Coil Spring Tech
is the formula for calculating the stiffness or rate of a coil
spring. 
K = Spring rate in pounds per inch
W = Diameter of the spring wire
in inches
G = 12,000,000 for steel springs (a constant)
N = Number
of active coils (number of coils that are free to move + 1/2 coil)
D =
Diameter of the coils measured to the center of the wire, in inches
Note the relationship between the number of active coils (N) and the
spring rate (K).
If you cut off half the active coils the spring rate
is doubled.
Applying the above to the stock springs on my 2000 Mustang convertible
yields:
| Front Springs | ||||
|---|---|---|---|---|
| W | N | D | K | Approximately |
| 0.65" | 8.75 | 4.25" | 398.63 | 400 lb/in. |
| Rear Springs | ||||
|---|---|---|---|---|
| W | N | D | K | Approximately |
| 0.55" | 7.5 | 4.625" | 184.99 | 185 lb/in. |
