Question 1

What spring index should I target for a practical design?

Accepted Answer

Aim for C = 6 to 10. Below 4, the spring is difficult to coil and prone to cracking. Above 12, the spring is prone to buckling and tangling. C = 8 is a good compromise between stress concentration (Wahl factor), manufacturing ease, and stability. Very tight springs (C < 4) require special mandrel coiling and stress-relieving.

Question 2

How do I account for spring ends in coil count?

Accepted Answer

Active coils (n) generate the spring rate. Total coils depend on end type: Closed and ground ends add 2 inactive coils (most common), closed ends add 2 coils, open ends add 0. So a spring with 10 active coils and closed-ground ends has 12 total coils. Free length = (n + 2) × d + deflection. Solid height = (n + 2) × d for ground ends.