Spring Arch is measured by splitting the center of the spring's eyes with a straight edge.
Then measure down from this line to the top of the Main Plate next to the center bolt (C).
(the Main Plate is the leaf with the eyes)
This measurement can be positive or negative. A spring does not care which way it is bent, they will work both ways.
Setting the spring upside down and measuring from the floor to the main plate or to the bottom of the spring is WRONG, WRONG, WRONG. So don't even try to give us a measurement done that way.
Why? Because the support for the vehicle is through the center of the eyes not on the top of the eyes. Plus steel thickness, eye sizes, eye shapes can vary. Plus, all the OEM's measure through the eyes and all our specs are through the eyes, so do it our way.
Free Arch is the term used for the arch when the spring is unloaded (off the vehicle).
Loaded Arch is when the spring is under load (on the vehicle with weight on the springs).
To make it easy for you we have created a cheat sheet, download our Leaf Spring Measuring Form