fox spring rate calculator

What this FOX spring rate calculator does

Spring setup can feel confusing because terms like wheel rate, shock spring rate, and leverage ratio get mixed together. This calculator gives you a practical starting point for FOX suspension setup by estimating how stiff your spring should be to hit your sag target.

It works for both front and rear:

• FOX forks (FOX 34, FOX 36, FOX 38, etc.) where leverage ratio is effectively 1.0.
• FOX rear shocks (DHX coil, Float X, Float X2) where leverage ratio matters a lot.

Core formula used

At static sag, spring force balances the portion of bike+rider weight supported by that end of the bike.

For a fork, leverage ratio is 1.0, so wheel rate and spring rate are basically the same. For a rear shock, the spring typically needs to be much stiffer than wheel rate because of frame leverage.

How to use each input correctly

1) Rider weight (with gear)

Include helmet, shoes, pack, water, and anything you ride with regularly. This makes a big difference.

2) Bike weight

Add complete bike weight as ridden (tools, bottle, inserts, etc.). You can keep this simple: close is good enough.

3) Load on this end of bike (%)

This is how much total system weight sits on front or rear at neutral stance. If unsure, start with:

• Fork: 45%
• Rear: 55%

4) Travel and sag

Sag amount is travel multiplied by sag percentage. Example: 160 mm fork at 18% sag gives 28.8 mm sag target.

5) Leverage ratio

Rear suspension kinematics are critical. If your frame has a 2.7 average leverage ratio, enter 2.7. If you do not know it, check frame documentation or use 2.6-2.8 as a rough starting estimate.

Quick baseline table

Use Case	Sag Target	Typical Feel
Trail fork	16-20%	Supportive pedaling, balanced comfort
Enduro fork	17-20%	More traction, still supportive in steep terrain
Rear shock trail	27-30%	Efficient and poppy
Rear shock enduro/downhill	30-33%	Maximum grip and compliance

After-calculator tuning process

The calculator gives a mathematically sound start, not a final setup. Fine-tuning on trail is still essential.

• Set sag first using your normal riding position.
• Run one familiar trail section repeatedly.
• Adjust one variable at a time (spring, then rebound, then compression).
• If bottoming too easily, increase spring slightly before adding too much compression damping.
• If harsh on chatter, reduce spring a little or reduce low-speed compression.

Common setup mistakes

• Using bodyweight without gear and ending up too soft.
• Ignoring leverage ratio on rear shock calculations.
• Chasing perfect parking-lot feel instead of trail performance.
• Changing pressure, tokens/spacers, and damping all at once.

FAQ

Is this only for FOX coil shocks?

No. The output is spring-rate based, so it applies to coil and air as an equivalent spring stiffness target. Air pressure still needs real-world tuning and can vary by air can size and progression.

Why do I get a different number than a brand chart?

Brand charts are generalized. This calculator includes your weight distribution and sag target, which can shift recommendations significantly.

Should I choose the exact value or nearest coil increment?

Choose the nearest available spring (often 25 lb/in steps), then fine-tune with preload and damping. Avoid using excessive preload to force the wrong spring to work.

Note: This tool is an estimate, not a safety certification. Always follow FOX setup guidelines, torque specs, and service intervals. If in doubt, consult a qualified suspension tuner.