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. |