Let me ask you a question outside of the prior post.
Why if you shorten a coil by preload does it make the ride height go up, yet a higher rate coil at the same height will not need as much preload to equate the same ride height?
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Okay, that's a reasonable question. Let me see if I can explain.
I will assume your question is directed towards motorcycle suspension?
Your bikes forks for instance has about 10mm of preload adjustment. However, when the spring is installed at the factory, it always has some preload dialed in by installing let's say, 315mm of spring into a fork that only has room for 300mm. Now you have 15mm of preload without using any of your preload adjustment.
With your preload adjustment backed all the way out, you sit on your bike which overcomes the 15mm or factory preload and compresses the forks let's say,,,,,,,,,,,,,,,,, 40mm (Rider Sag). The spring is no longer topped out against the spring stop and is compressed 40mm.
Here is were the fun stuff comes in :-). While you are sitting on your bike, crank in all of the 10mm of preload available to you in your preload adjustment. What happens? You guessed it, you can't compress the spring any further without adding weight. What takes place is by adding preload and pushing down on the top of the spring which will not compress without adding weight, you have raised the front of the bike by 10mm. You have sucessfully changed your sag number from 40mm to 30mm.
Setting your sag using preload is nothing more than pushing down on the spring in order to raise the front or rear of the bike. Or lower according to which way you wish to go.
Suspension is an art all it's own. Greek to most but once you get a grasp of how it functions and how twiddling all those little knobs affect your bike's behavior, it becomes great fun. If you are truely interested in digging deep into this mysterious creature, I'd suggest picking up a copy of "Sport Bike Suspension Tuning" by Andrew Trevitt. Andrew does a great job with lots of illustration photos accompanied by detailed explainations. Another good source is Dave Moss's dvd's on suspenson. Dave get's into detail on how to read your tires (My favorite section). Great info!
Oops, let me add one more thing directed to your question above.
You can only compress the spring using preload if the spring is topped out. Once you add the weight of the bike along with the rider, the fork and shock are no longer topped out so adding preload will not compress the spring further. However, adding preload will raise the bike.
