Date: Mon, 6 Jul 1998 02:29:45 EDT
Reply-To: SyncroHead@AOL.COM
Sender: Vanagon Mailing List <Vanagon@vanagon.com>
From: SyncroHead@AOL.COM
Subject: Springs, Spacers & Preload
Content-type: text/plain; charset=US-ASCII
Hi SpringHeads,
I can't belive I care enough to continue this thread, but I do.
I don't really want to show anyone wrong, but to help just a bit on spring
charactaristics. There have been a few excellent posts recently describing
the charactaristics of springs and such, so I won't go into much detail, but
hopefully allow a picture I made to do the talking.
Spring Rate:
Spring rate (K) is the constant that is used in spring equations to relate the
"stiffness" of a spring. Typically used as a "deflection/force"
(movement/weight) rate like inches/pound. In the drawing I used it's inverse
(200LbsF/inch) because 200 LbsF/inch is more intuitive than .005 In/LbsF.
(LbsF =Pounds Force). Spring rate does not change in a spring's application.
It is built into the spring by the spring's maker and is a function of the
length, material, wire diameter, & such.
Variable rate springs:
These springs do not have a constant spring rate, however their behavior is
still consistant. They have a curve that relates spring deflection and
applied force rather than the straight line of a typical spring.
Preload:
Preload is imparted on a spring before it's normal load (weight) is applied.
This is done by physically constraining a spring to a maximum length that's
shorter than it's free (unloaded) length. A 200 LbsF preload applied to our
example 200LbsF/Inch spring causes the spring to compress by 1". In this
condition the spring is pushing back aganst the preloading restrainty with
200LbsF.
Adding 1 LbF to the spring causes no additional compression.
Adding 100 LbsF to the spring causes no additional compression.
Adding 200 LbsF to the spring causes no additional compression.
Adding 201 LbsF to the spring causes the spring to move a tiny bit off the
preload restraint. At this point the preload restraint is no longer a factor
in the spring's response. It is History and the spring has no "memory" of
what came before now.
Adding 400 LbsF to the spring causes the spring to move ONE additional inch
off the preload restraint for a total of 2". Exactly the same distance as if
the preload restraint had not been there.
Stiffness "feeling":
I don't really know a better way to describe this, but it relates to the
preload and the force required to overcome it. If there is no preload, you
start compressing the spring as soon as you apply even the smallest force.
You can compress the spring a bit with 10LbsF, more with 100 & so on. This
creats the feeling of a soft spring since compression begins immediately.
With our example 200LbsF preload you need to have at least 200 LbsF to begin
creating additional compression to the spring. After you reach 200 LbsF the
preload has no effect.
Why this matters in a dirt bike & not a Vanagon:
On a dirt bike you routinely unload the suspension at put the preload in
effect. This happens while "catching air" over even a small jump or even
departing a bump when the wheel might maintain contact with the dirt, but just
barely. When reloading the suspension (landing the bike) with the weight of
the bike & driver, the suspension doesn't begin to move until the preload
threshold has been met. The definitely makes a preloaded spring "feel"
stiffer because you have increased the amount of force required to begin
additional spring compression. I speculate that a considerable amout of time
on a dirt bike is spent transitioning into and out of this preload threshold.
This will make for a considerable chang is the way the bikes suspension feels.
A off-road race-truck might create a similar situation if it "flies" over
jumps and bumps and then recompresses it's suspension upon landing.
In a Vanagon you rarely unload the suspension to the point of putting the
preload into effect. Speaking for myself at least, I don't fly my van off of
jumps. When the suspension limit is reached, it happens very slowly as I
"crawl" over a rock or tip the van forward into a wash unloading the rear
suspension. When the suspension is re-compressed it happens very slowly and
the van's weight and size are so great that this isn't even felt.
The drawing I made shows the following:
They are charts of force and spring length. The line that relates these is
the spring rate. In this example I choose a spring rate of 200LbsF/Inch
because I figure that the Vanagon's spring might be somewhere near that, but
mostly because it was a nice even number and worked well on the graph.
The first chart shows the characteristics of a completely unrestrained spring.
It's length varies from unloaded (free) length to a point where the coiuls
make contact with eachother. The spring rate line goes all the way between
these two points. Notice that 600 LbsF compresses the spring to 11".
The second chart shows the characteristics of the same spring when applied in
a vehicle with upper and lower limits that prevent it from completely reaching
it's free length or being compressed so far as to cause coil-to-coil contact.
Notice that the slope of the line (the spring rate) doesn't change, only the
endpoints. Also notice that 600 LbsF still compresses the spring to 11". The
weight of the vehicle does NOT ADD to the preload, but REPLACES it after the
preload threshold it reached
The third chart shows the characteristics of the same spring with greater
preload applied (perhaps with a "donut"). Again the line's slope the same as
befoe, just it's endpoint constraints have changed. 600 LbsF still compresses
the spring to 11".
Since 600 LbsF still causes the spring to be the same length, adding a "donut"
on top of the spring will raise the load (in this case, our vans) by the
thickness of the donut.
Please email me if you would like to see these chart's and I'll send you a JPG
file of them. I posted them on the www but for some reason they were not
sufficiently readable.
Hope I've helped.
Jim Davis
