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## Re: Formula for spring rates (part II)

 To: autocross digest Re: Formula for spring rates (part II) Scott Knight Sun, 15 Aug 1999 07:33:10 -0400
 ```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 ```
 Current Thread Formula for spring rates (part II), Smokerbros Re: Formula for spring rates (part II), Scott Knight <= RE: Formula for spring rates (part II), Kevin Stevens RE: Formula for spring rates (part II), Kevin Stevens Re: Formula for spring rates (part II), Don Elzinga