How to Set Up MTB Suspension Forks

Before you begin the suspension set-up, establish a generalized tire pressure and do your best to stick to it (though it may change depending on trail conditions). The more air pressure in your tires, the less the rolling resistance and chance of flats. Less air pressure in your tires will give you more traction, but also increase your risk of flats.

Check the sag. Wrap the zip tie around the upper fork tube. Push down on the bars five to 10 times to cycle your fork, then push the zip tie down to the fork seal. Wearing your normal riding attire, sit on the bike in a standard riding position. Have your helper hold you up or lean against a stationary object. Dismount the bike and measure the distance between the zip tie and the fork seal. This is sag.

Adjust your sag rate. General sag should be about 25 percent of your fork's total travel. For a more specific sag rate, you can use the following guidelines:
Cross-country (3-inch or less travel fork): 5 to 15 percent sag.
Trail bike (3- to 5-inch travel): 15 to 20 percent sag.
All mountain (5- to 7-inch travel): 25 to 35 percent sag.
Downhill (7- to 10-inch travel): 35 to 45 percent sag.

If you have a coil sprung fork you should have a preload adjuster on the top of the fork. To increase sag turn this knob counter clockwise. To decrease sag turn the knob clockwise.
If you have an air sprung fork you will need to attach the shock pump to the air valve on the fork in order to adjust the preload. Consult your forks owners manual for specific air pressure settings. If you don't have an owner's manual you can make minor adjustments and refer back to step two until the proper sage rate is found.

Set the forks rebound. Rebound is the rate at which the fork will spring back. Most forks have fast or plus and slow or minus. To adjust your rebound, turn the rebound knob all the way to fast. Next, add resistance until the fork compresses only once and returns to the sag position when you ride off a curb.

Adjust the compression damping. Compression damping controls the rate at which the fork is pressed together. When you land off a jump, hit a bump or brake for a corner the fork will compress. Most newer forks have a "lock out." With the fork totally locked out, the compression rate is minimal, making the fork stiff. With it fully open, the fork will be soft and squishy. There are settings between these that will allow you to customize the compression damping for varying trail conditions. Stiff compression is good for climbing hills and riding out of the saddle since it minimizes the amount of bob in the fork. Softer compression is good for going down hills and rough trail sections. Soft compression damping will absorb more bumps keeping your tire in contact with the ground and giving you more control and traction.
Try various settings and see what works best for you.