Advanced Calculator Inputs
Example Data Table
| Use Case | Corner Load | Travel | Sag | Motion Ratio | Preload | Approximate Result |
|---|---|---|---|---|---|---|
| Trail bike rear coil | 180 lb | 6 in | 30% | 0.32 | 0.15 in | 900 lb/in range |
| Enduro bike rear coil | 205 lb | 170 mm | 28% | 0.30 | 4 mm | 145 N/mm range |
| Light off-road corner | 300 lb | 8 in | 25% | 0.50 | 0.25 in | 500 lb/in range |
Formula Used
The calculator uses a linear spring model with motion ratio correction and preload:
Spring rate = Load ÷ [Motion ratio × ((Motion ratio × wheel sag) + preload)]
Wheel rate is estimated as spring rate multiplied by motion ratio squared. Shock stroke at sag equals wheel sag multiplied by motion ratio. Dual spring initial rate is estimated with the series spring formula: combined rate equals rate one times rate two divided by their sum.
How to Use This Calculator
- Select imperial or metric units.
- Enter the corner load carried by the wheel being tuned.
- Add extra carried load when luggage, gear, or cargo matters.
- Enter wheel travel and a sag percent, or enter exact sag.
- Use shock travel divided by wheel travel for motion ratio.
- Add preload distance and any firmer or softer adjustment.
- Submit the form and read the result above the inputs.
- Download the CSV or PDF file for setup records.
Fox Spring Setup Guide
A Fox coil shock works best when the spring supports the rider, bike, cargo, and terrain demands. The correct rate lets the suspension settle into useful sag without riding too deep. It also keeps the chassis stable during braking, pumping, and hard landings. A spring that is too soft may feel plush at first. It can bottom out often and reduce support. A spring that is too stiff may skip over bumps. It can also limit grip.
Core Physics
The calculator treats the coil as a linear spring. Linear springs follow Hooke's law, where force equals spring rate times compression. A rear suspension linkage changes that force through its motion ratio. This is why wheel sag and shock compression are different. The tool uses motion ratio, target sag, preload, and corner load to estimate a practical shock spring rate. It also reports wheel rate, shock stroke used, preload force, and rounded coil choices.
Advanced Tuning Use
Use the corner load field for the weight carried by the wheel being tuned. For bicycles, you can estimate rear load as rider plus bike weight times rear weight bias. For powersports or cars, use measured corner weight when possible. Enter wheel travel and desired sag percent. The calculator can convert that percent into sag distance. Then enter the motion ratio as shock movement divided by wheel movement. A lower motion ratio needs a higher spring rate.
Preload should be used carefully. Small preload values help remove looseness and set ride height. Large preload values can reduce available droop and make the suspension harsh. The existing spring check is useful when comparing your current coil. It predicts sag from the rate you already have. Dual spring fields estimate the initial combined rate for stacked springs before crossover. This helps compare plush initial travel with firmer later travel.
Practical Notes
Use results as a setup guide, not a final rule. Real suspension feel also depends on linkage progression, damper tune, tire pressure, terrain, and riding style. Recheck sag with full gear. Record each test. Make one change at a time. Export the table after each setup session so your best setting is easy to repeat. Keep notes simple, clear, and consistent.
FAQs
What is a Fox spring rate calculator?
It estimates a coil spring rate for a Fox-style shock setup using load, sag, preload, and motion ratio. It also reports wheel rate and setup values.
What does motion ratio mean?
Motion ratio is shock travel divided by wheel travel. It shows how much the shock compresses for each unit of wheel movement.
Should I use rider weight or corner load?
Use corner load when possible. For bikes, estimate rear load from rider, bike, gear, and rear weight bias. Measured load is better.
How much sag should I target?
Many coil setups start near 25% to 35% sag. The best value depends on travel, terrain, linkage, and personal feel.
Does preload change spring rate?
Preload does not change the physical spring rate. It changes starting compression, ride height, droop, and the force needed to begin movement.
Why is wheel rate different from spring rate?
The linkage changes force and movement between the wheel and shock. Wheel rate is spring rate adjusted by the square of motion ratio.
Can I compare an existing spring?
Yes. Enter the current spring rate and unit. The calculator estimates sag with that spring under the selected load and preload.
Are dual spring results final?
No. The dual spring value is an initial combined estimate. Crossover position, hardware, linkage progression, and setup details affect real behavior.