There is no big secret on measuring leaf springs, just a bunch of bad information.
Remember the guy back at the beginning of the Tech Section?
He has spread a lot of bad information that we are about to correct.
The WRONG way to measure is eye to eye. While this may seem to be the easiest, it is not accurate.
The measurement you get is only good as long as the spring stays at that arch.
As a spring flexes up and down, the distance between the eyes changes.
A 48 Inch (measured the correct way) with a 6 Inch arch will measure 46-3/4 Inches eye to eye.
The same spring with a 3 Inch arch will have an eye to eye measurement of 47-3/4 Inches.
Also if you would like your axle to be in the right spot, the location of the center-bolt is very important.
Measuring eye to eye will not tell where the center-bolt is located and the chances of your axle being in the right spot are not good.
So the ONLY way to measure the length of a spring is as though the spring was flat.
Follow the curve of the spring measuring from the center of the front eye back to the center bolt (A).
Then measure from the center of the other eye back to the center bolt (B).
These measurements are called the "Divisional Length" and are indicated as 2 different lengths.
Using the 48 Inch spring from above could have several "Divisional Lengths". 24 x 24, or 21 x 27 or 20 x 28 or whatever.
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.