If not please ask questions. I am trying to paraphrase
   so ordinary mortals (non-engineers) can understand it :)


   The University of Queensland School of Engineering. Quote:

   "Mechanical failure is generally interpreted in terms of change,
   some physical or chemical process having produced failure.
   However the improper location and guidance of components
   also constitutes an important mode of failure often ignored
   in mechanical engineering practice."

   Any object is said to have six degrees of freedom or movement.
   Rotation about the X, Y, and Z axises.
   Translation (movement) in the X, Y, Z directions.

   In order to hold any object from moving in any way requires only
   three connections. If a fourth, or more, connections are added
   additional loads and stresses are introduced that cannot be determined
   by simple means. A three legged stool is a classic example.
   It is trivial to calculate the loads in the three legs and
   the forces these loads apply to the floor. A four legged chair
   on the other hand is a much more complicated situation and it
   usually requires a finite element analysis computer program
   to resolve the individual loads and stresses. The fourth leg
   is therefore redundant and in fact considered undesirable.

   Paul Lamar ...No rotor no motor.

   A three legged round stool is more stable supporting a downward
   force only when that force is applied on the area included in the
   area defined by a triangle connecting the legs (since the legs are
   not attached to the floor).
   In this case, four legs have greater stability.  I have crashed &
   burned more than once on of those stools when trying to sit on the
   edge.  Also recall why the three wheel RTV's were replace by quads.

   Fortunately for us, the three point mount IS constrained, hence a
   better design.

   Joel
   Wondering why I get involved on subjects out of my Expertise.

Thanks Joel. Good thinking. Agreed.

You are absolutely right but for our purposes and audience IMHO it
is too much detailed.  Do you think I need to go down that
slippery slope and draw a 3 legged stool where the force is applied
on the area included in the area defined by a triangle connecting
the legs ?

The constrained word is good but for the average person I think
the connected word is better.

Paul Lamar ...No rotor no motor.

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