The frame assembly is the foundation of your home on wheels.
Its function is to connect the body with the chassis, just like the foundation of a house serves to anchor it to the land it stands on.
If a house is situated on a slope, a sandy beach, rocky ground, or perhaps even in an earthquake zone, in each case the foundation is different, in each case it adapts to the particular conditions of the substratum.
It's the same in vehicle manufacture: the connection between body and chassis must be adapted to the selected chassis, the body size and weight, and the purpose.
As to purpose, there are two fundamentally different categories: on-road use and off-road use.
For pure over-the-road operation—and thus for 99.9 % of all trucks—the subframe recommended by truck chassis manufacturers is adequate; the body is rigidly bolted to the chassis via the subframe, which introduces the little torsion generated into the body. This is quite sufficient for a normal haulage body with no cut-outs in the side walls and no interior finish. But if the body is a container for transporting gases or liquids, then for this type of body, in which the torsion would cause cracks, the chassis manufacturers specify a more sophisticated subframe that makes limited use of the three-point mounting principle—even for pure over-the-road operation—in order to keep chassis torsion occurring in road operation safely away from the body.
For off-road use, a simple subframe is far from adequate; even a subframe with three-point mounting would be a bad compromise.
The body specifications of Mercedes-Benz accordingly stipulate the compulsory use of a torsion-free floor unit for the Unimog, whereas subframes are deemed adequate for trucks—with or without all-wheel drive.
The fact that the engineers of Mercedes-Benz Unimog have developed a special torsion-free floor unit specifically for their chassis, with its great cross-country capability, shows the need for a sophisticated technical solution and is in the best tradition of that company.
The question whether such a floor unit is necessary only for the Unimog, or also for trucks in off-road use, can be reduced to this: do other laws of physics apply to the Unimog? Therefore, from the outset UNICAT has used torsion-free frame assembly for all the vehicles it builds.
How does the torsion-free frame assembly of UNICAT function?
The chassis should be suitable for cross-country driving and afford good directional stability and road adhesion at high speed. The flexibility designed into it by the manufacturer must not be restricted. Frame distortion ensures that the vehicle retains ground contact with all four wheels even in extremely rugged terrain. Only then can it move safely off-road.
The shelter, on the other hand, is designed as a sturdy box and should permit only minimal distortion. Of course, this likewise applies to the permanently installed furnishings and technical equipment.
If you've ever loosened ice cubes from their tray, you know what the effect of deforming the tray is: the ice cubes separate from it.
What is desired in that case—to make the ice cubes fall out of the tray—should be avoided at all costs where body and furniture are concerned!
So, chassis and shelter should be connected in such a way that no torsional forces at all are transmitted from chassis to body.
The torsion-free frame assembly of UNICAT, like the frame assembly of the Unimog, therefore always consists of one fixed bearing and 1 or 2 self-aligning bearings.
The fixed bearing acts to locate and fix the body in the middle and transmits forces in all directions.
The self-aligning bearings in the forward and rear third transmit forces only in certain directions so that the vehicle frame beneath can twist freely.
When frame distortion occurs, it is no longer possible to introduce body weight pressure evenly into the now twisted frame. In the UNICAT frame assembly, the forces are thus introduced into defined areas of the frame. The principal forces are introduced exactly where the rear axle is attached to the frame so that the load on the frame is even reduced.
The intended freedom of movement between frame and body also must be taken into account, of course, when pipes and cables are laid and the tanks and other frame add-ons are fitted.
All in all, this is a problem of considerable complexity that only can be optimally solved if body and frame assembly originate under the same roof—as at UNICAT.