It's to my understanding that unless you have modified the valving in your factory dampers, a linear spring would not be happy in those spring frequencies. Thus the reason that major manufacturers(i.e. eibach, h&r, tien, neuspeed) stray away from making a lot of linear rate lowering springs for street vehicles. That being said, there are some OE shocks that will work with a linear spring rate.
I do not have data to back up the fiesta ST side of things, but if you look at the major spring manufacturer lines of lowering springs, most of them(almost all of them) are a progressive spring until you start looking into the 2.5"ID springs to use on a threaded body coilover.
YMMV
I do not have data to back up the fiesta ST side of things, but if you look at the major spring manufacturer lines of lowering springs, most of them(almost all of them) are a progressive spring until you start looking into the 2.5"ID springs to use on a threaded body coilover.
YMMV
If you ask any real suspension techs, this concept does NOT work so well, IF you care more about that 'track day' type of performance, even on the street.
You may be correct in the factory settings of some dampers would not work all that great with aftermarket linear rate springs, but, Swift's factory; diameter/location/sized replacement springs ARE supposedly fully 'matched'/built to the ST's factory damper valving/settings (or so they claim) despite being linear rate deals.