I have FCM coil-overs with 300 lb/in front and 224 lb/in rear springs. According to Shaikh, the front damping is at 0.58, 0.65 and 0.41 at 2/5/10. Rear is 0.48, 0.44 0.31. Tires are (very used) Michelin Pilot Super Sport 215/45/17 on Enkei 17x8 wheels.
I live in Santa Barbara County, California. If you've been here recently, you know the county has not been maintaining the roads - potholes, settling at bridge joints, etc. There are some great roads here, though I can't enjoy them because the car isn't comfortable. I don't autocross or race, so the setup was designed to increase handling while maintaining comfort. Ride height is as high as the coil-overs will allow, roughly 13.5".
At highway speeds (50+) the rear end feels like a pogo stick over expansion joints. Makes no difference top up or down. Concrete highways are the worst example. Particularly bad bumps on the highway launch the rear end up and my head lands against the roof. Any bumps encountered in corners (taken aggressively) will move the rear end off line. It was this way when the coil-overs were new as well.
At slow speeds, the best example I can give is drainage dips. The shape of the ditch is smaller than a tire. The front end comes out smoothly while the rear end follows the profile exactly without compressing, forcing the rear end up quickly. I notice this whether I am going 10 mph or 25 mph. I was able to go through these dips faster with the stock (sport) suspension (when new) and by the time the original shocks were due for replacement, the stock suspension was showing the same symptoms.
I also notice a tendency to over-steer at all speeds. Rain slick roads require extra careful throttle application, especially on sanded-smooth agricultural roads that I rarely had traction on in dry weather. (How many of you can do burnouts in third gear at 25 mph with a stock car?) Although aggressive off-the-line starts on 'normal' roads just causes wheel hop.
Put another way, it seems the rear end doesn't have any compliance over these fast-occurring events. Any other suggestions for me and Andrew?
2007 Silver MX-5 PRHT, 6-speed Manual, Sport Package, Roadstersport Street Single w/Baffle, FCM Coilovers - "GT" setup with KBO & Ripple Reducer, Kosei K4R 16x6.5 wheels, Goodwin Underbody Braces