[VAC] Re: axles (springs and dampers)

[jon fitz <fitzjo1@xxxxxxxxx>]

Just a few points here for those interested in
pondering the finer points of suspension performance.

First the simple part.  Springs (torsion or otherwise)
produce a reactive force that is proportional to
displacement.  Simply put, their force is a function
of POSITION.

Now the trickier part.  Shocks are dampers.  Shocks
produce a reactive force that is proportional to the
RATE of displacement (mathematically referred to as
the first derivative of position).  Simply put, their
force is a function of SPEED (the rate at which the
POSITION is changing).

So, a pure shock absorber (not to be confused with an
air spring) will not affect the static position of the
trailer at all.  If it's not moving, the shock is not
producing a reaction force.

BUT, if you try to push the wheels up suddenly, the
shocks will work with the springs to limit how far the
wheels are displaced.  An example of this is a bump in
the road.  The more sudden (faster) the bump, the more
the shocks react.  The key to understanding the
reaction is to remember that the springs only respond
to POSITION, while the shocks only respond to the RATE
of change of the position.

So, shocks help to prevent bottoming out, but not by
changing ride height.  They are especially helpful
with bumps because they do most of their work at the
start of the bump, while springs do more work at the
top of the bump.

On a related note, we recently did a crude experiment
to compare the dynamic response of my old Overlander
(26') to a new Safari.  My wife jumped up and down
inside the unit while attempting to match the natural
frequency of the suspension.  We only got about 1/2"
of travel out of the old suspension.  Much of this was
in the tires.  And so, we were convinced that the
springs must have gotten stiffer with age.  Much to
our surprise, the new unit had almost the same
response.  Although the travel was slightly more, it
was so little difference that it was hard to quantify
with a tape measure.

We also noted that the natural frequency of both units
was very high in comparison to a typical car or even
our E250.  For those of you familiar with the
mathematics, this is a clear indication that the
spring rate (stiffness) is higher in relation to the
mass.  Thus, depsite the poor resolution of the test,
it was qualitatively obvious that the new unit is
built with a very stiff suspension.  This relationship
also accounts somewhat for any weight differences
between the two units. 

I found this particularly interesting because my
primary concern was fatigue.  Even if the unit never
bottoms out, it could be ruined by an overly stiff
ride.  But, it seems as though there has been little
change in my old 68 in this regard.

Jon in SC
68 Overlander