Steven,

Rainer is exactly correct - as I splained in an earlier message - but for
grins - giggles and guffaws, consider the following analogy that is exactly
correct from a mechanism/physics perspective.

You, Steven are on a pogo-stick - remember these things - the sprung stick
with footweights that you can bounce around on - they also have a lower stop
that results in spring preload, but preload is of no matter as you will see.
OBTW - because you have perfect balance, you can stand on this stick virtually forever.

So Steven, you're standing on the stick and suddenly get the urge to do a
daring feat - you pogo upstairs - yep, upstairs. The bottom of the spring is
now elevated (so are you - aren't you).

Next. because you are a mechanical dude, you want your pogo to ride higher, so
you put some washers in the pogo between the spring and the lower spring
bracket. Do you ride higher when at rest on the pogo - of course you do - the
spring preload is greater, but that doesn't matter because your constant
weight compresses the spring to the same length.

Because:

1) The free length of the spring hasn't changed

2) The spring constant hasn't changed

3) Your weight hasn't changed

If  you wanted to really get into spring application/design, (do it!) go to a
college library and get a book on Mechanical/Machine design - go talk to
someone in a Mechanical Engineering department. A reference that you may find
in a college library: Marks Standard Handbook for Mechanical Engineers; McGraw
Hill -  If you draw a freebody diagram with the forces and geometry, you can
fairly precisely calculate the deflections for any load condition. Its not
rocket science!! (no calculus or differential equations required) And then
have a beer or two.

Best Regards,
Ken Stich

BS Mechanical Engineer
MS Aerospace Systems Engineer
80 Vanagon-L
85 Prelude Si
57 Beetle
 ===========================================================================================
Dr. Rainer Woitok wrote:
>
> Hi all,
>
> On Mon, 1998-06-29 23:12:10 -0700, Steven X. Schwenk wrote:
>
> > ...
> >                                                              (1) does
> > inserting a donut compress/shorten the spring?
>
> Definitely  NO!   Provided the length of    the spring under  the normal
> weight of the van isn't constrained by the shock (which it shouldn't be)
> the length  of the  spring  depends ONLY on  its  material, its geometry
> (unloaded) and the weight to  which it is exposed.   Thus, if the  lower
> end of  the spring  is  two inches higher  due to  the doughnut and  the
> weight is the same, the upper end will be two inches  higher, too.  This
> means the van will ride two inches higher.
>
> Sincerely
>  Rainer
>
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