A few probably pedantic observations on accurate spring calcs:
The formula is k = Gd4/[8nD3]
Where:
k = constant, pounds of load per inch of deflection
G =
modulus of rigidity of spring material, pounds per square inch
d = wire diameter, inches
n = number of active coils, which is the number of coils subjected to flexure (always less than the total number of coils)
D = mean coil diameter, inches = Outer Diameter - Wire Diameter
As the rate is dependant on the
fourth power of the wire diameter, you need to know this very accurately as any error will be greatly magnified. Forming the spring may cause the wire to be out of round and any coating will also cause an error. What I do is measure in several places then try to make an educated guess what gauge wire was used and use that as the probably correct number.
The rate is dependant on the
third power of the spring diameter, so again measurement accuracy is important, and you are likely to find the OD is greater at the ends than in the rest of the spring.
Lastly David says total active coils are 1 less than total coils.
Most online calculators I have seen assume 2 less, and a few assume 3 less.
This clearly varies with the spring pitch among other things.
Using a "2 less" online calculator for one of the AVO coilover springs resulted in a rate higher than the manufacturers spec. Close examination suggested that 2 whole inactive coils was an overestimate. Changing this number to 1.5 coils inactive gave a better match to the spec.