Date: Mon, 29 Jun 1998 23:12:10 -0700
Reply-To: "Steven X. Schwenk" <sxs@SCHWENK-LAW.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: "Steven X. Schwenk" <sxs@SCHWENK-LAW.COM>
Subject: Pre-loading Rear Suspension :~o
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I will kill this thread if requested. However, it is an important topic
and the issue at hand remains unresolved. This post proposes a means to
resolve the dispute. It's worth the effore because if adding a spacer
pre-loads the springs, then there's a cheap, relatively easy and
accurate way to tune the suspension...balance it...and yes, improve it.
In fact, there should be no dispute that you can accommplish these goals
by pre-loading each wheel to the optimal level of spring
pre-load/suspension sag given the weight the wheel bears and the damping
provided by the boges. The stock set up is not the optimal set up, but
is too soft and unbalanced...The the damping of the boges really isn't
that bad...just needs a little more spring. The issue, then, is, does
inserting a spacer pre-load the spring or not, or must something else be
done instead?
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.
The majority opinion is erroneous in that it assumes that inserting a
spacer between the spring and top perch is the same as simply placing a
spacer on top of a free-standing spring ... which just lifts the van and
does not change the spring at all. 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. 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.
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.
We can resolve the controversey by answering two questions. (1) does
inserting a donut compress/shorten the spring? (2) If so, is a spring
that is compressed or pre-loaded or shortened by, say, two inches (the
width of the spacer) any different in its characteristics/stiffness than
a stock, uncompressed spring? Alternatively phrased, isn't it true that
the last, say, 8 inches of travel of a spring with 10 inches of travel
is stiffer than the first 8 inches of travel? By stiffer, I mean it
will provide less sag under the same weight, will be slightly harder to
compress and will rebound slightly quicker....and will feel frimer.
Once these two issues are resolved, we can either move on to determining
the optimal amount of pre-load for each wheel, or figure out how to
pre-load if not this way.
steve
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