MTB Coil Spring Rate Calculator
Estimate a starting coil spring rate for your mountain bike shock using rider/system weight, kinematics, sag target, and preload.
Note: This is a practical starting-point calculator for a linear coil spring. Final setup depends on frame progression, riding style, and trail speed.
How this coil calculator MTB tool helps
If you are switching from air to coil, replacing a stock spring, or dialing in a new bike, the biggest question is usually: what spring rate do I need? This page gives you a quick way to estimate a rate that should get you very close to your target sag.
Coil suspension is loved for traction, small-bump compliance, and consistency on long descents. But unlike air shocks, coil shocks do not let you fine-tune spring force with a pump. You must pick a physical spring rate (for example, 400, 425, 450, or 500 lb/in), so choosing the right one matters.
Inputs explained
1) Rider + bike weight
Use realistic ride-ready numbers. Include water, pack, shoes, helmet, and anything you normally carry. More complete numbers produce better spring estimates.
2) Rear wheel travel and shock stroke
These values create your average leverage ratio:
- Leverage ratio = Wheel travel / Shock stroke
- Example: 150 mm rear travel / 55 mm stroke ≈ 2.73
A higher leverage ratio generally means the shock sees less force for the same rear-axle load, which changes the spring requirement.
3) Target sag (%)
Common starting targets:
- Trail / all-mountain: 27% to 30%
- Enduro: 28% to 32%
- Bike park / freeride: 25% to 30%
4) Rear weight bias (%)
This is the percentage of system weight carried by the rear wheel in a neutral standing position. A typical value is 60% to 70%. If you are unsure, 65% is a reasonable default.
5) Preload (mm)
A little preload is normal, but excessive preload is not a substitute for the wrong spring. Most setups stay in the low range, often around 1 to 3 mm.
Formula used in this calculator
The script estimates rear-wheel load, converts through leverage ratio to shock force, and solves for spring rate based on sag compression and preload.
- System weight force (N) = mass (kg) × 9.80665
- Rear force (N) = system force × rear bias
- Shock force at sag (N) = rear force / leverage ratio
- Required spring rate (N/mm) = shock force / (sag shock travel + preload)
- Conversion: 1 N/mm = 5.7101 lb/in
The output includes a nearest standard spring suggestion in 25 lb/in increments and predicts sag for nearby options.
How to fine-tune after the first ride
Check sag first
Set rebound and compression to neutral, cycle the suspension, and measure sag in ride position. If sag is much higher than target, go up one spring step; if too low, go down one step.
Balance front and rear feel
A correct rear spring can still feel off if fork pressure or tokens are mismatched. Keep your bike balanced by confirming fork sag and support too.
Use damping for behavior, not spring-rate fixes
If spring rate is correct, then tune damping for traction, pop, and control. Avoid masking a wrong spring with extreme damper settings.
Common mistakes riders make
- Using body weight without kit and pack.
- Ignoring bike weight on a heavy eMTB.
- Assuming every frame uses the same leverage behavior.
- Running too much preload to force sag numbers.
- Changing spring and damper settings at the same time (hard to isolate results).
Quick FAQ
Is this calculator accurate for progressive linkages?
It is a strong starting point, but progressive curves and riding speed can shift the ideal spring one step up or down.
Should I choose exactly the calculated spring?
Choose the nearest available rate, then validate sag on trail. Many riders prefer either the exact nearest option or one step firmer for support.
Do I need different springs for bike park vs trail riding?
Some riders run one spring all season. Others keep two springs and swap based on terrain and riding style. If you do both long pedaling days and jump-heavy park laps, dual springs can make sense.