831

Installing Your Fork | Before You Ride | Setting Fork Air Pressure | Adjusting Rebound | Climb. Trail. Descend. | Adjusting Trail Mode | Adjusting Fork Travel | Service Intervals | Important Safety Information

	travel	3.9 in./100 mm (Internally adjustable to 110 or 120 mm)
	model features, adjustments	Factory FIT CTD with Trail Adjust: Kashima coated or hard anodized upper tubes, 1.5" tapered or 1 1/8" steerer, lever actuated 3-position Climb/Trail/Descend, 3-position Trail Adjust, air spring pressure, rebound
	lower leg options	15QR thru axle system, post style disc brake mounting
	spring	air
	riding styles	4X, dirt jump

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 Fork Air Pressure

To get the best performance from your fork, occasionally it is necessary to readjust fork air pressure.

NOTE:  Setting sag is desired and necessary for proper fork performance during general XC/Trail riding. The 831 fork is not intended for general XC/Trail riding use, but for four cross racing, slalom, or dirt jumping. Therefore, the air pressure is generally not set by obtaining proper sag, but is intended more for proper riding feel and bottom out control. The 831 fork comes with a pressure setting of 100 psi from the FOX Factory.

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, then remove the pump.
4. Use the Air Pressure Troubleshooting table below, for fine-tuning the air pressure for proper fork performance.
5. Screw the blue aircap back on, and go ride.
   
   

   Air Pressure Troubleshooting
   Symptom	Remedy
   Excessive bottoming	(+) air pressure in 5 psi increments
   Harsh ride; full travel not utilized	(-) air pressure in 5 psi increments

Adjusting Rebound

The red rebound adjuster knob is located at the bottom of the right fork leg.

Rebound controls the rate of speed at which the fork extends after compressing. Turning the knob clockwise (in) slows down rebound; turning the knob counter-clockwise (out) 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) 6 clicks.

Climb. Trail. Descend.

The blue CTD lever lets you to switch between the Climb, Trail, and Descend ride modes. Each mode is most suitable for each specific type of terrain, providing best performance and riding enjoyment with your fork. With some experience, you can experiment with using Trail mode when descending, or Descend mode for flat or undulating trails.

NOTE:  With full CTD suspension, front to rear suspension balance is best achieved by running the fork in the same ride mode as the rear shock (Climb with Climb, Trail with Trail, Descend with Descend).

Climb Mode

Rotate the CTD lever fully clockwise to set the fork in Climb mode, which is most useful for climbing and sprinting. In Climb mode, the fork will "blow off" in the event that a big hit is encountered. This setting was formerly known as “lockout.”

Climb Tune Factory Settings

There are two pre-set Climb Tune mode settings offered for Factory and Performance model forks: medium and firm.

Refer to the Climb Tune decal located on the crown next to the CTD lever, to determine which factory pre-set climb tune your fork has:

	Medium factory settings will be found on longer travel forks (130 mm travel and above for 26 forks, and 120 mm travel and above, for 29 forks)
	Firm factory settings will be found on shorter travel XC-type forks

Climb Tune factory settings can only be altered through complete disassembly and re-bleeding of the CTD damper system. Contact your local dealer or FOX at 1-800-FOX-SHOX for more information. Additional service information can be viewed at http://service.ridefox.com.

NOTE:  The fork may cycle a couple of times after you engage Climb mode. Once complete Climb mode is achieved, the fork may continue to move 3 - 5 mm. This is normal, and does not affect performance.

Trail Mode

Rotate the lever to the middle setting of the blue CTD lever to engage Trail mode. Trail mode offers significantly less compression damping than climb mode, which is good for a light pedaling platform when pumping undulating terrain, and for preventing excessive travel in technical riding situations (such as low-speed drops).

Descend Mode

Rotate the lever fully counter-clockwise to set the fork in Descend mode, which is similar to Trail mode but with significantly less low-speed compression damping for the most resilient ride when descending.

Adjusting Trail Mode (FIT CTD Trail Adjust Only)

Factory FIT CTD Trail Adjust model forks feature a three-position Trail Adjust control that regulates low-speed compression damping only while you are in Trail mode.

NOTE:  While it is possible to readjust FIT CTD Trail Adjust when you are in either Climb or Descend mode, Trail Adjust changes will only be felt in Trail mode.

For a firmer low-speed compression setting in Trail mode, turn the black Trail Adjust dial clockwise. For a lighter setting, turn the Trail Adjust dial counter-clockwise.

TIP: This setting is designed to work in conjunction with the rear shock, keeping in mind that front to rear suspension balance works best when the fork is in the same ride mode as the rear shock.

Adjusting Fork Travel

Travel on your 831 RLC fork can be changed by rearranging the internal travel spacers. After changing travel, check the fork for proper operation before riding. If there is noticeable play in the fork or if it makes strange sounds, disassemble the fork and check for the complete number and correct orientation of spacers.

NOTE:  The 831 fork comes from the factory preset to 100 mm of travel. The fork travel can be increased to 110 mm or 120 mm, but it cannot be increased beyond 120 mm.

Tools Required

• 32 mm 6-sided socket
• 10 mm socket
• Small screwdriver
• Torque wrench
• Oil drain pan
• 1.5 mm hex key wrench
• Plastic-faced hammer
• Measuring container w/ cc or mL increments

Supplies Required

CAUTION:  With the model year 2010 and newer 831 fork, it is absolutely essential that the air spring assembly be removed and installed via the bottom end of the upper tube. Your failure to do so will most likely cause damage to the air piston seal.

1. Remove the blue air cap from the top of the left fork leg. Let the air out of the fork. Remove the left top cap with a 26 mm socket 6-point socket wrench.
2. Remove the bottom nut on the air side with a 10 mm wrench and discard the crush washer. Re-thread the bottom nut 3 or 4 turns onto the air spring assembly shaft protruding from the base of the lower leg. Do not thread the bottom nut so far back on that the air spring assembly shaft protrudes from the bottom of the nut. With a plastic mallet, gently tap the bottom of the bottom nut to free up the air spring assembly from the lower leg. Make sure you are not hitting the bottom of the air spring assembly shaft with the mallet. Remove the bottom nut and allow the oil to drain.
3. Remove the rebound knob from the bottom of the damper side, and repeat step 2 of this procedure with the damper side. Be careful not to damage the protruding rebound adjuster assembly. It may be helpful to use a deep well 10 mm socket to reach the bottom nut after you have re-threaded it on, and then tap the hammer on the socket to loosen up the damper.
4. Slide the lowers down and off of the upper tubes.
5. Turn the fork upside down.
6. Remove the first snap ring from the bottom of the air side lower leg, and pull out the negative spring plate.

   NOTE:  The groove around the inside edge of the plate must be facing the inside of the leg when it is installed.

7. Remove the inside snap ring.
8. Pull the air spring assembly out through the bottom end of the upper tube. Refer to the images below to add or remove the appropriate number of 10 mm spacers to achieve your desired amount of travel.

   NOTE:  Spacers snap onto the air shaft in between the negative spring guide and the topout plate.

   Travel Spacer Orientations

   

9. With the fork still upside down, lubricate the U-cup seal on the air piston with FOX FLOAT Fluid and re-install the air shaft assembly into the upper tube from the bottom end. Be sure to orient the U-cup seal as shown in the air piston seal orientation drawing below. Be sure that the lip on the air piston seal is positioned towards the top.

   Air Piston Seal Orientation

   

10. Push the air spring assembly halfway into the upper tube. Do not push the assembly so far that it hits the threads at the top of the upper tube.
11. Re-install the inner snap ring and negative spring plate into the upper tube. Be sure the groove around the inside edge of the plate is facing inward.
12. Install the outer snap ring.
13. For the air side, measure and pour 30 cc of FOX Suspension Fluid into the upper tube through the space between the negative spring plate and air shaft.
14. For the damper side, measure and pour 30 cc of FOX Suspension Fluid into the upper tube in the space between the upper tube and damper.
15. Re-install the lowers halfway onto the upper tubes. Do not turn the fork right-side up to do this, or the bath oil you just poured in will spill out. Be sure the arch brace is positioned on the same side as the front of the crown.
16. On the air side, push the air shaft assembly up until the shaft comes through the bottom hole of the lower leg. Push the lower leg down, if the air shaft assembly does not make it through. You can use a pick tool to guide the air shaft assembly through the bottom hole. Install the bottom nut with a new crush washer and tighten the nut to 50 in-lb torque.
17. On the damper side, install the bottom nut with a new crush washer, and tighten the nut to 50 in-lb torque.
18. Re-install the rebound knob and tighten its set screw to 11 in-lb torque.
19. Turn the fork right-side up. Pour 5 cc of FOX FLOAT Fluid on top of the air piston.
20. Lubricate the o-ring on the air topcap with FOX FLOAT Fluid.
21. Re-install the topcap and tighten to 220 in-lb torque.
22. Pump up your fork to the desired pressure and cycle it several times to check for proper operation.
23. Re-install the blue air cap.
24. You're done, so get out there and ride!

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FOX Factory Inc.