Variable rate springs are used in a wide variety of suspensions - including cars, trucks, and recreational vehicles - to improve both safety and ride quality on all types of terrain. Unlike standard springs, variable rate springs exhibit different stiffness properties (or multiple load rates) as they are compressed. To assure that these springs were performing as expected, we measured each load rate using an
SF Series spring tester and
Partner™ Spring Testing Software.
The load vs. height curve for a simple dual rate spring is shown in Figure 1. The key to successfully testing a variable rate spring is the precise definition of the transition point that is characterized by a gradual change in slope between one rate to the next. Very simply, the method uses the spring rate at heights above the transition point (Rate 1), and the spring rate at heights below the transition point (Rate 2) to define two lines that extend past the transistion point as shown in Figure 2. The intersection of these two lines can be used to precisely define the transition height (Lt), and transition load (Ft).
This method can be duplicated as many times as needed to satisfy requirements for other types of variable rate springs. Partner Spring Testing Software can also be configured to calculate multiple transition points for springs with more than two spring rates.