Originally Posted by aosborn
No, not referring to the preload. When I input the weights and angles for my car using a 6.3" travel shock and a 12" spring, performance mode recommends a 150# spring with .47 preload. Then when I input the same numbers but change to a 4.1" travel shock with a 10" spring, performance mode recommends a 200# spring with .52 preload. I don't understand why when changing spring lengths, the spring rate needs to change.
I think what they are doing is giving you the fully extended preload setting for each spring rate. To end up at the proper "at ride height" dimension with the different rate springs with the same given load, there might be packaging constraints (length of coil-over threads for example) figured into their formula that would require you to have a different rate to give you enough bump and rebound travel and still be within the adjusment range of the shock.
It is somewhat of an akward calculator, but I think it will get you very close to the right rate, though their ride catagories seem to be a bit on the stiff side. I would think going one catagory softer than what they recommend would be a good idea. If that makes your car a little on the softer side in springing, make up for it with stiffer anti-roll bars.