Solid Axle - How to Measure For a Spring
There are two different methods you can use to determine which spring length you need:
Method A: If you are using an axle with vertical perch bolts holes, you simply measure center to center of the spring perch bosses (taken from the top side of the axle) and deduct 5”. This will be the length you need on a spring-over application.
Method B: If you already have your axle, perches and shackles, you can assemble them and situate the shackles so they are horizontal. You can then measure from the centerline of one shackle pin to the opposite one as shown to the right. This will give you the length of spring you need.