Date: Fri, 26 Jun 1998 00:37:42 -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 Springs re Syncro Suspension (long)
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kenstich wrote:
> Steven,
>
> I've annotated your message - hopefully to add clarity.
>
> Steven X. Schwenk wrote:
> >
> > You're right...I am going beyond where I need to to make my point, though, which
> > is that inserting a donut above the spring does not raise the van per se, it just
> > increases spring tension and thus reduces suspension sag.
>
> NOT true - with the addition of the 'donut', the weight of the vehicle has not
> increased, therefore the compressive force on the spring(s) is unchanged,
> therefore the spring deflection due to the compressive force is unchanged ==>>
> The van rides higher by the thickness of the donut.
Well, we seem to be talking about the same thing, at least. :~) Consider, however,
the "compressive force" on the spring before you put weight on it. At that point, with
the shock absorber fully extended, it is easier to see that inserting a spacer between
the spring and the top spring perch indeed compresses the spring beyond what it would
be without the spacer. Moreover, at any given point in the travel of the shock, the
spring will be slightly more compressed with the spacer than it was without it. It is
this effective lowering of the top spring perch that compresses the spring, not any
weight factor.
The space the spring fits into (the distance between the top and bottom spring perches)
does not expand to accomidate the spacer. Rather, the rear shock at full extension
deterrmines that distance. Adding a spacer does not cause the shock to lengthen
more...it is already fully extended. Instead, the spacer decreases the distance
between the top and bottom spring perch by the width of the spacer, and thus compresses
the spring by the same distance over what it would be at any point of
travel/compression without the spacer.
> I suspect that to only
> > increase height without also increasing spring tension, you would have to lift
> > the vehicle at the point where it attaches to the suspension.
>
> The point of load bearing from the chassis to the wheels is the spring(s) -
> instead of tension, this is actually a compressive load on the spring(s)
>
> ..I have not thought
> > this part through, the comment about extending the shock mounts is the beginning
> > of a guess on how to do that and was made prematurely.
> > steve
> >
>
> Since the chassis rides higher with donuts installed, depending on the
> thickness of the donuts and also the age-related spring deformation (metallic
> creep results from loading over time) "sag",
"Sag" is not a deformation, but the amount the suspension compresses by the weight of
the vehicle. The suspension is designed to have sag and must to perform properly. Put
another way, if you have no sag, your springs are way too stiff and will inhibit the
dampening effect of the shocks, among a host of other problems. If you have too little
sag, your suspension will be too hard, rebound too quick and traction will be
diminished. If you have too little sag, the suspension will be mushy...too bouncy and
may even bottom out.
> one may or may not need to
> install longer shocks to compensate for the added height. One should measure
> to determine the necessity of changing shocks. It is fairly critical that the
> suspension stops limit travel - not the shocks!
It would not be advisable to simply put longer shocks on. The trailing arm travel is
limited to not much more than the travel of the stock shocks. The geomotry of the
trailing arm will also change. The shocks do in fact limit fully-extended
travel...which you get only when air borne...or with the wheels off the ground. Unbolt
the shock, and the trailing arm drops...nothing else holds it.
> Best Regards,
>
> Ken Stich
> Former Mechanic
> BS Mechanical Engineer
> MS Aerospace Engineer
> 80 Vanagon-L
> 57 Vintage Oval Bug
I am beginning to feel like the list heretic on this one ... perhaps about to be
burned at the stake for repeating my dissenting mantra over and over ... If I could
draw a simple triangle to represent the rear suspension, it would be easy to show what
the spacer does and that it does in fact compress...and thus pre-load ... the spring.
steve
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