Dont' be confused about stiffness though. Adding preload to a spring does not alter the spring rate 'stiffness'. It only does just as it's name suggests... pre-load the spring.
Let's say a spring has a rate of 2.5 lbs/in. and has a total compression length of 3 inches. That means it takes 7.5 lbs. of load for it to compress completely. If you put a load of 2.5 lbs on it, it will compress 1 inch. Now if you put 1 inch of preload on the spring you essestially shortened the spring by one inch, however the total length of the shock has not changed. The shock is now at full length except it is contantly under 2.5 lbs. of load. At this point the shock will not compress until there is more than 2.5 lbs. of load put on it. So basically if you have two shocks with the same 2.5lbs/in. springs and 1of them has 1 inch of preload and you put 2.5 lbs. of load on both of them, only the shock w/o the preload will compress 1 inch. The other will not compress at all. If you continue to add load (weight) to the shocks past 2.5 lbs. they will compress at equal rates. That is to say if you add an extra 2.5 lbs. of weight to each of them on top of the 2.5 lbs. already there they will both compress 1 more inch. I hope I didn't make that too confusing. :shrug:
