There are a lot of misused terms out there. This is explaining the phenomenon, not discounting it. No offense to Frankenyota but you are actually propagating a more myths.

Preload adjustment in the motorcycle world needs to be understood correctly so you don't missapply it here. You increase the ride height in anticipation of a heavier rider. The rider is the extra pre-load, the motorcycle does not gain weight. We call it a preload adjustment because we are compensating for the anticipated weight but it is important to distinguish between a larger person on a motorcycle and a truck that has not gained any weight and therefore preload.

A bit about spring rates. There is no such thing as a "soft zone".(this is a myth) Torsion bars are linear springs as are coil springs. (I'll post a graph when I get home.) Progressive rate springs are different in their geometry and I'll talk about it in a minute. The spring rate of the metal does NOT change at ALL.. EVER... unless plastic deformation occurs or you re-heat treat the metal. (plastic deformation does occur over a long period of time assuming you don't exceed the yield strength of the metal) it is a fixed property of the metal. To change this, you need to basically bend the spring (strain hardening) or heat treat it to obtain different characteristics. The spring simply responds to force but does not in itself change.

Now progressive springs us different geometries as I said. For example, you can have a cone shaped coil spring. Here, as you compressed each coil it only moves to the point you are resting on the coil below. If you make the coils thicker, the spring rate effectively gets higher. I say effectively because the mechanical properties of the metal are the same, we just changed the geometry. (again it's the root cause behind the perception thing)

You might *percieve* a difference in ride quality and that perception is correct, it's the root cause of the phenomenon that is being adressed here.

Frank.