There are a few basic forces at work in any fork or shock that must be understood before you can make successful changes from the stock settings.

Springs
The first thing a tuner needs to understand is that for the most part, SPRINGS primarily just hold the bike up. They return the bike to a pre-set attitude or sag any time the suspension is compressed. Springs are NOT the POWER in a suspension system! Shimming springs or replacing the stock springs with stiffer ones only changes the sag. The sag can be tailored to different rider weights by changing the spring either stiffer or softer, but really only affects the ride-height or sag, nothing more!

Hydraulics
The second thing is Hydraulics. This is really easy to understand as long as you remember that a fluid will NOT compress like a gas will, so a fluid is always going to be constant. Take for example a syringe with a needle on the end. The needle represents a valve. The bigger the needle, the FASTER you can push the plunger, and the smaller the needle, the SLOWER you can push the plunger. Its all about the SPEED at which the plunger, or forks, can be pushed. The valving in your forks work the same way as the needle. It restricts the flow of the fluid. In a fork or shock that is externally adjustable, all your doing by turning adjusters, is increasing, or decreasing the TIME it takes to push a given volume of fluid through the valve. If you change to a heavier weight fluid, but leave the valve as before, it will take LONGER to pass the same amount of fluid through the valve as before with a lighter weight fluid or oil. This works in both directions as in the compression and the rebound strokes. On most modern forks and shocks these valves are adjustable with the clickers. Remember that it's all about the SPEED at which the suspension compresses or re-bounds.

Air-Spring
There is a third thing to consider. This is the air-spring. The amount of AIR between the oil and the top cap inside the fork. This air-spring stiffness can be changed by lowering or raising the oil LEVEL in the forks. Whether the fluid is oil or water, it doesn't matter for the air-spring. The air spring is NOT affected by the weight of the oil or the valve setting and adding air ONLY affects the spring rate, kind of like an air shock on a car. It increases the load carrying ability just like a spring does. Adding air will ONLY decreases the sag. Air is also affected by temperature and altitude, so it's NOT a good way to change the sag. It's NOT constant! Don't add air! Make all your adjustments by mechanical means. Use the Schrader valves to bleed-off the excess air pressure. Use shims to stiffen the forks or go to a stiffer spring if needed.

Compression
If the forks or shock COMPRESS too FAST, then they will bottom out. If they compress too SLOW, then they wont use the full stroke, causing a rough ride, and a push in the front-end when your turning.

Rebound
Rebound is really misunderstood by a lot of people. Rebound controls the SPEED at which the forks or shock returns to its pre-set ride-height or sag, after it compresses. If the forks rebound too FAST, then the bike will push in a turn. If the rear rebounds too FAST, then the rear of the bike will kick-up sometimes throwing the rider over the bars, or at least, smacking him in the rump. The object to setting rebound is to get the wheel back on the ground as soon as possible, WITHOUT adverse effects.

Experiment with your suspension and expect excellence. Just remember what does what and know what your trying to fix before you start throwing money at it. Almost all suspension problems can be fixed with very little money and some trial and error. Remember that it's all about the SPEED at which things happen. If you think about it like that then you will be able to identify what needs to be fixed. Good suspension really only has one job, it needs to give the rider the plushest ride possible, over varying terrain, while allowing total control. Anything else is NOT acceptable!

Credits: Article written and submitted by New Old Stuff and edited by 4Strokes.com