Date: Wed, 1 Jul 1998 13:21:02 -0600
Reply-To: Gary Shea <shea@GTSDESIGN.COM>
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: Gary Shea <shea@GTSDESIGN.COM>
Subject: Re: spacers and sproings
In-Reply-To: <359A6AAA.AD31042E@Schwenk-Law.com>
Content-Type: TEXT/PLAIN; charset=US-ASCII
I can't believe I'm this stupid, but I'd like to stick my .02"
of spring travel in here. I've been listening to some of this
thread and it seems to me that if we accept Steve's observations
about bikes, then we have to accept that the bikes do not have
the same kind of springs that we compressed in Physics 101.
That seems entirely possible.
A 'normal' spring takes some amount X of
force to compress it one inch. If the spring is already
compressed 1 inch or 4 inches, an additional force of X
applied to the spring will compress it exactly one more
inch... assuming you aren't about to compress the spring
flat or break it or something.
To put it a bit more simply, if you have a normal spring that
compresses one inch when you place 150 lbs on it, then if you
put another 150 lbs on it, it will compress another inch. I'm
going to call these 'linear' springs.
The spring that Steve is talking about doesn't seem to work
this way, and it has been pointed out in this thread that such
springs do in fact exist. Apparently they are found on
off-road motorcycles. Call these fancy springs 'non-linear'
springs.
The next question is: what kind of spring does the Vanagon
have? A 'normal' spring, or a non-linear spring? Got me.
I'd certainly expect a 'normal' spring, but I'm not in a
position to know for sure.
Now on to pre-loading. If I have a spring, and I
stick a piece of threaded rod through it and assemble together
some washers and nuts and such and clamp the spring down
an inch shorter than it normally is, I've pre-loaded it.
Suppose I have a non-linear spring, like is on the motorcycle,
which at its full extension requires 150 lbs to compress it
one inch. Now I clamp it down an inch. A load of 150 lbs.
placed on this pre-loaded non-linear spring will now compress
it LESS than an additional inch.
Now suppose I have a linear spring, the common everyday Physics 101 kind
of spring, and I compress the spring down 1 inch, and then apply
that 150 lbs, and the spring compresses 1 more inch. Now I tighten
up the nuts and compress the spring down 2 inches. Applying the
150 lbs will once again compress the spring 1 more inch! So
pre-loading is meaningless with a linear spring. Make sense?
I've at least got myself convinced so far.
So the two questions that I see as being relevant here are:
is the Vanagon spring non-linear, and if it IS non-linear,
is addition of a spacer truly pre-loading, i.e., forcing the
spring into additional compression before any load is placed
on it? If both of those questons are answered in the affirmative,
then we are seeing useful results from pre-loading the springs.
If the answers aren't yes and yes, then we're seeing another effect
and need to work out a better explanation.
I'm hoping that my re-explanation of springs will help someone,
anyone figure out what's going on. If not, well, at least it
wasn't written in anger or with the intention of making anyone
else look stupid ;)
Gary
-----------------------------------------------------------------
Gary Shea shea@xmission.com
Salt Lake City http://www.xmission.com/~shea
