Because mountain biking deals with some rough terrain, it's important that you make sure your shocks are properly adjusted to match your biking needs. Most mountain bike shock systems that use air pressure have adjustment knobs that you can use to tweak your shock system to achieve the appropriate preload for your bike.
When it comes to mountain bike shocks, the preload refers to the amount of sag the shocks allow when the bike is at rest with only the rider's weight bearing down on it. Determining the proper preload is important because if it is too great, it will take more energy to move the shocks, resulting in a harder and desensitized shock system. If it's too low, your shocks won't be supportive when you need them to be. A downhill mountain bike's preload sag should be about 40 percent of the shock's total sag, meaning when you're on the bike at rest, the shocks should compress about 40 percent of the way to the bottom.
Positive Preload Springs
The positive preload springs are the main spring found in spring-based shocks and are responsible for keeping the bike level while you are riding over rocky terrain. When you increase the preload on the positive fork, the top spring will have more force behind it then the negative spring and will result in a lower sag. It's important to keep these two springs balanced or your bikes shock system will become uneven and potentially dangerous if you're biking on rough terrain.
Negative Preload Springs
The negative preload springs on mountain bike shock forks are responsible for repelling the force generated by the positive preload shock spring, keeping the two balanced and keeping the sag from becoming too great or from the shocks compressing too quickly. Negative preload springs can also be found in air shocks, but they have a different role than with spring shocks. Negative springs in air shocks are small springs that push against the main spring to get the spring moving as air shocks sometimes down will pick up the sensitive suspension changes caused by small bumps.
While spring-loaded shock systems are easily adjusted using knobs found on the shock forks, air shocks are a little more difficult to adjust for the right preload. The systems use positive and negative air chambers to balance out the pressure level inside of the shock system. These systems work like a piston with the actual shock being reduced by the air pressure built-up inside. To adjust this type of shock system, you have to alter the air pressure in the shock system or change the amount of lubricant oil found in the chamber.