The Fox 40 Factory suspension fork is a high-performance, dual-crown downhill fork built for aggressive gravity riding. Engineered with precision and equipped with top-tier features such as the GRIP2 damper, FLOAT EVOL air spring, and Kashima coating, this fork is tailored for maximum performance. However, unlocking its full potential requires proper installation, precise tuning, and regular maintenance.
Tools Required
- 5 mm and 6 mm Allen keys [paid link]
- Torque wrench [paid link] (2–10 Nm range)
- Shock pump (up to 300 psi)
- Suspension grease and appropriate lubricant
- Clean rags and isopropyl alcohol
- Rubber mallet (for crown race installation)
- Digital calipers (for measuring sag)
- Travel indicator o-ring (or zip tie)
Installation Overview
Before tuning, correct installation is crucial. The Fox 40 Factory is a dual-crown fork and requires a compatible downhill frame with a 1.5” head tube and proper axle spacing (110×20 mm Boost DH).
Steerer Tube Preparation
Ensure the steerer tube is cut to the correct length and chamfered. Install the crown race with a mallet and install the fork into the head tube. Torque the upper and lower crown bolts to manufacturer specifications (typically 5 Nm for the upper and 6 Nm for the lower).
Brake and Wheel Setup
Attach a compatible 203–220 mm rotor and post-mount brakes. Mount the wheel and ensure the 20 mm thru-axle is torqued to 17 Nm. Align the caliper by loosely mounting it, squeezing the brake lever, and tightening bolts evenly.
Initial Air Spring Setup
Setting Sag
Use a shock pump to inflate the FLOAT EVOL air spring. Recommended starting pressure varies by rider weight:
- 140–160 lbs: ~75–85 psi
- 160–180 lbs: ~85–95 psi
- 180–200 lbs: ~95–105 psi
Check sag by using an o-ring or zip tie. Aim for 15–20% sag (27–36 mm on the 203 mm travel fork). Adjust pressure in 5 psi increments to fine-tune.
Volume Spacer Tuning
The Fox 40 Factory allows tuning via volume spacers. More spacers make the fork more progressive, resisting bottom-out; fewer make it more linear.
- Aggressive riders: 3–5 spacers
- Flow trails or lighter riders: 0–2 spacers
Remove air before opening the top cap and adding/removing spacers. Always re-lubricate the seals upon reassembly.
Damping Adjustments
High-Speed Compression (HSC)
HSC manages big hits and g-outs. Start at the factory midpoint (8 clicks from closed) and adjust based on trail feedback. Add compression if you’re bottoming out too easily.
Low-Speed Compression (LSC)
LSC controls mid-stroke support and fork dive. Begin at 10 clicks out from closed and increase damping for berms or tech climbs.
High-Speed Rebound (HSR)
HSR affects how the fork recovers from big impacts. Start at the midpoint (6 clicks out) and tune for terrain speed.
Low-Speed Rebound (LSR)
LSR tunes the fork’s return on small, fast hits. Begin at 8 clicks and adjust based on traction and control feedback.
Common Set-Up Issues and Troubleshooting
Fork Not Returning Smoothly
Often caused by incorrect rebound damping or low air pressure. Verify pressure and damping settings, then check for dirt or contamination in the air shaft.
Fork Feels Harsh Over Small Bumps
Too much compression damping or over-inflated air spring is the likely cause. Reduce LSC and slightly lower air pressure to improve sensitivity.
Bottoming Out Easily
This suggests insufficient air pressure or too few volume spacers. Add 5 psi or install an additional spacer.
Oil or Grease Leaks
Inspect the dust wipers and seals for wear. Replace lower leg seals if leaking persists. Regularly clean and lubricate the seals with suspension-specific grease.
Routine Maintenance
Every 20 Hours
Wipe stanchions and clean wiper seals. Apply light suspension oil to keep seals lubricated.
Every 50 Hours
Perform a lower leg service. Remove lowers, clean internals, and replace bath oil (Fox recommends 20 cc of 20wt Gold per leg).
Every 125 Hours
Full damper and air spring service, including damper bleed and air spring inspection. Replace seals and re-lubricate components.
Customization for Performance
The Fox 40 Factory is widely regarded for its customizability. Riders can tailor their suspension feel through:
- GRIP2 Damper Tuning: Offers independent control of HSC, LSC, HSR, and LSR.
- Air Spring Curve Adjustment: Allows linear or progressive tuning with spacers.
- Offset Crowns: Available in different offsets to alter steering feel.
Compatibility with Drivetrain and Components
The 40 Factory is non-integrated with drivetrain parts, but it affects cockpit setup. Ensure:
- Handlebar Diameter: 31.8 mm or 35 mm clamps work.
- Brake Compatibility: Direct mount for 203–220 mm rotors.
- Axle Standards: 110×20 mm Boost DH only.
It is not compatible with flat-mount brakes, lightweight XC wheels, or 1.125” steerers.
Comparison with Similar Models
| Model | Damping System | Weight | Travel | Use Case |
|---|---|---|---|---|
| Fox 40 Factory | GRIP2 | ~2,820g | 203 mm | World Cup DH |
| RockShox Boxxer Ultimate | Charger 3 | ~2,600g | 200 mm | Downhill racing |
| Manitou Dorado Pro | TPC+ | ~3,000g | 203 mm | Freeride, DH |
The 40 Factory offers the most advanced damping system (GRIP2), known for its independent tuning. While slightly heavier than the Boxxer, it provides better small-bump compliance and adjustability.
FAQs
How often should I service my Fox 40 Factory fork?
Basic lower leg service is recommended every 50 hours of riding. Full rebuilds should occur every 125 hours or annually.
Can I use a Fox 40 Factory on an enduro bike?
No. The Fox 40 is designed specifically for downhill bikes with dual-crown compatibility.
What’s the best air pressure for a 170-lb rider?
Start at 90 psi and fine-tune based on sag (aim for 30 mm). Always adjust in small increments.
Is the GRIP2 damper better than the older FIT4?
Yes. GRIP2 offers independent HSC/LSC and HSR/LSR adjustments for better performance and tuning flexibility.
Can I change the fork travel?
No. The Fox 40 Factory is fixed at 203 mm. Travel changes require internal modifications not supported by Fox.
Why is my fork creaking during hard hits?
Check the steerer tube and crown interface, as well as headset spacers. Creaking may also stem from loose bolts or worn bushings.
Conclusion
Tuning your Fox 40 Factory suspension fork is essential for achieving peak downhill performance. By setting sag accurately, adjusting damping parameters, and maintaining it regularly, you ensure responsiveness, comfort, and durability. Whether you’re racing or pushing limits at the bike park, a dialed 40 Factory transforms your ride with unparalleled control and support.