Four Point Support Plane
The four point support plane is the bearing surface defined by the tips of the four jacks. Geometrically, three points define a plane; the fourth point makes the system statically indeterminate. For this reason, with four-jack support the load distribution depends on the jack lengths and the torsional stiffness of the body together.
A three-legged table never rocks, because three points always define a single plane. With four legs, however, one of them can end up outside the plane established by the others; then either the leg sits unloaded or the body twists to make contact.
- Unloaded jack: it carries no load, that corner of the vehicle sways and moves in the wind.
- Overloading jack: it raises the vehicle, the opposite corner unloads and the body twists.
- The right situation: all four jacks in contact and each carrying a measured share of the load.
This is why, in the first stage of the automatic levelling cycle, the jacks are lowered one by one and the ground contact of each is recorded separately: all four are loaded against a known reference. When planning the jack layout, keep the rectangle formed by the four points as large as possible; as the footprint widens both the load shares get smaller and the stroke needed for the same angular correction decreases.
Context in which this term is used: RV automatic levelling guide