Calculator Inputs
Example Data Table
| Setup Type | Rider + Gear | Bike Weight | Travel | Stroke | Sag | Typical Coil Range |
|---|---|---|---|---|---|---|
| Trail bike | 190 lb | 34 lb | 5.5 in | 2.0 in | 30% | 400 to 550 lb/in |
| Enduro bike | 205 lb | 38 lb | 6.7 in | 2.5 in | 28% | 450 to 650 lb/in |
| Light downhill bike | 175 lb | 42 lb | 8.0 in | 3.0 in | 30% | 350 to 500 lb/in |
| Heavy freeride setup | 235 lb | 43 lb | 7.5 in | 2.75 in | 27% | 600 to 800 lb/in |
Formula Used
The calculator estimates the rear sprung load, shock leverage, shock sag, and required spring rate. It uses these main formulas:
Total loaded mass:
Total mass = rider weight + gear weight + bike weight
Rear sprung mass:
Rear sprung mass = total mass × rear bias - rear unsprung mass
Average leverage ratio:
Leverage ratio = rear wheel travel ÷ shock stroke
Wheel sag and shock sag:
Wheel sag = wheel travel × target sag percentage
Shock sag = wheel sag ÷ leverage ratio
Shock force:
Shock force = rear wheel load × sag leverage ratio
Spring rate:
Spring rate = shock force ÷ (shock sag + preload)
The final result is converted into N/mm, kg/mm, and lb/in. The nearest coil value is rounded using your selected coil step.
How to Use This Calculator
- Choose imperial or metric units first.
- Enter rider, gear, and bike weight.
- Add rear weight bias. Use 60% when unsure.
- Enter rear wheel travel and shock stroke.
- Leave manual leverage blank unless you know the exact ratio.
- Set target sag. Many rear shocks use 25% to 33%.
- Add preload and linkage progression if known.
- Press calculate. Review the result above the form.
- Use CSV or PDF buttons to save your setup.
Rear Shock Spring Setup Guide
Why Spring Rate Matters
A rear shock spring supports the rider and bike. It also controls ride height. The correct spring keeps the bike balanced. It lets the suspension move freely. It also helps the tires stay planted. A spring that is too soft rides low. It can bottom out often. A spring that is too firm rides high. It may skip across rough ground.
Sag Is the Starting Point
Sag is the amount of travel used when the rider sits on the bike. Rear suspension usually needs enough sag for grip. Trail bikes often feel good near 30 percent. Enduro bikes may use slightly less. Downhill bikes can vary by frame design. This calculator uses sag as the core target. It then estimates the spring needed to reach that sag.
Leverage Ratio Changes the Result
The rear wheel moves farther than the shock shaft. This difference is the leverage ratio. A higher ratio needs a firmer spring. A lower ratio needs a softer spring. Some frames have progressive links. They become firmer deeper in travel. The progression input gives a practical adjustment for that behavior.
Preload Should Stay Reasonable
Preload compresses the spring before riding. Small preload helps remove looseness. Too much preload can hide a soft spring. It can also reduce grip and small bump comfort. If the calculator needs excessive preload, choose a firmer spring. If sag is too low with little preload, choose a softer spring.
Use the Result as a Workshop Baseline
The result is a strong starting point. Real testing still matters. Tire grip, damping, frame kinematics, riding speed, and terrain all affect feel. Start with the nearest spring. Check sag in full riding gear. Then ride a familiar trail. Adjust preload, rebound, and compression carefully. Change one setting at a time. Record each change. A clear setup log makes tuning faster.
FAQs
1. What is a rear shock spring calculator?
It estimates the coil spring rate needed for your rear suspension. It uses weight, travel, stroke, sag, preload, and leverage ratio to create a practical setup value.
2. What sag percentage should I use?
Many bikes work well between 25% and 33% rear sag. Trail comfort often improves near 30%. Aggressive riding may need slightly less sag.
3. What is leverage ratio?
Leverage ratio compares rear wheel travel with shock stroke. A 150 mm bike with a 55 mm shock stroke has about a 2.73 ratio.
4. Should I enter manual leverage ratio?
Use manual leverage when your frame maker provides a known average ratio. Otherwise, leave it blank and the calculator will estimate it from travel and stroke.
5. Why include rear unsprung weight?
Rear unsprung parts are not fully supported like rider mass. Subtracting them gives a better sprung load estimate and improves spring rate accuracy.
6. What does progression mean?
Progression describes how the suspension firms up through travel. A progressive frame may need a different spring feel than a linear frame.
7. Is the nearest coil always the best choice?
Not always. The nearest coil is a practical starting point. Choose softer or firmer based on sag check, bottom-out resistance, grip, and trail feel.
8. Can this calculator replace real suspension tuning?
No. It provides a strong baseline. Final setup should include sag measurement, trail testing, damping adjustment, and notes from real riding conditions.