Date: Tue, 30 Jun 1998 06:33:56 -0700
Reply-To: Mark Drillock <drillock@EARTHLINK.NET>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Mark Drillock <drillock@EARTHLINK.NET>
Subject: Re: Pre-loading Rear Suspension :~o
Content-Type: text/plain; charset=us-ascii
Steven X. Schwenk wrote:
>
> 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.
steve
Sorry Steve, I thought that Jim Davis' drawing ended the preload
dispute showing as it does that the upper limit of the suspension has no
effect on the static load on the spring. I agree with you that the
spacer improves the performance of the suspension. I think that your
misconception is based on your use of a motorcycle as your model. The
motorcycle model is a very good match to the syncro rear suspension
except for one critical part. The vanagon at rest always loads the rear
spring enough to compress it down from the shock travel limit. On a
motorcycle, particularly one designed for dirt riding, the rear
suspension is against its upper limit stop(the shock) unless a rider is
actually sitting on the seat. If you have someone sit on the bike and
compress the rear spring with the preload adjustment set to soft, then
compare what happens as you increase the preload setting, you should be
able to see that the rider is lifted by the distance of the preload
adjustment(a variable spacer). This adjustment causes a very real change
in the suspensions behavior as would putting a spacer in the vanagon.
The dispute seems to be that you think there is some other force
compressing the vanagon spring other than the weight of the van. Unless
the van is up against the suspension stop(and it isn't when its parked),
gravity is all there is.
Mark Drillock
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