Date: Tue, 30 Jun 1998 12:48:42 -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: Re: Pre-loading Rear Suspension :~o
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Derek:
I can say "wrong" too. And you, my friend, are wrong. I have explained in great
detail why. Don has now chimed in with an equation that shows why the spring is
stiffer if pre-loaded. Simply saying "wrong" does not cut it. If you do not
address the issues, there is no discussion and no progress.
Look at the YZ manual. http://www.concentric.net/~Sxs/preload.shtml
Are they wrong, too? If not, what's the difference between lowering the spring
perch on the YZ and lowering the spring perch on the syncro? And if adding a
spacer does not accomplish the same on the syncro, what would accomplish pre-load
on it? After all, I thought the issue was how do we do this to tune the
suspension and thus improve it, not who's right and who's wrong.
steve
Derek Drew wrote:
> My comments are sprinkled below...
>
> At 11:12 PM 6/29/98 -0700, you wrote:
> >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.
>
> your confusion begins with the above sentence
>
> >By inserting the spacer between the spring and the top spring
> >perch, you compress the spring by the width of the spacer,
>
> only when there is no weight on the spring. under almost all normal
> conditions, you do not compress the spring by the width of the spacer
>
wrong: the spring's characteristics are fixed and constant...once you pre-load
it, you change it's properities, with or without weight on it. As Don noted, it's
like using up one coil of the spring without changing its length.
> >and it is
> >compressed that much further at any given point in the travel over what
> >it would be without the spacer
>
> wrong. it is only compressed further when the shock is at full extension.
>
Wrong: it is compressed further the entire range of travel of the shock over what
a spring with no spacer is.
> >...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.
>
> The conceptual trap here is to visualize the suspension completely
> unloaded.
>
The conceptual gridlock is to refuse to consider the change in spring properities
standing alone, before any weight is put on it. Wouldn't inserting a stiffer
spring result in less sag.
> >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
>
> wrong
>
Why/how
> >because the spring is stiffer
>
> wrong
>
> why/how
> >because it starts out
> >already slightly compressed.
>
> wrong. The word "because" above appears to be at the root of your problem.
>
that tells me a lot
> >..just like putting a stiffer spring in a
> >scale...the weight sinks it less.
>
> wrong
>
why/how?
> >We can resolve the controversey by answering two questions. (1) does
> >inserting a donut compress/shorten the spring?
>
> It does when the shock is at full extension; otherwise it does not. The
> shock is rarely at full extension unless Jim Davis, Tim Smith, or myself is
> at the wheel driving on uneven terrain.
>
what happens o change the spring, then, between the time it is completely
un-sprung and when weight is put on it.
> you think the extra compression somehow gets stuck on the spring and
> applies to the spring even after the shock is compressed; this does not happen
>
No...it's properities are completely different than the un-pre-loaded spring...no
matter how little or how much weight is on it.
> >(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.
>
> meltdown
yours, not mine.
>
>
> >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
>
> pre-wash the pre-load
Gladly...enjoy your over-sprung suspensionsteve
> ________________________________________________________
> Derek Drew New York, NY & Washington, DC
> drew@interport.net
> '90 Syncro Westfalia...
> ...seen off-road at http://www.tiu.net/~des/vw/drew/index.html
>
> Also seen at: http://www.pipeline.com/~tforhan/swrange.htm
> http://4wd.sofcom.com/VW/snow.html
> http://www.pipeline.com/~tforhan/swmassn.htm
> http://4wd.sofcom.com/VW/Campers/Campers.html
>
> Note: most valuable Vanagon sites on the planet (for owners) are:
> http://gerry.vanagon.com/cgi-bin/wa.exe?S1=vanagon
> ftp://gerry.vanagon.com/pub/
