ok it may sound like a dumb question, but i really dont know the answer. do the stock springs sag because their spring rates have dropped? or has the metal started to bend under the weight of the vehicle, but still retains the spring rate? thanks.

 

Spring rate should stay close (unless they see extreme temps - eg -40 deg will make the metal harder). The sag is caused by slip in the crystaline structure. So the springs should be shorter unloaded.

 

They sag because of constant weight on them, and constant flexing over the years. Larger heavier cars will tend so sag more than lighter weight cars. Age is a big factor.

 

As David said the spring will eventually sag due to fatique of the metal, but the spring rate will remain nearly the same.

For a given diameter of wire you can make a spring softer by adding coils. When the spring sags and becomes shorter but with the same amount of coils, it must also lose some of its rate. Unless your talking more than an inch or so it probably ain't much though.