Date: Tue, 30 Jun 1998 12:19:25 -0700
Reply-To: David Bayer <bayer@SYBASE.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: David Bayer <bayer@SYBASE.COM>
Subject: Re: Pre-loading Rear Suspension :~o
>| The majority opinion says inserting a spacer only raises the van height
>| by the thickness of the spacer without affecting spring or suspension
>| characteristics. The dissenting opinion says, no, that's wrong; when
>| you insert the spacer between the spring perches, you compress the
>| spring within the perches, so that even with no weight on it, it is
>| compressed further than with no spacer at any given point in the
>| suspension travel, and hence stifferr than without the spacer.
Ok, you've got us. Now just what precompresses the spring? More
weight? A fixed spring perch distance (I believe this is how the motorcycle
suspensions work)...
>| The majority's mistake is in not
>| taking into consideration that the spring is confined within spring
>| perches of a limited maximum length that does not change when the spacer
>| is added.
And just what limits this? We need facts here not conjucture. Call
your first expert witness... (sorry, it's not friday, I am just getting
caught in the tone of this post). The spring is not limited anywhere in the
intermediate travel, just at the top and bottom bump stops. Without some form
of limitation (and therefore a source of force to provide additional
compression beyond the weight of the van), the spring will not be hitting
the bottoming out stop (unless you add a very large spacer). The spring
therefore sees no difference between spacer and no spacer (again unless you
add a very large spacer).
With that said, there a number of things that 1/2" could change
in the system. The first and most noticable being the dampening of the
shocks - especially the "overloaded" gas adjust shocks (did you take these
off?). The gas adjust shock is precharged to provide additional force - I
remember this quite well when I put a set on my 72 - they wanted to expand
to the full length instead of just resisting motion. When one adds distance
to the at rest location of the KYB gas adjust - one will end up compressing
the spring a bit more (as the spring now takes up the weight that the shock
used to support).
>| By inserting the spacer between the spring and the top spring
>| perch, you compress the spring by the width of the spacer, and it is
>| compressed that much further at any given point in the travel over what
>| it would be without the spacer...and it is this compressed spring's
>| characteristics you are starting out with, not the same, free standing
>| spring with just a spacer sitting on top of it.
Just how did you determine that the spring is additional compressed?
Did you take measurements before and after and find the height of the van to
be equal? If the body of the van is lifted, by the same amount as the spacer
than your argument falls apart. If the body of the van lifted a bit than
the answer lies somewhere in the middle. This exercise in logic has too many
holes (all revolving around
>| The conceptual key here is to visualize the suspension completely
>| unloaded. Now put the full weight of the van on it. How much does it
>| sag? Next, insert the spacer...now, when you put the full weight of the
>| van on the suspension, how much does the suspension sag?...less than
>| without the spacer because the spring is stiffer because it starts out
>| already slightly compressed...just like putting a stiffer spring in a
>| scale...the weight sinks it less.
I deffer to an eariler post on the source of misunderstanding. The
motorcycle spring situation seems quite different in that the motorcycle has
some sort of mechanism for tensioning the spring - there is a steel bar
running to the top spring post and if I am visiualizing it correctly, the
preload nut simply moves the limit of the spring perch - I may have this
wrong so go look at your motorcycle more closely. Take the thing apart
and compare it to the vanagon and tell us what you find (I don't have any
suspended bike or I would have done this already).
>| (1) does
>| inserting a donut compress/shorten the spring?
The way this is going on, the dissenting side will forever be
crossexamined until hard evidence is brought into the game. And likewise,
I will mostly work through the physics of the the otherside's argument...
The physics say that you might have a very small shift if you lift one
end and not the other, but other than that, to the spring sees no change in
forces applied to it until you hit the limit of travel... You might see
a difference in the spring compresssion if you are using an "overcharged"
shock like the gas adjust which adds expansiver force into the system...
dave
