Differences between Linear vs. Progressive Springs
Standard springs from car manufacturers are generally linear springs (one defined spring rate per inch) and progressive springs (constantly increasing rate springs or dual-rate)
The difference between the two are as follows:
Linear springs have equal distance between each coil throughout the entire length of the spring. This means that when the spring compresses, the applied force corresponds in a linear fashion. For example, if one applies a 10kgf load to a spring with a rate of 10kgf, the spring will depress by 1mm. If another 20 kgf is applied, the spring will depress another.
Progressive Springs, on the other hand, have unequal distance between each coil throughout the length of the spring. Usually, there are only 2 pitches that “progress” in weight that result in either a smoother or stiffer handling. On one end, the coils are closely wound and then wider on the other. Once the spring is compressed, the stiffer the suspension becomes as it compresses each subsequent length.