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F-Series FIT Terralogic®

Installing Your Fork | Before You Ride | Setting Sag | Adjusting Rebound | About Terralogic® Brassmass™ | Adjusting Terralogic® Threshold | Service Intervals | Important Safety Information


F80 RLC: 3.1 in./80 mm
F100 RLC: 3.9 in./100 mm
F120 RLC: 4.7 in./120 mm


Terralogic® threshold adjust, air spring pressure, rebound

spring/damper type

air/Terralogic® FIT

intended use


Installing Your Fork

Be sure your fork is properly installed before proceeding. If your fork came pre-installed on your bicycle, continue to the next section.

Before You Ride

  1. Check that quick-release levers are properly adjusted and tightened.
  2. Clean the outside of your fork with only mild soap and water, and wipe dry with a soft dry rag.

    Note:  Do not use any solvents or de-greasers, as these products can cause serious damage to paint and anodized parts (upper tubes, knobs, steerers).

    Do not spray water directly on the seal/upper tube junction. Do not use a high pressure washer on your fork.

  3. Inspect the entire exterior of your fork. The fork should not be used if any of the exterior parts appear to be damaged. Contact your local dealer or FOX Racing Shox for further inspection and repair.
  4. Check your headset adjustment. If loose, adjust it accordingly to your bicycle manufacturer's recommendations.
  5. Check that all brake cables or hoses are properly fastened.
  6. Test the proper operation of your front and rear brakes on level ground.

Setting Sag

You can also view a Flash video on Setting Sag.

To get the best performance from your fork, it is necessary to set and adjust sag. Generally, sag should be set to 15 – 25% of total fork travel.

  1. Unscrew the blue aircap on top of the left fork leg to expose the Schrader valve.

  2. Attach a FOX Racing Shox High Pressure Pump to the Schrader valve.
  3. Pump your fork to the appropriate setting from the Suggested Air Spring Pressure Settings table below, then remove the pump.
  4. Unless your fork already has a sag setting o-ring on an upper tube, install a zip tie to an upper tube. Slide the o-ring or the zip tie down against the scraper lip of the fork dust seal.
  5. Dressed to ride, position your bike next to a wall or table to support yourself. Without bouncing, mount your bicycle. Assume a normal riding position for at least 30 seconds, allowing the suspension to fully settle.
  6. Dismount your bike without bouncing downward, without further moving the o-ring or zip tie. Measure the distance between the seal and the zip tie. This measurement is your sag value.
  7. Compare your actual sag value to your target sag value in the Sag Setup table below.

    If your sag measurement value is less than the Sag Setup table value
    , screw on the pump fitting, note the current air pressure setting and depress the black bleed-valve to reduce the gauge pressure by 5 psi. Measure sag again and repeat this adjustment, if necessary.

    If your sag measurement value is greater than the Sag Setup table value
    , screw on the pump fitting, note the current air pressure setting and pump to increase the gauge pressure by 5 psi. Measure sag again and repeat this adjustment if necessary.
  8. Screw the blue aircap back on, and go ride.


    Suggested Air Spring Pressure Settings

    80 mm

    100 mm

    120 mm





    125 - 135




    135 - 145




    145 - 155




    155 - 170




    170 - 185




    185 - 200




    200 - 215




    215 - 230




    230 - ≥250




    Sag Setup


    XC/Race FIRM

    All-Mountain PLUSH

    80 mm (3.50")

    12 mm (0.50")

    20 mm (0.75")

    100 mm (3.90")

    15 mm (0.62")

    25 mm (0.98")

    120 mm (4.70")

    18 mm (0.69")

    30 mm (1.19")

    Sag Troubleshooting



    Too much sag

    (+) air pressure in 5 psi increments

    Too little sag

    (-) air pressure in 5 psi increments

    Excessive bottoming

    (+) air pressure in 5 psi increments

    Harsh ride; full travel not utilized

    (-) air pressure in 5 psi increments

Adjusting Rebound

The rebound adjuster knob is located at the top of the right fork leg. This knob has 18 clicks of adjustment range.

Note:  Before you set rebound, after taking note of where your Terralogic® threshold adjustment is set (how many clicks out from clockwise in), turn the blue Terralogic® threshold knob counter-clockwise fully. This step allows you to adjust your rebound speed more easily, free of any potential interference from Brassmass™.

Rebound controls the rate of speed at which the fork extends after compressing. Turning the knob clockwise slows down rebound; turning the knob counter-clockwise speeds up rebound. As a starting point, turn the rebound adjuster knob all the way clockwise (full in) until it stops, then turn counter-clockwise (out) 12 clicks.

Knob Setting
(clicks out from full in)

Setting Description

Tuning Tips

Setup Tips


Slow Rebound

Too slow and your fork will pack down and ride harshly.

If you increase your spring rate or air pressure, you will need to slow down your rebound


(Factory setting)

Average Rebound



Fast Rebound

Too fast and you will experience poor traction and wheel hop.

If you decrease your spring rate or air pressure, you will need to speed up your rebound setting.

Terralogic® Brassmass™

The Terralogic® damper contains a weighted brass inertia valve called Brassmass™, which freely slides on a shaft to open and close compression flow ports in the damper.

The Brassmass™ assembly automatically responds to the impact of bumps or potholes in the trail, opening the compression flow ports and allowing the flow of damping oil. The Brassmass™ assembly does not respond to rider input (pedaling, sprinting, G-outs), keeping the compression flow ports closed and maintaining the pedaling platform or the feeling of fork lockout. With the patented timer circuit constantly in control as you ride, the transition between maximum pedaling efficiency and low speed damping control is nearly instantaneous.

The 15-position blue Terralogic® threshold adjust knob located on the bottom of the right fork leg allows you to fine-tune your pedaling platform balance anywhere between lockout-firm to fully open plush damping (or zero pedaling platform).

Adjusting Terralogic® Threshold

Terralogic® threshold is an adjustable compression blow-off valve that bypasses the Brassmass™ assembly, adjusted with the blue knob located on the bottom of the right fork leg. This knob enables you to regulate compression platform when the Brassmass™ assembly is closed.

Note:  If you have just set your rebound, return the blue Terralogic® threshold adjust knob to fully clockwise in, before making your Terralogic® threshold adjustment.

To make pushing through the fork travel harder and increase the pedaling platform, turn the threshold adjust knob clockwise. Turning the adjust knob completely clockwise makes the fork feel locked out—until the Brassmass™ activates from the force of impacting a bump in the trail terrain.

To make pushing through the fork travel easier and decrease the pedaling platform, turn the threshold adjust knob counter-clockwise. Turning the knob fully counter-clockwise bypasses the effect of the Brassmass™ assembly entirely. The threshold adjust knob has fifteen (15) clicks of adjustment.

Knob Setting
(clicks out from fully clockwise in)

Setting Description

Suggested Application

Setting Characteristics


Very firm pedaling platform

XC racing and trail riding

Offers the greatest pedaling efficiency, while sacrificing some low-speed traction.


Moderately firm pedaling platform

XC racing and trail riding; all-mountain

Offers less pedaling efficiency while increasing low-speed traction.


Light pedaling platform Trail riding, all-mountain Further increasing of low-speed traction, while still providing a light pedaling platform.


No pedaling platform

Downhill shuttle runs

Offers maximum low-speed traction, without any pedaling platform.

In the table below, Fox offers a suggested range of Terralogic® threshold adjustment settings per fork travel, as a starting point for your experimentation. As no two riders are the same (weight, strength, riding style, terrain choices, or simply that personal preference with riding "feel"), Fox encourages you to spend some time exploring the possibilities with Terralogic® threshold adjustment, to discover that "sweet spot" when it just feels right to you.

Fork Travel

Suggested Terralogic® Threshold Settings
(clicks out from fully clockwise in)

3.15–3.94/80–100 0-4
4.72–5.12/120–130 4-8
5.51–5.90/140–150 8-12

When It Counts.

When It Counts.

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