I guess shock valving type is another aspect to add to the aquation.
But surely a "progressive stacked shim" comes into play with shock piston speed, the 'stacked shim' will blow-off at pre-selected sudden and/or high piston load/speeds. A stacked-shim cannot relieve damping at low speed/loads and then be heavier damping at high speed/loads. Although I guess if the a low-speed valving circuit could have limited passage relief to relieve low-speed restriction but it be negligable at high speed/loads?
But then to have variable valving based on high & low shock speeds, why would you need multiple springs?
If a different shim-stack could be utilized on the first half of the shock stroke when the light spring is in effect, and then another shim-stack in the second half of the shock stroke to cater for the heavy spring, then that would work.
I agree that certain older Euro bikes had the best suspension back in the 70's and they had dual-rate springs BUT with technology back then you only had to build a better valving mechanism and the suspension would be better no matter what springs you were using.
I think the variables that matter when it comes to using dual springs are .....
* Valving stack.
* Travel of the primary spring.
But in an old-school shock with just plain orrifice control and not stacked-shim things become even worse.
Now, high & low speed compression damping is just that 'compression damping', the rebound damping (for a given temperature) has only to be set to deal with the strength of the spring ..... but what spring? A stacked-shim on rebound, is there really such a need on a non rising-rate (non linkage) suspension? Unlike compression that has to deal with various size & shape bumps, the rebound is always mid-speed (spring strength). To put in a stacked-shim is to relieve damping on sudden loads (reduce hydraulic-lock), the opposite to what you need with dual springs (it would relieve damping for the strong spring and keeping it for the light spring!). Which again gives me my problem, too much rebound damping for a light spring, causing the suspension to pack-down against the stiffer spring.
I think my brain is hurting. Hopefully the penny-will-drop for me soon and all will be clear? ;-)