I'll be interested in how this turns out. Be sure to leave feedback once it's up and running. The fork is one of only a few things I'm going to want to change on this beast.
There is a difference between true progressive springs and dual rate (or three or four rate) springs, and the longer one is not a progressive. The picture shows three different winding rates, so it would be a three rate spring, which with shift to a higher rate in each of two steps, rather than progressively, as in a curve. True progressive springs are either wound with all coils of the same diameter, but progressively wider with each coil, or they are wound with a tapered or "beehive" shape, with each coil a larger diameter than the last. The other thing is that from any given point in the spring's compression, it will remain at one single spring rate until the next coil binds (runs into the one above or below it and ceases to be effective). I'll explain.
First, you have to understand that a coil spring is nothing more than a compact form of torsion bar. when you compress one, you are twisting, applying torsion to, the wire. With wire of any size, the longer the wire the less twisting it 90 degrees at one end will twist each 1mm section of it, so the longer torsion bar has a lower rate than the shorter one (which is why cutting springs down makes them stiffer). Thus, a coil of any length with fewer windings has a higher rate than one with more, as does a coil with smaller diameter coils compared with one that is larger. That's coil diameter, not wire diameter.
So, looking at the new longer spring, you see there is one very tight wound section at the lower end. That's lowest spring rate. Then there's another les tightly wound section, and another comprising most of the length that is looser yet. That top one is the stiffest. Now, how does that work?
A dual rate spring is nothing more than stacking two different rate springs on top of one another, so imagine there's a 200 inch pound spring with a 100 in/lb spring sitting on top. The 100 in/lb spring will coil bind after two inches of compression, and there is no load on the combined spring to start with. When a 100 pound weight is placed on top of the spring set, the 100 in/lb spring is compressed one inch as a result, but the same weight is also bearing on the 200 pound spring, so it gets compressed a half inch as well. The total compression resulting from the 100 pound load is 1.5 inches. Dividing the 100 pound load by the amount of resulting 1.5" compression gives the rate as 67 inch pounds. That's actually less than either of the two spring rates used in the combo because of the compound compression.
If the load is increased to 200 pounds, the rate continues to be 67 in/lbs, but only until the 100 pound spring coil binds, at which point it becomes just a stack of spacers for the 100 pound spring. So once the combined spring is compressed 3" under this load, the rate jumps up immediately to 200 in/lbs, because that's the only active spring in the stack.
Progressives are the same. Because all coils bear the load in common, they all compress under it according to their individual rate, and the rate rises only as each lighter rate coil binds up against it's neighbor.
