Autocross to win is a good read.

We are out of topic but did you realize that your 5/5K Ohlins springs are 200mm while the "standard" 4/3K springs are 230mm length?

Ohlins spring 48020-07 (40Nmm): 65mm/230mm
Ohlins spring 48020-03 (30Nmm): 65mm/230mm

Ohlins spring 48010-11 (50Nmm): 65mm/200mm

I know they do it due to softer springs but 30mm is quite a lot of suspension (bump+rebound) travel difference!
In my humble opinion for road use and bad surfaces as yours and mine suspension travel is critical factor no matter the setting you have.
I set zero toe rear and some toe out front, steering response is little bit improved but when I fit the Ohlins I'll try some more toe out front and same camber front-rear, may be is my driving style or our slippery roads but rarely I felt my car understeering.

Spring length doesn't limit travel unless you preload to the point of coil bind or have a spring that is way too short. Thanks for all the tips on books folks.

Length of spring does not correlate 1:1 with the travel of the spring even using the same stiffness.

A 65mm ID 5k Eibach spring of 200mm length has 5.06" (128.5mm) of travel
A 65mm ID 3.5k Eibach spring of 230mm length has 5.93" (150.6mm) of travel
So a 30mm shorter spring that's 1.5k stiffer loses only 22.1mm of travel.

A 65mm ID 5k Eibach of 250mm length has 6.12" travel or 155.4mm so only a 26.9mm increase in travel despite adding 50mm of spring length.

Source:
https://eibach.com/de/en/products/mo...-spring-system

The way the spring is wound to achieve the stiffness is everything and every manufacturer is a bit different, wire diameter and coil spacings drive the springs performance and manufacturers aren't going to build every spring optimally they'll make a compromise to hit the ~$70 price point that people buy linear rate springs at. Spring travel isn't critical if you have a decent enough quality spring, to fully load that 5k spring and compress it 5"-6" you're looking at almost 2x the load that the car sees statically in the front, 3x for the rear.

Now I have to dust off some textbooks and see what that works out to in lateral g's... roughly speaking of course.

And? The stiffer spring will compress less and doesn't need to be as long.

Taking my numbers from the other thread the actual load on the spring at ride height is around 360kg. That compresses the 3k spring 120mm, and the 5k spring only 72mm. Turning that around, if you give the 5k spring 30mm of rear pre-load there will be only 42mm of droop, which I think might not be enough. Generally you want to have at least as much or more droop travel as bump (if your spring rates and travel will allow that). So I would take out some of that preload, maybe go somewhere in the 0-10mm range.

Yup. The preload helped with bottoming a bit but contributed to the post bottom-out buck in the rear. I think the shock is topping out and thats the buck I feel. Thats why I'm trying to learn more about tuning before continuing with my random, confused, and inexperienced tuning. I'm going to take everything to factory recomended settings after control arm install and play with it some more in the summer after I at least have some more idea of whats going on. I have as much fun playing with settings and seeing outcomes as I have fun driving anyways so no loss for me. I got this car as a toy and its serving its purpose well.

Of course 30mm length difference doesn't equate to absolute same travel difference except we talk about the same spring shape.
Number of coils/size/diameter/material play their role (http://www.reliablespring.co.uk/calculator.htm) but why Ohlins choose 4/3K springs at 230mm and available only in BRZ platform then?
I believe the softer springs can loaded easily to their limits as OEM suspension that works on bump stops.

PS: In your example the two 65mm 5K Eibach springs with 50mm length difference are same size/diameter/number of coils?
Otherwise we can't compare their travels.

Do you know the length of bump stop as to calculate the bump/droop for each different spring?

PS:I meant suspension travel favors bad roads

Can I just say, it’s awesome to see someone taking an academic approach to cars. I’m taking physics and auto shop at my school next semester for the same reasons hhaha.

LOL when I started tinkering with cars, "How to Make Your Car Handle" wasn't that old...

It's a great book to get a good foundation on suspension tuning. Most of the information, though old, is still applicable. I think it is outdated wrt tire technology, though.

Another good book is Race Car Engineering and Mechanics by Paul Van Valkenburg.

I'll have to go check out RCVD...