time for us to call André, whom we had met some years earlier, when he was driving his Man truck with wife and two kids between the Wadi Rum desert and the Red Sea. André in turn called a good old friend of his who in the past decades had prepared many trucks for the Paris-Dakar race. We met André at the airport as he was about to fly out of Europe, saw him off with our warmest thanks and kept preciously his sketch drawn on a cafeteria napkin...
What we liked the most about André's sketch and explanations was that this was not a new idea, but an old one, a very old one indeed. One that had been tested over and over, through the years. It turned out that months later, I ran into an old truck which indeed showed the same principle...
Click on pictures to enlarge them.
...so we decided to go this route, imagined a few improvements and called it "a two line pivot floating subframe".
We would have a rigid steel subframe, articulated at the rear of the chassis beams.
Most of the time, the subframe would sit flat on the chassis, on two "lines". When off road, any twist of the chassis would see the load weight spread on one "line" and one "point" (articu-lation), rather than at all times on three points only.
The subframe would not sit directly on the chassis beam: between subframe and chassis, we would "sandwich" two wooden beams to spread the unit load as much as possible. The wood would have to be a bit soft to accommodate rivet heads, etc... again, to avoid any concentration of load. And should be pest as well as mold resistant. So, let's thank again the young carpenter who pointed us to Californian Red Wood!
The "dump truck" like subframe would have its opening angle limited by an adjustable steel cable. The sideways/lateral movement would be countered by two lateral plates.
The frame located behind the cabin and the flat bed was removed. So were the mudguards, and the license plate frame.
Our Angel and partner in crime. Chris, master welder, who makes hot rods, restores beetles and builds side cars... helped us turn an idea into an expedition vehicle. Here we are about to start the two line pivot floating subframe with Chris's dimensioning and design. On the floor, the boxing plates to be welded on the chassis beam prior to the welding of the articulation plates.
Left, welded boxing plate. Right, the wood beams and the main frame steel beams are sitting on the truck chassis beam to help position the articulation plates before they are welded.
Left, welded articulation plates. Right photo shows the maximum opening angle. It is much more than what we actually need but it gives more room for a man to perform future construction steps. In the end, a retaining cable will be mounted to limit opening angle.
Bottom left, simulation of chassis flex, far less than a real life situation since there is no load on the truck (axles don't touch their respective bumpers) and furthermore, the rear axle is flat (not crossing the opposite side of the front axle).
Top left. The space "A" illustrates the angle between the front of the floating subframe and the front of the truck chassis (just behind the cabin).
Right. "B" shows the subframe lift over the truck left chassis beam.
As expected, the subframe right beam was sitting perfectly all over the right chassis beam (over the right wood beam).
A little rest... before fitting the sliding plates waiting on the mudguard.
How do we keep the front of the subframe from sliding sideways?
Left: black nylon sliding plate attached to the truck chassis beam.
Right: steel lateral plate, welded to the subframe. The arrow points to a hole drilled for a future grease tit.
The steel subframe was first degreased, then it received a primer and later two coats of black special anti-rust paint.
To limit opening angle between subframe "A" and truck chassis "B", a stainless steel cable runs several loops which act as a "spring". The length and tension is adjustable in "C".