[949 Racing]

Xidas were mentioned here so I though I'd chime in.

Everything else being equal, spring rates need to be tailored to roll moment. Roll moment is basically how much force is trying to roll the suspension over. Bigger, stickier tires generate more roll moment.

The other factor is CG (Center of Gravity) vs RC (Roll Center). Cars are designed with RC below CG. Imagine the RC is a virtual pivot down the centerline of the car. The CG is above that so when you turn, the CG generates a moment (leverage) over the RC. The farther the CG is from the RC, the more leverage it has.

With pretty much any car suspension, the RC drops when you lower the car. Because control arms can not be infinitely long, the RC will drop faster than the CG. Meaning that when you lower your car 2", your CG drops exactly 2". Your RC however, might drop 3 or 4 ", depending on suspension geometry. Why does this matter? Because, as we noticed earlier the further the CG is above the RC, the more leverage it has. This effectively lowers rolls stiffness. Lowering your 86 2" is the exact same effect as putting smaller sway bars on.

Combine this reduction in roll stiffness with a greater roll moment from bigger tires, then reducing total bump travel because it's lowered, and you have the recipe for lotsa bottoming. This is why such high spring rates are required to keep a lowered car with sticky tires off the stops.

So think about the roll moment you plan to tune in (how much grip from wheels/tires) and how low you will run it when choosing spring rates.

The 86, in our opinion actually has quite a bit of roll stiffness relative to other cars in this segment. Also has very high bounce frequency OEM, meaning fairly stiff springs. This is why our Street spring pack (175/150) for the Xidas is so soft. We actually had to regress from OEM a bit to get the ride quality we were after. Those soft rates still work on track but only with medium grip tires and with the damping adjustment cranked up. Not a great track set up but still fun if you don't have a full track wheel/tire setup and are focused on comfort.

The Race spring pack (500/500) was balanced for 255/40/17 Super200 tires on 17x9 and about stock weight. Just enough spring to just kiss the stops here and there on the bumpiest tracks. They're pretty busy on the street but liveable if you are after max grip with your race wheelset.

As someone else mentioned, valving, shock length, bumpstop shape/durometer/length all play a role in compliance, ride height and grip. I'm still not sure how so many 86 owners run 3-4" below stock. Out 255's smash into the chassis when we got that low unless we ran springs so hard that the suspension stopped working. Compliance = grip. The stuff Blub will soak up with impunity still amazes us.