I realized that many people are very confused about what this term (ZPS, SSD, etc...) means.
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Pre-Load...
Simple Definition:
The difference between a spring's free length and it's installed length.
Pre-load can be numerically represented in length, or the weight it would take to compress the spring to that length.
Pre-Load and Sag are not interchangeable terms. Pre-load is often used to control sag, and hence is often confused with sag. On a standard shock (non-ZPS/SSD) Pre-load is set to adjust the sag to the proper ride height.
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So,
Zero Pre-load (ZPS) technically means that the spring on the shock is not copressed at all when installed. If this were true, it would basically rattle in it's hardware (loose). Most of these types of set ups are really just shocks that have very soft initial springs that allow for very low ride height, and a lot of sag. There is no specific magic to these shocks, or any sort of special mechanism involved. Most shocks can be set up this way. It's just a matter of using a combination of springs that allow the bike to sag to a low ride level.
Most high end shock companies can set your shocks up to sag a little or a lot. It should depend on what you want. Some people prefer a lower ride height. There are some serious advantages to this. Some people however, prefer a higher ride height. There are some advantages to this as well.
There are some other folks on this board who have a lot of suspension knowledge too. If you feel you can explain this any better, please add your comments. This really seems like a term that is confusing many riders.
