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Specialized® Inertia Valve (IV) Shock

General Maintenance | Before You Ride | Setting IV Sag | Adjusting Rebound | Adjusting Brain Fade™ | Important Safety Information

Figure 1: Stumpjumper™ IV Shock, 26" & 29" Wheel:

Figure 2: Micro IV Shocks: Epic™ M-XL, Epic™ Small, Epic™ 29, ERA™


standard air sleeve, inertia valve damping, lightweight chassis, Brain Fade™ adjust, air spring pressure, rebound adjust



intended use

all-mountain, cross-country

The IV rear shock family is supplied as original equipment (OE) with the following Specialized® bicycle models:

Setting IV Sag

To set sag:
  1. Turn the Brain Fade™ and rebound adjustment knobs all the way counterclockwise, to their minimum position.
  2. Slide the travel indicator o-ring on the shock body to the dust wiper lip.
  3. Ideally with all your gear as you would ride (don't forget your water bag if you're using one), mount your bicycle next to a counter or wall to steady yourself, letting your full weight settle and distribute itself as it normally would when riding.
  4. Carefully dismount your bicycle, and measure the distance from the dust wiper lip and the travel indicator o-ring.
  5. Refer to the SAG Setting Guidelines table below for your optimum sag setting. Add air pressure in 5 PSI increments with the FOX high pressure pump, and repeat steps 2 through 4 until you achieve your desired sag.

    Bicycle Model
    inches (mm)

    Eye to Eye
    inches (mm)

    SAG Setting Guidelines

    Shock Travel
    inches (mm)

    SAG Measurement, % of shock travel
    inches (mm)
    inches (mm)

    Stumpjumper™ 26‡

    7.75 (196.85)

    1.93 (49.00)

    0.48 (12.25)

    0.58 (14.70)

    Stumpjumper™ 29‡

    7.75 (196.85)

    1.75 (44.45)

    0.44 (11.10)

    0.53 (13.33)

    Epic™ M-XL

    7.36 (187.00)

    1.85 (47.00)

    0.46 (11.75)

    0.56 (14.10)

    Epic™ Small, Epic™ 29, ERA™

    7.28 (185.00)

    1.69 (43.00)

    0.42 (10.75)

    0.51 (12.90)

    ‡ With the Stumpjumper™ shocks, the eye to eye measurement extends from the center of the shaft eyelet to the flat on the body end.

Adjusting Rebound

Rebound controls the rate at which your shock returns after it has been compressed. The proper rebound setting is a personal preference, and changes with rider weight, riding style, and terrain. A basic rule of thumb is that rebound should be as quick as possible without kicking back hard, bucking you off the saddle.

The rebound knob has 8-10 clicks of adjustment.

For slower rebound, turn the red adjuster knob clockwise.

For faster rebound, turn the red adjuster knob counterclockwise.

Adjusting Brain Fade™

All IV rear shocks are equipped with Brain Fade™ technology, which gives you the ability to adjust your shock from firm to plush to match your riding terrain. The inertia valve automatically switches from your pre-adjusted Brain Fade™ setting to a fully plush feel when the rear wheel hits a bump.

For a firmer ride setting, turn the Brain Fade™ adjustment knob clockwise.

For a softer ride setting, turn the Brain Fade™ adjustment knob counterclockwise.

Before You Ride

  1. Inspect the entire exterior of shock; the shock should not be used if any of the exterior parts appear to be damaged. Contact your local dealer or FOX for further inspection and repair.
  2. Check that quick-release levers (or thru-axle pinch bolts) are properly adjusted and tightened.
  3. Check that your headset is properly adjusted. If it's loose, adjust it accordingly to the manufacturer’s specifications.
  4. Check that brake cables or hoses are properly fastened and adjusted.
  5. Check that the front and rear brakes operate properly on flat land.

General Maintenance

Note:  IV rear shocks are not 100% user serviceable, meaning that if seals or rings are found to be in need of replacement, this level of service must be done by FOX Racing Shox service, or by an Authorized Service Center.

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