Dave, The tyres are Hoosier 315/30 18's on all 4 corners.
It shouldn't make any difference if they were 185/70 14's [if they could fit]
The distance from the axle centrelines to the top of the arches would still be the same so the rake would be the same [just the ride height changes]
Absolutely agree, HOWEVER, the spec you gave was from ground to fender arch, and tire OD
will affect that number. The FR500C manual lists two places to take measurements from to determine "proper" ride height, and they are tire-independent. I'm not certain where it was up front, but I believe it was a height differential between the bottom of the ball joint and the center of the FCA forward pivot bolt. I do remember that in back it was the distance between the top of the axle tube and the frame arch. I do remember the manual recommending the fabrication of "go-no/go" sticks for each for rapid setup.
I didn't know about the extended ball joints so I just crawled under to have a look, Yes it has them.
These ball joints alter the instant centre and roll centre not the ride height. That is determined by the distance between the upper platform and the stub axle.
Back to my car, I'm learning new things about it all the time. The level of detail is amazing and would be cost prohibitive to replicate.
My particular car was campaigned by "Blackforest Motorsport" in Florida.
Yessir, the FR500 program came out with three extremely well-sorted cars, developed by Multimatic. FR500C came first for the Grand-AM series, the FR500GT was built to euro-spec, then the FR500-S was developed for the proprietary Mustang Challenge. Well built, well-sorted race cars, all of them. In theory, you COULD build a "privateer" version without buying one, but as you mentioned, the labor involved would be insane to try to duplicate. If you WERE building a car, you could certainly take some design cues from the program, though, and even use some of the specific parts to help speed you along. I have a FR500-S chassis harness in mine, as it was more "cost effective" to buy the de-contented harness than it would have been to do the work myself. WAY too many hours! The front roll center correction is a perfect example of how the cars are designed and built. The development team had their crap together, and came up with a well-engineered package The lowered ride-height, taken in isolation, would have wrecked the camber curve, the front roll center (and thus roll couple with the rear) etc. The Sachs dampers are shortened, and they relocated the front roll center by extending the studs on the ball joints, which in turn necessitated the extended studs on the tie rod ends to correct the resulting bump-steer... Take any one of the pieces in isolation and you have drama. All three? Functioning race car.
For autocross purposes, I think you should be focusing on individual front and rear ride heights and letting the amount of rake fall wherever it may once you've found the ride heights that work best for you. It's things like the front vs rear geometric roll center heights and the amount of rear axle roll steer that will affect transient handling, not the side view appearance.
Absolutely agree.
As a datapoint that might be useful, lateral accelerations that peak above 1.3g lateral are entirely possible for an S197 on 285/35-18 MPSS tires (25.9" tall) with the car still on its OE springs. So OE ride heights, OE rake, about 0.6" lower than OE CG height and roll center heights. Just stiffer sta-bars, Koni yellows, and about -1.8° camber.
Norm
This, right here, is a telling point. If this is your first outing on a new tire, plan on using the outing to figure them out. Pressures, temps, etc. If the car has been doing "fine" on the current chassis setup, I would avoid any changes simply to keep the variables down. Otherwise, you'll be hard pressed to tell if the oversteer you're experiencing is due to rear pressures, rear roll-steer, or damper/bar tuning. Change one thing at a time. If this is your first event, period, then just focus on getting used to the environment and the style off driving you're going to need to get through the course.