Date: Tue, 30 Jun 1998 11:06:42 -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: Springs and spring rates
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Don:
THANKS! Finally someone is focusing on what I am, which is how compressing the
spring with a spacer (in effect reducing the number of coils without changing
length) afffects the characteristics of the spring. I am proposing to compress the
spring only slightly, maybe one inch maximum. This will not unduly stress the metal
of the spring or other suspension components. Unfortunatley, the syncro springs do
not have adjusters...so all we can do is effectively lower the spring perch via a
spacer. I first did this on my yamaha mini enduro back in 1972...it did not have
adjusters either...Jim Varnes, whose son now sits in fifth place in the Grand
National Championship series, taught me the trick of using spacers to make the
adjustment. Anyway, the spacers will not fall out and or deform and are in fact
designed to operate under these conditions...they are the same sapcers in there
now. I expect I will have to shave a couple of them to get the thickness I want,
though.
The reason to do this is that the one-spring-fits-all appraoch of VW results in
poorly tuned suspension. Too much time, extra parts and extra cost for the factory
to do it right.
steve
Don Gibbons wrote:
> Alot of this stuff flying around concerning springs is just wrong.
>
> If you put spacers in between the spring coils you WILL raise the ride height
> slightly. You will also reduce the number of active coils in the spring. The
> equation for the spring rate of a coil spring:
>
> k= d^4 G/8 D^3 N
>
> d is the wire dia, D is the mean spring dia, G the modulus of rigidity, and N is
> the number of active coils.
>
> Placing a spacer (rubber or otherwise) in the spring will create a slight
> "preload" in the active part of the spring. It will also make the spring rate go
> up.
>
> The stress in a spring:
>
> stress = K 8FD/pi d^3
>
> K is a correction factor, and F is the force on the spring.
>
> So you put a number of spacers in the spring. This make the spring rate go up.
> This means that for a given bump or spring deflection the force in the spring
> goes up too. This makes the stress go up just as well. Not good.
>
> The relationship is linear. Reduce the number of active coils in the spring in
> half, the spring rate doubles and the stress also doubles. This practice may be
> OK for racing where the parts can be inspected after only a few hours use. Most
> racing classes that allow spring adjustments do it by adjustable spring perches,
> not spacers.
>
> Spacers are not the thing to do if you have hundreds of miles to cover off road
> or only look at your springs every 3 months.
>
> Pawling NY
>
> Cars I have: 93 Eurovan MV
> 75 Westy
> 73 Super
>
> Cars I had: 89 GTi 16v
> 87 Syncro
> 83 Rabbit GTi
> 82 Vanagon Diesel (w 83 GTi engine)
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