Thanks for the reply.
Would you happen to have measurements, or estimates of min/max ID/OD of the top and bottom seating surfaces of the front vs. rear shocks & mounts? I'm attempting to determine if I can use repurposed OEM springs of different sizes.
In my old miata race car, the springs weren't fully seated at full droop in the coilover sleeves, but it never seemed to be an issue. Perhaps the sleeves helped keep the springs from moving around side to side too much at full droop. I remember reading about crude droop limiters too (chains). It sounds like you feel the springs need to be seated in this context. Perhaps damage/odds of unseating are lower at tracks and with lower mileage vs. a street car.
It’s especially important that the rear springs stay seated as they are divorced from the shocks, hence free as a bird. If they unseat in droop, they can literally fly out the back of the car where a coil-on-spring setup will at least stay roughly in place, especially if there’s spring is tight around an accordion boot
Unseated is never good, which is why helpers were invented, coil bind is also bad but you can get away with odd stuff on racetracks to skirt weird rules, in ways that don’t fly on a long term installation on a street car.
Getting into actual suspension mechanics, Unseating also does weird things to suspension behavior if you actually encounter a big enough bump/dip, with zero spring force for part of the travel, you essentially lose tire contact with the road briefly. Not end of the world on a straight, but bad for overall grip
Here are my messy notes on the stock spring measurements.
Front 2019 spring measurements
5.25” Dia, 4.25” Ends ~12” Free Length, 7/16” wire, 3.5-4 Live Coils 0.49”
Rear 2019 Spring
4.25” Dia, 3” Ends, ~11.5” Free Length, 0.435 Wire, 5.5-6 live coils
2014-2016 Springs
Front 0.505” wire
Rear 0.440” wire 11” Free length[/COLOR][/FONT][/SIZE]
Other thing to add is that the plastic bearing above the flange of the front spring boot is tapered, hence the 80mm bearing used as a spacer. The OEM accordion boot is tapered in that area. The OEM front springs are actually slightly larger than 3.0” ID, but the fact that they end in a “pigtail” rather than a closed end like the coilover spring means they actually sit slightly differently, in such a way that the 3.0” closed spring is a good enough fit
The problem you will find is that OEM springs (which I did investigate somewhat) are about 99% designed to use simple linear springs with various seat sizes, not always the same top and bottom. At the vehicle design stage, rather than use something like a helper, someone like Ford can simply move the entire spring seat and select shock droop travel to work with exactly the spring rate and length desired. The problem is that in order to use a stiffer spring than factory design, you have to start with a shorter spring to get the same compressed height (for ride height)
“Conventional” 5.0” OD on at least one of the seating surfaces is more common. It’s pure weird luck that our stock setup uses such small seat diameters with barrel springs. It’s odd enough that if you look at the lower front perch, it’s adapted from a strut designed to accept a larger lower spring seat (which you may find a way to use, but that will further increase the required droop length.
I did go down the OEM spring rabbit hole as you’re talking about, but seemingly springs are pretty bespoke for each model of car, and short of visiting a junkyard and taking lots of measurements, information on spring dimensions are extremely sparse.
