Adjusting Fox mountain bike shocks primarily involves setting the sag, which determines how much the suspension compresses under the rider's weight, and tuning rebound and compression damping.
Understanding Sag
Sag is the most critical adjustment for proper suspension function. It sets the initial position of your suspension, ensuring it can both absorb bumps (compress) and maintain tire contact on the ground (extend). Setting the correct sag is essential for optimal performance and handling.
Setting Sag on Your Fox Shock
To accurately set sag, you'll need a shock pump and a measuring tool (like a ruler or measuring tape).
Here's a common method to set sag:
- Determine Required Sag: Fox provides recommended sag ranges for different riding disciplines (e.g., Cross-Country, Trail, Enduro). Check your bike manufacturer's guidelines or Fox's documentation, but a common starting point is 20-30% of the shock's total travel.
- Find Shock Travel: You need to know the total travel of your rear shock. This is often listed on your bike's specifications or the shock body itself.
- Measure Uncompressed Length: As mentioned in the reference, one way to measure is to know your shock's travel and then measure the uncompressed shock length from eye to eye. Alternatively, measure the total exposed shaft length when the shock is fully extended and unweighted.
- Inflate to Base Pressure: Use a shock pump to add air to the shock, consulting Fox's base pressure recommendations if available for your shock model and rider weight.
- Prepare for Measurement: With the shock fully extended, push the rubber O-ring (sag ring) down the shock shaft until it rests against the wiper seal.
- Mount the Bike: Carefully and slowly mount your bike in your riding gear. Assume your normal riding position (sitting or standing, depending on what the manufacturer recommends for sag measurement). Avoid bouncing.
- Dismount Carefully: Gently dismount the bike without bouncing the suspension excessively.
- Measure Sag: Measure the distance the O-ring has moved from the wiper seal. This is your sag measurement. The reference mentions measuring your expose shaft length after compression to determine travel used. The difference between the uncompressed exposed shaft length and the compressed exposed shaft length is the sag.
- Adjust Pressure:
- If the sag is less than recommended (O-ring moved too little), release air from the shock.
- If the sag is more than recommended (O-ring moved too much), add air to the shock.
- Repeat: Push the O-ring back down and repeat steps 5-9 until you achieve the desired sag measurement.
Sag Measurement Example
Let's say your shock has 50mm of travel, and you aim for 25% sag.
Desired Sag Amount = 50mm * 0.25 = 12.5mm.
After mounting and dismounting, the O-ring should be approximately 12.5mm from the wiper seal.
| Sag Percentage | Riding Style Recommendation | Shock Travel Used |
|---|---|---|
| 15-20% | Cross-Country (XC) | Less |
| 20-25% | Trail / All-Mountain | Moderate |
| 25-30% | Enduro / Downhill (DH) | More |
Note: These are general guidelines. Always check your bike or shock manual for specific recommendations.
Other Key Adjustments
Once sag is set, you can fine-tune the shock's damping characteristics.
Rebound Damping (Red Knobs)
Rebound controls how quickly the shock extends after compression.
- Too Slow: Suspension packs down on repeated bumps, becoming harsh.
- Too Fast: Suspension springs back too quickly, potentially throwing you off balance or losing traction.
- Adjustment: Usually controlled by a red knob. Turn clockwise ('+' or 'Slower') to slow rebound down, counter-clockwise ('-' or 'Faster') to speed it up. A good starting point is often found by compressing the shock and quickly releasing your weight – the shock should return quickly but without kicking back violently.
Compression Damping (Blue Knobs)
Compression controls how easily the shock compresses under impact or rider input. Fox shocks often have different levels of compression adjustment (e.g., Open, Medium, Firm levers or multiple clicks of adjustment).
- Low-Speed Compression (LSC): Affects suspension movement from rider weight shifts, pedaling, and rolling bumps.
- High-Speed Compression (HSC): Affects suspension movement from sharp, high-speed impacts like rocks or drops.
- Adjustment: Usually controlled by blue knobs or levers. More compression (clockwise / '+') makes the shock feel firmer, resisting movement. Less compression (counter-clockwise / '-') makes it feel softer and more active.
Always refer to your specific Fox shock model's manual for detailed instructions and recommended starting points for rebound and compression settings.
