How to Set Suspension Sag on a Mountain Bike

How to Set Suspension Sag on a Mountain Bike
Sag is an important setting on a mountain bike that uses either front or full suspension. It is basically a measurement of how much your suspension compresses when you sit on the bike. Sag should be set so that it is 20 to 25 percent of the total travel of your suspension. Setting the sag correctly ensures that your bike's suspension responds the way it was designed without being too stiff or bouncy. Setting the sag is merely a matter of taking a quick measurement and adjusting your preload accordingly.


Difficulty: Moderate

Front Suspension Fork

Things You’ll Need:
  • Zip ties
  • Ruler or measuring tape
  • Calculator (or good old-fashioned pencil and paper)
Step 1
Attach a zip tie to one leg of your fork. You want it firmly against the seal where the stanchion meets the lower fork leg. The stanchion is the piston-like part of the fork that moves up and down into the lower part of the fork. Be sure the zip tie is tight.
Step 2
Sit on the bike. With the bike firmly secured in a rack or against a wall, sit down as you normally would with your feet on the pedals and weight distributed as it would be if you were biking. Stay seated for about a minute or so to allow the shock to settle in place.
Step 3
Get off the bike.
Step 4
Measure the distance traveled. The zip tie will have traveled up the stanchion. The space in between the zip tie and the seal on the fork leg is your current sag---it reflects how much the fork compressed when you sat on the bike. Measure this with a ruler or measuring tape. Be sure to use measuring units that correspond to how the travel on your fork is denoted, probably millimeters or inches.
Step 5
Do the math to get the sag percentage value. In this article, Z is defined as the space you just measured from the zip tie, and T is the total travel of your fork. To calculate, it's simply: Z/T x 100.
Step 6
Adjust your preload. If you're lucky, your calculation will have come out to be 20 to 25 percent and your sag is already good. If the percentage is less than this amount, you have too little sag and you'll want to decrease the preload, which will allow the spring to compress more when you sit down. If it's more, you'll want to increase the preload, which will make the shock firmer. For coil shocks, turn the preload knob clockwise to increase the preload and counterclockwise to decrease. For air shocks, add air with a shock pump to increase, let air out to decrease.
Step 7
Retest the sag to ascertain the 20 to 25 percent target.

Setting Rear Suspension Sag

Step 1
Measure the rear sag if you have a full suspension mountain bike. The rear shock may already have an O-ring attached that can be used in place of the zip tie. Push zip tie or O-ring to the seal between stanchion and main shock.
Step 2
Sit on the bike in your normal position.
Step 3
Get off and measure the distance that the zip tie or O-ring moved. Calculate percentage. Note that rear shocks don't have the straightforward travel that forks do; if your rear suspension offers 5 inches of travel, for instance, this is 5 inches of wheel travel, not shock travel. To calculate sag, you'll need to use shock travel, also known as stroke or stroke length, not wheel travel. Refer to your owner's manual to identify the shock travel or measure the stanchion on which the shock slides down. You still want your sag at around 20 to 25 percent.
Step 4
Adjust the preload on the shock accordingly.

Tips & Warnings

Set the sag when you first get your new bike and check it periodically.
Check your bike or suspension manual for recommended sag, as this can vary by mountain bike type and manufacturer.
In general, cross-country riders may consider less sag, as they want to keep the bike more stable for long ascents, while free ride and downhill riders could benefit from more sag.
If you are using a coil shock and cannot get the appropriate sag by preload adjustments, you may need a heavier or lighter spring.

Article Written By Joe Fletcher

Joe Fletcher has been a writer since 2002, starting his career in politics and legislation. He has written travel and outdoor recreation articles for a variety of print and online publications, including "Rocky Mountain Magazine" and "Bomb Snow." He received a Bachelor of Arts in political science from Rutgers College.

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