Came across a good article hitting on a few basic but very critical variables in expedition vehicle design."Most people give very little thought to how the body of their campervan is attached to the chassis.
Imagine a rigid box bolted directly to a perfectly rigid chassis.
Now imagine driving along a slightly uneven road. There is no problem; the suspension absorbs the relative movement of the wheels keeping them all in contact with the ground. Of course there is limit to how uneven the road can be. Beyond a certain point the suspension will not be able to keep all the wheels on the ground. (True off-road vehicles are far better at this than on-road vehicles.) Even with permanent four wheel drive, unless you have differential lock, you will loose all traction with one wheel off the ground.
However no vehicle has a perfectly rigid chassis. In the case of true off-road vehicles the chassis may even be designed to twist significantly as part of the suspension. In the case of on-road vehicles the chassis will twist simply because it is not rigid enough not to. If the rigid body of a campervan were bolted directly to the chassis then it would be twisted each time the vehicle was driven over rough ground and the body would soon buckle or crack. For this reason the body of a conventional campervan is usually mounted on rubber blocks. However these may well be insufficient to prevent transfer of "twist" from the chassis to the body when the vehicle is driven over rough roads.
The real solution to this problem (found for example on all Unimog campervans) is a torsion-free sub-frame. This is in effect a second chassis mounted above the real chassis at either three points or more commonly two pivots at right angles. These type of mounting rely on geometry, not elasticity, to avoid the transfer of twist. However the torsion-free sub-frame found on Unimogs is expensive, heavy and raises the cabin by 200mm.
Nearly as good as a torsion-free sub-frame, and a great deal cheaper, is to mount the cabin of the campervan on two parallel rails that rest on the chassis. At one end (often the rear) they are bolted directly to the chassis. At the other end they are attached with springs, that may allow as much as 100mm of vertical movement!
Conventional campervans, driven for long distances over corrugated roads are often torn apart by a combination of twist and vibration. Making the body stronger may reduce the damage done by vibration (for example making the furniture of thicker wood and fixing it to the floor, walls and roof) however this will also make the body more rigid and thus more susceptible to damage by being twisted.
A good overland campervan should have a strong body (cabin) attached to the chassis by an appropriate torsion reducing mounting.
"
