The Math
With all the foreplay out of the way, it's time to finally get into some number crunching. Calculating the optimal spring rate for any car can be expressed in the following equation, where "F" is the free length of the spring, "L" is the loaded length of the spring, "R" is the baseline spring rate, and "T" is how much the shock collapses at ride height.

(F - L) R = Spring Rate

For the sake of illustration, let's presume that a 500 lb/in spring (R) measuring 8.94 inches (F) compresses down to 6.50 inches (L) at ride height. The difference between the two spring height figures is 2.44 inches, which yields a product of 1,220 when multiplied by the spring rate of 500 lb/in. When matched with a shock featuring 4.25 inches of travel, setting it up for a 50/50 cornering application requires the shock to collapse 2.13 inches (T) at ride height. Finally, dividing 1,220 by 2.13 results in a quotient of 572.77, which is the ideal spring rate for this application.

In this example, the baseline spring rate of 500 lb/in was too soft, which resulted in a shock that collapsed more than the desired amount at ride height (2.44 inches). Not only would this spring move the shock outside its sweet spot, it would also bias its travel too far toward rebound, not leaving enough travel for optimum compression valving and potentially degrading ride quality. Although it may seem trivial, the extra 73 lb/in as calculated in Chassisworks' formula would effectively decrease the amount the shock collapses at ride height to hit the target sweet spot of 2.13 inches. That yields an equal amount of shock travel in each direction for compression and rebound (50/50), which is perfect for many road course or autocross setups.

Coilover Street Handling Drag Spring Street Handling Drag
Shock 60/40 50/50 40/60 Free Max. Center Min.
Travel (T) Spring Travel Used At Ride Height Length Installed Spring Length At Ride Height
2.80 1.12 1.40 1.68 7 5.88 5.60 5.32
3.50 1.40 1.75 2.10 7 5.60 5.25 4.90
4.25 1.70 2.13 2.55 9 7.30 6.88 6.45
5.15 2.06 2.58 3.09 12 9.94 9.43 8.91
6.15 2.46 3.08 3.69 12 9.54 8.93 8.31
7.15 2.86 3.58 4.29 14 11.14 10.43 9.71
Use above values for "T" in calculation table. Compressed spring length will be close to above values when using the correct baseline spring rate.