W (mm) D (mm) N G K (kg/mm) K (lb/in) Notes

Wire Gauge Spring diameter Number of coils Modulous of ... Spring rate
(calculated)		 Spring rate
(calculated)
Racetech .95 5.14 29.6 29.25 8420 0.968 54.12
Racetech .90 5.09 30 29 8420 0.902 50.625
Racetech .85 5.01 30.17 27.33 8420 0.884 49.572 ????????
Racetech .80 4.87 29.9 30.1 8420 .78 43.5
Progressive - initial 5.28 29 48 8420 0.699 39.206
Progressive - static 5.28 29 34 8420 0.986 55.350 Static sag, for a 170 lb person, roughly, it's quite progressive
Progressive - final 5.28 29 26 8420 1.290 72.380
1998 F3 - initial 4.8 30.8 27 8420 0.708 39.738
1998 F3 - final 4.8 30.8 18 8420 1.062 59.606 After 3.7" compression, including preload
Hawk - initial 4.51 30.9 28 8420 0.527 29.575
Hawk - final 4.51 30.9 16 8420 0.922 51.756 After 4.4" compression, including preload 

         G * W^4
K=G = ----------------
        8 * N * D^3


Formula is in the spreadsheet, but it's something like
where:
K = Spring rate
G = Youngs modulus (stiffness), depending on units
W = Wire diameter
D = coil diameter (center of wire to center of wire)
N = number of turns

Typically, either closed ends or closed and ground ends are specified due to the greater area of contact between the spring and its base.

Effect of end treatment.

Plain Ends Closed Ends Plain Ends
Ground		 Closed Ends
Ground*

Active Coils, Na

Nt

Nt-2 Nt-1 Nt-2