They're technically dual rate springs, which is exactly as it sounds. The spring has 2 different spring rates for the 2 sections that are wound differently. The tighter part completely collapses when the car is on the ground and effectively is not in play. They call that part a tender section. That part allows the car to be lowered, but still keep the spring captive where it's supposed to be. Once on the ground, the other part takes over and that part is linear. A progressive spring does not necessarily have to have differently wound sections. It can have even coils throughout the spring and still be progressive.
Here's a link with comparisons of aftermarket springs to take a look at if you have some time to burn:
https://store.redshiftmotorsports.com/category-s/134.htm
Interesting, thanks for the clarification. I've actually seen that page you linked, good review of those springs and visualization of force vs compression but it didn't explain the basic concept of having a weak ("tender") section that simply keeps the spring in place when unloaded. You knocked that out in one sentence.
So that section will coil bind, and limit the amount of travel, keep the spring from getting unseated, while the rest of the spring is linear or progressive, but typically stiffer than stock to make up for the limited amount of travel? Is that right? This is why you can't just lop off a coil or two off the stock spring and call it a day. Makes sense.
I wonder how they get evenly spaced coils of the same diameter to be progressive or linear. I always assumed that having different coil spacing and diameter dictated linear vs progressive. Never messed with coil springs much. My M3 has Eibachs over Koni yellows, which works really well and was recommended to me as a popular street set up. On my old corvette springs were stock. It had a very unique transverse spring made of some carbon-composite material, which also acts as a sway bar. I wanted to rip it out and do coilovers at some point, but sold it and bought the M3. Basically never had a chance to fully understand the ins and outs of coil springs. I had a '95 integra that I swapped to a full Type R suspension I bought from a local importer (struts, springs, LCAs, sway bars, braces, etc), and that was a remove and replace affair, no thinking necessary. Again great result but the only choice I had to make was to do it or not to do it...
Here I'm debating Swifts for handling versus Eibachs for comfort. In both cases I want the lower CG and less roll. I thought all lowering springs were generally stiffer than stock, which would reduce roll, but many people say the Eibachs actually feel softer than stock, and there's MORE roll on turn-in - maybe due to being progressive? Doesn't sound like a perfect solution. Swifts seem better in that respect, but the Fiesta is already so stiff that I'm a bit weary of going significantly stiffer. I hate having to make blind choices. And I don't want to buy both sets just to find out which I like more
Is there a comprehensive review of each set for our cars, by someone who tried both over stock shocks? I saw Ron's post, but he's going off other people's experiences and it boils down to comfort vs handling between Eibachs and Swifts... Those obviously mean different things to different people.
For example I tried the AWR RMM, softest bushing (71 or whatever) - uninstalled it the next week. The cons outweighed the pros by far, for me, in a daily driver. Very solid and well made, price was right. Just not for me. Very thin bushing. Going to try a COBB or Mountune, something a good amount thicker and softer. Many reviews said it's "perfectly streetable" but I'd say it's a track-only part. Mirrors and everything else including my fillings jiggled like crazy at idle with AC on. Ruined the car, honestly. Basically everyone has their own threshold of what is acceptable in a commuter, and I'm wondering which spring will suit my taste given that experience.