[Kobura]

Apparently there are two types of car springs in common use: Those with a constant spring rate and those with a progressive spring rate. The progressive rate springs become stiffer with compression, the constant rate ones don't.

I may have not conveyed the reasoning behind my planned approach to modifying my BDR rear suspension very clearly because I gave a superficial summary of the spring-mass-damper system analysis. No differential equation, no exponential function, no imaginary number, etc. However, I think that my planned approach to achieving a more supple suspension is intuitively obvious in that a soft spring rate with minimum damping seems like the way to go rather than a stiffer spring with over damping.

What makes my situation difficult is that there doesn't seem to be a source of the BMW 'beehive shaped' rear springs with a softer spring rate. Moreover, the selection of a shock absorber is kind of a crap shoot in that the damping factors for shocks is not stated/known. I know that a couple of CC contributors have shown photos of modification of their BDR rear suspensions to coilovers which allows great freedom in spring and shock selection, especially with variable compression and extension damping shocks. However, I have not reached the point of spending that kind of money to solve my issues...yet.