WIP

Foreword

Setups are all the buzz, the only key to lap time must be within a secret combination of all settings - but hold on for a second. What can we actually do on the setup page?

As for Assetto Corsa Competizione it is first difficult to make general statements. While there are elements that will have the same or similar impact on all cars, there are also parts that will have a different outcome on different cars. All cars have different suspension construction or different aero maps for example.
Second, what we are given in the game on the setup page on each slider is a limited range of possible settings. That means a damper on 0 is not a damper that is "off" - it is just the softest possible setting the game allows. To give a random example, if a damper can provide a resistance within 10N and 1000N, the changeable range we get in ACC might only span across the values from 600 to 650. We eventually do not know. This is similarly true for ride heights, we can't go to 30mm and we can't go to 100mm on most cars. So, while we have plenty of options to play with, they are all already limited by default, which narrows down our options, but it also narrows down our room to screw things up completely. A high setup therefor does not become "a tractor" just because the setting is on one end of the slider. It might still be very low considering the capped off ride heights we do not see. 80mm is a very typical ride height on the Nordschleife and some cars by BOP get limited to not go below that in the real world depending on the series they are driving in - so the maximum in ACC is the absolute minimum in real life and as well in iRacing on the Nordschleife for example.

Tl;dr: It's a matter of perspective. Do not shy away from what appears to be extreme values - cause they might simply not be that extreme afterall. 

All things written here will treat the settings how they are working in the game. This will also not go into technical details, but stay on a level that is applicable quickly and hopefully for everyone

The Basics

1. Finding a direction.

There are several parts we can adjust and depending on the track layout and character they will play different roles and become more or less important in the hierarchy of settings. Therefor, we should first get a rough overview of the track. You can ask yourself things like:

• What's the average speed around the lap?
• how many slow speed corners are there (up until ~100 kph)?
• how many medium speed corners are there (up until ~150)?
• how many fast speed corners are there (roughly between 150 and 200)?
• especially important for fast corners: How do you approach them? Do you have to brake or do you accelerate through them?
• how many straights are there and how long are they in relation to the overall track?
• are there heavy braking zones (and how many)?
• is it a track with a grippy surface or slippery surface - or is the weather making it a slippery surface?
• Are there a lot of bumps or high kerbs you need to use?
• What car am I using and what are its general traits?

Surely there are more questions to ask. And the answers to those questions can lead to something like this: 
There are a lot of kerbs and bumps, so we'd like a soft setup. However, the track has a lot of high speed turns and we need aero stability with a rather stiff setup. Traction is very low, so we need a soft setup. Also our driver is not very experienced and needs a stable, more predictable stiff setup. There is a very long straight so we want top speed, but there are also a lot of fast turns that need downforce.

You can see, you will have contradicting directions for the same track and now its all about finding the right compromise.

2. Assessing the current setup

While of course there are often values you can be sure you want to adjust (like brake bias or pads), before starting to dial anything in, it's probably worth doing a few laps first and just listen to the car in the sense that you want to extract its behavior in various driving situations.

a) It is heavily driver dependent if you need to do more laps, but if you already know the track and car, then for now its fine to do 4-5 laps, I prefer doing the setup for medium fuel loads ~50-60L as that will lead to less behavioral shifts in both Q and Race direction. This will make sure the tires reach their likely final temperature and pressure. during the lap have half an eye on the temperature and whether or not we need bigger or smaller brake ducts. When a tire turns yellow inside, its likely we want to increase the brake ducts, if it turns blue decrease the brake ducts.

b) Go back to the pits and check your latest pressure readings and adjust them accordingly to reach around 26.5 on all 4 tires (e.g. if the latest pressure reads 26.3 add 2 clicks). We're not trying to achieve perfection here in the first step, just making sure nothing is entirely off. If you make changes to the brake ducts, add or remove another 2 clicks of pressures on that axle. Brake ducts are not linear, so sometimes its 1, sometimes 3 clicks of pressure to compensate. The step between ducts 2 and 3 seems rather impactful

c) go out again on a fresh set. Now we are looking at the behavior more closely. We always need to differentiate between fast and slow corners (and sometimes medium speed, too). Judging how the car behaves into corners when braking, coasting, how much it rotates in the middle of the turn and how it behaves onto the throttle. The big problem here is differentiating driver and setup error. If the driver has wrong approaches to a corner, then some behavior will not be tweakable with setup changes. So first make sure you are driving more or less correcly. Focus mainly on where your car is on entry, where it is the slowest in a corner, where it is the tightest in a corner and from where the throttle is applied. If this matches a benchmark closely, we can be sure we're assessing setup rather than driving issues. Now we are simply checking if the car is oversteery or understeery in all situations on that track

Checklist

• Behavior into slow corners (understeery, oversteery?)
• Behavior into fast corners (understeery, oversteery, specifically while braking and turning in, think Pouhon on Spa) 
• Brake lift-off behavior slow speed 
• Brake lift-off behavior high speed
• coasting behavior slow speed
• coasting behavior high speed
• throttle application slow speed
• throttle application high speed
• late exit behavior slow speed
• late exit behavior high speed
• direction changes slow speed
• direction changes slow speed

d) You don't actually have to write it all down I think. And we don't have to access every single corner and situation. As a start its fine to focus on those situations where the behavior really sticks out. So we are not looking for a bit of over or understeer that you can work around as a driver - we are looking for the too oversteery and too understeery situations, which reduces what we need to think about at once. 

3. Deciding what to adjust

Generally, for anything high speed, we need to think in terms of aerodynamics first and mechanical elements second. For anything slow speed we are looking at mechanical elements and weight balance first. Mechanical adjustments are easier to do, as their impact is less dynamic. In turn the impact of aero adjustments is very dynamic and more delicate. Additionally there is an intersection of aerodynamics and mechanics, in particular between ride height and the ride height controlling settings like springs and bump stops. All other settings will also have impact in every situation, but each has situations with big and situations with less impact. 

Aerodynamic impact (in order of magnitude)

• Ride heights
• springs (especially dynamic ride height)
• bump stops (hard[er] ride height limiations)
• roll bars (assuming that roll impacts aero in the game - aka ride height difference between inside and outside of the car)
• dampers

Mechanical impact (in order of magnitude)

• springs
• roll bars
• ride heights
• dampers
• bump stops (the stiffer the springs, the less impact of the bump stops)

 Intersection (fast turn in, cornering, exit)

• Bump stops
• springs
• ride height

Ok ok, wait a second. Everything matters everywhere, where do I even start? We first have to add another layer of complexity: The car.
Each car in Assetto Corsa Competizione has a different aero map. Here is an article dealing with the various aero effects and you should continue there if it isn't already clear. Every car also has a different weight distribution, but here it's enough to distinguish between front, mid and rear engine. 

That means that in order to do any changes in regards to aero or mechanical balance its important to know your car first. Clicking through the ride heights and wing settings will give you a good indication how sensitive the aero map is to ride height in general and how (im)balanced the aero develops with rake.