Danger random brain dump follows (full disclosure I also have a head cold so mileage will vary):
“true” multi spring setups are very complex and generally require a long threaded shock body to allow placement of stops to control transition of the effective spring rate. At least I’ve had a hard time even getting the math to work out between available height (minus couplers, cages, etc), required travel (for street application), and workable effective spring rates for a street application. Softer spring rates just need more room for travel and preload, where there’s more flexibility with limited travel and preload of stiff race applications. Some manufacturers also make a small inner spring sometimes called a bump or helper spring which will replace a rubber/foam bumpstop and add to the effective rate of the main spring (allowing for a slightly softer main spring) but these require compatible dampers, cups, and travel height. Most of these I’ve seen are made for large shaft dampers (aka pimp shocks).
What you commonly see on (Honda) coilover setups is a keeper/tender spring (generally a flat profile coil on top of the main spring) in combination with a short high rate main spring. The keeper springs have an extremely low rate (iirc 0-30lbs/in) and really only serve to ensure short main springs remain seated at suspension drop, when the main spring fully unloads and would otherwise float. These don’t come into play with ride comfort since the keeper springs are completely compressed as soon as the suspension is under any weight.
For comfortable street suspension progressive or dual rate springs make sense by the numbers only as long as the transition height aligns correctly with effective weight and preload for desired ride height. The difficulty here is that most designers don’t have the ability to wind springs themselves for each application so don’t have much control over locating transitions. What’s packaged will vary in suitable and will have limited use in adjustable height setup as properly locating transitions requires a fixed preload.
I think a lot of the perception of progressive setups come in here since many report the progressive portion of the spring is fully compressed/doesn’t come into play under vehicle weight or in some cases once the spring is mounted on the damper. For example my H&R Street springs had the equivalent of maybe one coil to transition once mounted on the damper so would only have come into play at near full drop.
The valving/design of the damper will play a large role in ride for a given spring rate. These designs generally have a sweet spot and don’t account for significant variance in variable/multi-rate spring setups. This also applies to soft springs on high-end (generally race oriented) dampers which can yield a worse ride quality than when paired with a stiffer spring.
Also I’ve had issues getting spring rates below 350 to work (up front) in a lowered street application again due to required preload, travel, and bind (usable height of the springs). Limited clearance for barrel shaped springs also raises the potential for bowing on narrow diameter springs as they increase in length.
From what I’ve heard Koni (yellow) sport dampers suffer with a spring much above 550lbs/in. They also seemed poorly suited (bouncy/too stiff) on all but the softest setting with the H&R Street springs I ran (no published rates but some where close to OEM). Koni sports should do well with rates 350-500 unless re-valved.
Then there’s bump stops
To actually put some suggestions out there, personally (not trying to spark a debate, there are near infinite options, these are suggestions for a starting point, just where my head is at again personally):
For something that’s comfortable in primarily street use, OEM is probably best. Always do bushings first and see if you really need/want more.
If lowered (keep it moderate, the geometry does suffer as you go lower) but street use, I’d suggest GC coil conversion sticking around 350 rates in front, 300 in the rear. Probably Bilstein shocks (Koni sport have a larger diameter shaft which requires modification of upper shock mount bushings and dust covers or aftermarket mounts, don’t skip dust covers unless you like replacing shocks).
If primarily street but sees some track use too then take a look at bump stops as spring aids (common in the Miata space) as I believe this is the easiest/most approachable/lowest cost way to produce/tune an effective progressive setup. This approach can work with aftermarket or OEM suspension but you’ll need to replace bumpstops as they age.
If looking for even more option to get aggressive you can swap the Bilstein for a Koni or other adjustable sport shock, still use GC sleeves (but need Koni specific model) and street rate springs and pick up a set of spring rubbers and block coils to stiffen things up as desired before hitting the track.
