2. Diameter of the spring: That is in fact 'the mean diameter of the spring', achieved by subtracting the diameter of the wire from diameter of the spring:

The overall outside diameter of the spring (mm) - diameter of the wire (mm)

When diameter of the suspension spring increases, the spring rate decreases.

3. Number of Active Coils (length / height of the coil spring): Determination of the active coil number varies according to spring design. Total coil number -2 for springs with both ends closed. Count the total coils -1 for springs with one end closed and one end open.

As the number of active coil decrease, the spring rate increases.

Normal Springs has a fixed spring rate.

Step Linear Springs are springs which have 2 different spring rates.

Progressive Springs have a variable spring rate.

Please refer to our article about suspension springs for further information.