Originally Posted by Godless Commie (Post 1057270)

Having said all this, I still need to come up with data such as number of coils and coil spacing for an 8" spring with an inside diameter of 2.5".
Desired spring rate is 300, 275 and 225 lbs, respectively.
Available wire diameters are 8, 8.5, 9, 9.5, 10 and 10.5 mm. (Al spring steel)

Originally Posted by Joe Perez (Post 1057271)

But seriously. I think that the mathematical approach is probably going to yield more accurate answers than something involving a bathroom scale. The basic idea behind the scale contraption is a sound one, but I don't think I'd go that way myself from a practical standpoint.

Originally Posted by sixshooter (Post 1057272)

I guess you could use the formula for that, then.

Originally Posted by Godless Commie (Post 1057278)

Joe, please refer to post #19.

Originally Posted by sixshooter (Post 1057113)

I have a better idea. Use mathematics and a set of dial calipers to do it the right way. A formula exists for determining spring rates. I've seen it published a number of times, including in the book by Herb Adams called "Chassis Engineering". I'm sure it is on teh interwebz somewhere.

Suspension Coil Spring Rate Design Equations Formulas Calculator

Another one:Spring Constant Calculator
Another one: Helical Spring Calculators - Spring Index, Spring Rate, Shear Stress, Deflection
Another one: Coil Spring Rate Calculations by Wallace Racing

FORMULA:

spring rate =

modulus of spring steel X wire diameter^4
--------------------------------------------------------------------
8 X number of active coils X mean coil diameter^3

*modulus of spring steel = 11,250,000 pounds/inch^2 = 78,500 newtons/millimeter^2

I have successfully tested this formula on stock Miata springs of a known rate and aftermarket springs of a known rate. I then used it to determine the approximate rates of some additional springs I had around. The modulus of spring steel is a constant.

Originally Posted by Joe Perez (Post 1057282)

Doh.

Please refer to the timestamps on posts 19 and 20.

Originally Posted by sixshooter (Post 1057113)

I have a better idea. Use mathematics and a set of dial calipers to do it the right way. A formula exists for determining spring rates. I've seen it published a number of times, including in the book by Herb Adams called "Chassis Engineering". I'm sure it is on teh interwebz somewhere.

Suspension Coil Spring Rate Design Equations Formulas Calculator

Originally Posted by JasonC SBB (Post 1057297)

Sprign rate is a function of diameter to the power of 4. A 1% error in thickness will give you a 4% error in spring rate. Paint will give you a 1% error.

Originally Posted by Savington (Post 1057311)

20ft.lb on a 500ft.lb spring. It will at least put him in the ballpark. If they were decent springs they'd be labeled anyway.

Originally Posted by Godless Commie (Post 1057270)

A formula tells you what the result "should be".
It does not take into account factors such as defective materials, aging, sloppily applied thick paint layer that could affect the caliper reading for wire diameter, treatment failures, etc....