Trial and Error may work, but I prefer to streamline the process a bit and employee factual data in the mix. It's still a bit of trial and error, but with quantifiable, measurable results.
This is a multistep process and will still be time consuming. It will however get you every last bit of grip you can get from the Spring Rates. Beyond that, you will need to factor in the weight transfer (controlled by dampers and ARBs) and camber/toe settings to fine tune even further.
Step 1:
To start, determine your "recommended" spring rate mathematically using the following formula:
- (Total Weight x (Weight Balance Percent / 100)) / Ride Height = Spring Rate
This will give you your starting point. The results are based on the total weight of the car, the amount of weight over each end of the car and the ride height of each end. It does NOT take into account aerodynamic load.
Step 2:
With your tire compound of choice, run several laps with the car either in a race, or in Test Drive/Time Trial mode, or any other mode that allows you to save a replay. Export the replay to MoTec Data format and review the info in MoTeC i2. You will specifically be looking at the Later G's and Longitudinal Gs. Simply put, the higher the numbers, the more grip you have.
Step 3:
Adjust spring rates and repeat Step 2. You are looking for improvements (increases) in both G-Force numbers. If the Gs have gone up, you have gained more grip. If not, then you have gone the wrong way
Generally speaking, harder compound tires need softer springs to gain grip, while softer compound tires prefer a stiffer spring rate.