The Fox 40 Factory suspension fork stands as a pinnacle of high-performance downhill mountain biking equipment. With its advanced GRIP2 damper, Float air spring, and exceptional adjustability, this fork demands precise maintenance and regular inspection to ensure optimal function. This DIY guide will take you through every necessary step for a thorough inspection of your Fox 40 Factory fork, tailored for riders who demand reliability on every descent.
Tools Required
- Torque wrench [paid link] with appropriate bits
- 2mm, 4mm, and 5mm hex keys [paid link]
- Suspension pump
- Isopropyl alcohol and clean lint-free cloth
- Torque spec sheet from Fox
- Oil pan or catch container
- Nitrile gloves
Understanding the Fox 40 Factory Design
The 40 Factory is a dual-crown suspension fork specifically engineered for downhill use. It features a Kashima-coated stanchion surface to reduce friction and enhance durability. The GRIP2 damper offers high and low-speed compression and rebound damping adjustments. A Float air spring reduces weight while delivering progressive ramp-up. These features create a highly tunable system, which makes regular inspections critical for maintaining performance.
Preparation for Inspection
Before starting any inspection, place the bike in a work stand with both wheels removed. Depressurize the air spring using the valve on the top of the left leg. This ensures that internal components are not under load, which can cause inaccurate readings or damage during inspection. Clean the fork legs using isopropyl alcohol to prevent debris from entering the internals during the procedure.
Inspecting the Stanchions and Lowers
Carefully examine the stanchions for signs of wear, scoring, or discoloration. Any imperfections in the stanchion coating can lead to seal wear and internal contamination. Look for oil residue or dirt accumulation near the wiper seals, as these indicate potential seal failure. Gently compress the fork and observe for any binding or stiction, which suggests internal wear or contamination.
Seal and Bushing Health Assessment
To assess seal integrity, wipe the stanchions clean and compress the fork several times. If oil appears above the dust wipers, the seals may be compromised. Inspect the bushings for lateral play by applying pressure at the bottom of the lowers while holding the stanchions. Any noticeable movement suggests worn bushings, which must be replaced to maintain performance and safety.
Air Spring Inspection and Service Check
Remove the air cap and connect a suspension pump to verify the air pressure aligns with rider weight and preference. An unexpected drop in pressure since the last ride may indicate a slow leak. For a deeper inspection, remove the top cap and inspect the air spring for contamination or wear on the shaft or seals. A clean and lubricated air spring is essential for responsive fork travel.
Damper Functionality Check
The GRIP2 damper should provide consistent and adjustable rebound and compression. Set all adjusters to the middle of their range, then cycle the fork to feel for smooth and controlled damping. Lack of resistance, noise, or inconsistent behavior may signal internal fluid loss or damper malfunction. A full damper rebuild may be required if abnormalities persist.
Torque and Fastener Verification
Inspect all external bolts, including axle pinch bolts and crown pinch bolts, for proper torque. Use a calibrated torque wrench [paid link] and refer to the Fox specification sheet. Over-torquing can damage components, while under-torquing may lead to unsafe operation. Pay special attention to the crown-to-steerer interface and lower leg arch bolts, which are critical for structural integrity.
Lubrication and Dust Wiper Maintenance
After cleaning, apply suspension-specific lubricant to the stanchions and cycle the fork to distribute it under the dust wipers. This prevents premature seal wear and reduces friction. If dirt continues to accumulate after lubrication, consider replacing the wipers and foam rings to restore a proper seal.
Common Setup Issues and Troubleshooting
Many riders experience symptoms such as harshness, lack of small bump compliance, or excessive dive. These issues often stem from incorrect air pressure, poor damper setup, or internal contamination. For harshness, reduce low-speed compression slightly. If the fork feels too soft, increase air pressure incrementally and recheck sag.
Performance Optimization Tips
For aggressive riding, set high-speed compression to resist bottoming while maintaining adequate rebound to avoid pack-down. Monitor how the fork performs in different terrain. Use sag gradients or o-ring movement to measure travel usage and adjust volume spacers or air pressure accordingly. Consistent maintenance of damper oil and air seals also contributes to long-term performance.
Customization Considerations
Fox offers air volume spacers and custom tuning for riders seeking a specific feel. Adjusting these can drastically change fork behavior under load. Riders can install additional volume spacers to increase ramp-up, ideal for bigger hits. Always reset damper settings after hardware changes to dial in performance.
Compatibility with Other Drivetrain Components
The 40 Factory uses a 20x110mm thru-axle and supports direct-mount disc brakes. Ensure brake caliper adapters match rotor size. Check headset compatibility for dual-crown installation, particularly bearing cup size and steerer tube length. This fork is commonly paired with coil or air rear shocks, and tuning should match both ends of the bike for balanced suspension behavior.
Comparing 40 Factory to Similar Forks
Compared to the Fox 38, the 40 Factory offers superior stiffness and damping for gravity-focused applications. Riders choosing between the two should consider ride discipline—enduro versus downhill. RockShox BoXXer Ultimate is a direct competitor, offering similar adjustability but with a different feel in damping characteristics. The 40 Factory is often preferred for its refined small bump sensitivity and precise steering under high loads.
Inspection Interval Recommendations
Fox recommends lower leg service every 40 hours of riding and a full damper and air spring service every 100 hours. Riders pushing hard in wet or dusty conditions should increase inspection frequency. Consistent inspections not only protect your investment but also ensure predictable fork behavior in demanding conditions.
Conclusion
Regular suspension fork inspections are essential to maintain the performance and safety of your Fox 40 Factory. This guide equips you with the steps and checks required to evaluate each component properly. Addressing issues early prevents damage and ensures that your fork continues to deliver elite-level downhill performance.