drharveys - FSO wrote on 12/29/07 at 22:19:52:It took me a while to get a handle on just how the pre-load works. As I see it now, it adjusts the range of movement of the suspension to match the load.
So, in effect, adding pre-load doesn't shorten the spring so much as it extends the shock when it's under load. This allows us to tune the suspension to the load.
OK, all you experts -- do I have it right?
I've assembled many types of suspensions for different HEAVY vehicles. Spring loaded; stacked rubber; gas over oil (nitrogen); and the pre-load principal is always pretty much the same.
Stop thinking of the changing length of the shock, although that is sometimes a byproduct of "pre-load." The important thing is to think of the spring's action as having been "pre-compressed" (interchangeable with "pre-loading").
If a spring's action has been pre-compressed to a weight of 100#; then it will not sag until more than 100# of down-force has been applied. When you change the position of the spring pre-load on a rear shock, or raise the gas pressure in a gas shock, you are simply raising the weight factor which must be applied to the shock before it begins to compress.
Then you get into rebound/recovery rates of the internal oil and valving / passages which control the rate at which the shock may move in either direction. That is why some shocks drop easily, but raise hard, or vice versa.
It is the interplay between progressive spring rates and the adjustable pre-loads and variable valving; especially in gas over oil shocks, that can make suspension tuning a black art.
Addition: I should have mentioned that you ARE getting a grip on the basic principal of suspension theory. I just wanted to explain more and take you farther.