How do I decide which springs to buy?
The springs on my S4 need to be replaced - the left side is 1 1/2 inches lower than the right side. The goal is to have a car that is fun to drive but recognizes that roads have dips, bumps, cracks, and holes. It appears that a lot of springs offered are designed for racing and "fast street" use. My understanding is that lowering springs significantly change the roll center's (RC) location and might even be detrimental to the S4's handling. Handling is determined by the relationship between the RC and CG, not by a low CG. I would need to model the suspension to understand the impact of a changed RC.
If vendors provided spring rates I would not be be writing this post. Unfortunately, they don't and they don't provide information wire diameter, coil diameter and number of coils that can be used to calculate the spring rate. If anyone has information on spring rates or information that can be used to calculate spring rates, it would be appreciated if you would share it.
The S4's factory springs provide the Spider with a natural suspension frequency of 1.1 cycles per second (cps) in front and 1.4 cps in back. I would like to have a front suspension frequency of 1.4-1.5 cps. The Spring Database I downloaded from the alfabb says most of the front after market springs have a spring rate of 1100-1200 lbs/in which produces a front suspension natural frequency of 1.8-1.9 cps. It alludes to the Alfaholics front springs' spring rate being between 600-800 lbs/in.
To come up with the natural suspension frequency I assumed a 50/50 weight distribution, calculated the front suspension motion ratio (MR) to be 2.5, and the rear suspension MR to be 1.19. The approximate sprung mass supported by each front wheel is 535 lbs. and approximate sprung mass supported by each rear wheel is 500 lbs. I used the Spring Rate Calculator rev3 spreadsheet for the calculations.
I'll update the spring database over the next few weeks with the listed spring's natural frequency.