|Preload - What does it do?
Preload is perhaps the most misunderstood facet of suspension tuning. Many people think that by adjusting preload that they are stiffening or softening their suspension. Nothing could be further from the truth. Adjusting preload does nothing to your spring rate. (technical explanation below). All it does is change your ride height and your sag.
So, given that, what should you do with those adjusters on the forks?? Basically, just use them to set your sag correctly.
For more info on the procedure for that go to Setting
Sag On Your Street Bike
You can also use preload to make small changes to the ride height. Generally it's better to move the fork tubes up or down in the triple clamps to accomplish this, but on some bikes that's not possible. In those cases a little compromising between the perfect sag and the perfect ride height may in order. Sag is important though, so don't go any more than 10mm in either direction.
A little thought experiment.
Let's assume that we have a pair of fork springs, each with a rate of 50 lbs/inch. The pair then have a combined rate of 100 lbs/inch. That means that it takes 100 lbs. to compress the fork the first inch, another 100 lbs. for the second inch, (total of 200) and so on. Also assume that the bike and rider together place a 300 lb load on the front end. If the fork is assembled with zero preload then the springs will compress 3 inches and the fork will compress 3 inches total. How much additional force does it take to compress the next inch?? 100lbs. Now say we add an inch of preload. The first 100 lbs of bike weight don't cause the forks to move, the next 200 lbs. make it compress 2 inches. The spring though is still compressed 3 inches and it still takes 100 lbs of force to compress the next inch.
Well, what about progressively wound springs, wouldn't preload matter there?? Nope, same theory applies. The fundamental concept is that the spring has to compress a certain total amount in order to support the weight of the bike, and it doesn't matter whether or not some of that initial compression is from preload.
If you've really thought this through you may be thinking, "Hmmm, the steel spring's not the only thing holding the bike up, there's also the air pressure inside the forks." That's true, but air is just a progressive spring, and combining it with the steel straight-rate spring doesn't have any magical effect.