My FYI on springs... The springiness (modulus of elasticity) of the steel never significantly changes regardless of heat treat or alloy.

What does change is the yield point (the point of permanent deformation).

Over time the springs take a set reducing preload. The steel itself still has the same "spring". Heat allows them to relax as does cycling them for a lot of miles.

Short story? As long as the springs are the correct design all you need to measure is the free length.

Kevin

87 Turbo Intercooled Raider, roller cam, torsen rear diff, LSD front diff, lockup auto with modified converter, V6 brakes, low transfer case gears...