Smokerbros@aol.com wrote:
> > I can't seem to find anyone local with a spring rate tester that I can use,
> > and neither of my Carroll Smith books seems to have the formula for spring
> > rate. Anyone have it handy?
>
> And I forgot a couple of things... Is a spring with a pigtail (coil diameter
> gets smaller at one end) figured any differently?
>
> What do you do with the portion that's ground flat on one end? Start
> counting coils where it ends?
Interestingly, a couple of years ago, I went through the same thing you
are and ended up finding the formula in a couple of places. Instead of
memorizing it or writing it on a sticky note and pasting it to my
monitor, I made a web page out of JavaScript that will actually do the
calculations for you in both lb/in or N/m. All you have to do is feed
it the measurements of the coil spring. The trickiest part is figuring
out the 'active' coils. The active part is everything that is not
touching the car when the spring is installed. The formula doesn't work
directly on progressively wound springs since the number of active coils
changes with compression.
You can take the webpage and save it off locally on your drive to use it
like a calculator since it is all self-contained (hint: that is not the
only one on the site). Here is the URL:
http://www.mich.com/~sknight/js_coil_spring_rate.html
As far as doing a pigtail spring, I am not exactly sure unless I had one
to test against the calculator, but I would imagine that you will come
pretty close by just doing the active coils thing and using the big part
of the spring for the diameters. From what I have seen, it looks like
it takes about 3/4 of a coil on either end for the spring to get to the
big diameters, but the actual radius of the coil during that time is
still closer to the big part than the little. Your calculations will
likely end up making the spring look slightly stiffer than it really is,
but I doubt by very much. Certainly it would be consistent from spring
to spring as you measure them, but would be off slightly to the
manufacturer's rating. If they have been on the car for any length of
time, they will have sagged and the free length will be a little short
anyway.
BTW, if anyone does have good formulas for progressive and pigtailed
springs, I would be interested in them as well. If they are useful
enough, I could just pop them up on the website like the one I already
have.
Good luck.
--
Scott Knight mailto:sknight@mich.com
http://www.mich.com/~sknight IRC:SS396man
'95 Black Impala SS
'94 Ducati 900SS CR
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