Now that I'm older than I was when I started building this car I've been looking into softening my suspension up a bit for future trips with the wife and incoming child. I pulled up the info from BC on the Swift springs for my coilovers and while looking for deals on the springs I found this site http://www.streettunedmotorsports.com/p ... mm_z65.htm
which has the tables of all the specs of all the Swift springs. According to BC our coils need a Z65-178-xxx up front and Z65-203-xxx rear (the format is [diameter (mm)]-[length (mm)]-[rate x10]). The 185 apparently has 60/40 weight distribution, which made BC's suggested 8kg front/4kg rear seem soft in the rear to me. What I didn't realize when I first bought the coilovers is that because the rear springs are longer, the spring rates listed by BC are more balanced than they sound.
According to the tables:
Suggested fronts - Z65-178-080 (8kg springs) - max load 2,064lbs
Suggested rears - Z65-203-040 (4kg springs) - max load 1,228lbs
60/40 = 1.5 (weight distro)
2,064/1,228 = 1.68 (load distro)
Since that's the softest option for that length of rear that's about as balanced as it could be without reducing stroke. I assume you could go to shorter springs and move the perches up but I don't know what effect that would have.
Looking at what I put in:
Front 10kg - max load 2,551lbs
Rear 8kg - max load 2,259lbs
2,551/2,259 = 1.13
Oops. That explains why the rear feels quite stiff to me. I'm leaning towards going to the suggested spring rates so that I can keep the stability in the corners while making some of the crappy roads around here more bearable.
If any of my logic or math here is wrong please correct me, I'd prefer to only have to change this once.