Originally Posted by cal1320
On a street car, you would normally want the shock in the center of travel at ride height. With a 12'' spring, you have 6'' of shock stroke (3'' to center of shock stroke). A 10'' spring will have 4'' of shock stroke(2'' to center of shock stroke).
Using the same front end weight with both springs, the 12'' spring will need less spring rate so it can compress the additional 1''.
I have always used 2/3 travel in bump (compression) and 1/3 travel in rebound (extension) as a basic rule for designing mounts and specing out shocks. The length of the spring won't affect the rate necessary. The same load is applied to a 10" or a 12" coil-over with everything being equal other than shock length. The only real difference we are talking about is more travel with a longer shock which is what I would recommend.
With that being said, If the rate I am after falls between two available choices, I go with the softer spring and just add pre-load to make up the ride height. By going this route, having the longer coil-over and spring is good so you will have plenty of travel without the spring binding up.