K-Tech Style Spring Rate Calculator
Estimate rear shock spring rate from rider load, sag target, and linkage ratio.
Quick Hooke's Law Converter
Already measured force and deflection on a bench? Convert directly to spring rate.
What this calculator is for
This page gives you a practical way to estimate a motorcycle rear spring rate in the same spirit as common suspension setup tools used by riders and tuners. If you are searching for a K-Tech spring rate calculator, this tool helps you get a solid starting number before you order a spring or book a suspension service.
The calculator is intentionally simple: it focuses on rider load, desired sag, and linkage ratio. That keeps it fast and useful for garage setup sessions.
How the spring rate estimate works
Core idea
A spring supports load. If you know how much force is acting on the rear and how much movement (sag) you want, you can estimate the required wheel rate. Then you convert wheel rate to shock spring rate using the square of the linkage ratio.
- Rear load (N) = total rider mass × gravity × rear bias
- Wheel rate (N/mm) = rear load ÷ target sag
- Spring rate (N/mm) = wheel rate × (linkage ratio)2
Real bikes are not perfectly linear, so this is an estimate. It is still very useful for selecting between available spring options.
Input guide for better results
Rider + gear weight
Use fully geared-up weight where possible: helmet, suit, boots, hydration pack, and anything you normally carry. Underestimating this value leads to choosing too soft a spring.
Rear load bias
This is the percentage of your rider load effectively supported by the rear suspension. A value around 55% to 65% is common depending on motorcycle geometry and rider position.
Target rider sag
Typical road setups often land around 90–105 mm at the rear wheel; track and race setups can vary. Use your preferred setup baseline from your manual or tuner.
Linkage ratio
Linkage ratio is wheel movement divided by shock movement. If your bike has 150 mm rear wheel travel and 50 mm shock stroke, that ratio is about 3.0:1.
Understanding the output units
The result shows three common formats:
- N/mm (SI engineering unit)
- kg/mm (often used in suspension catalogs)
- lb/in (common in some aftermarket systems)
This makes it easy to compare your calculated value against available spring rates from different suppliers.
If you enter your current spring
The tool also estimates your expected rider sag with your current spring rate. That gives a quick “too soft / too stiff” check:
- Estimated sag higher than target: spring likely too soft.
- Estimated sag lower than target: spring likely too stiff.
Final setup always requires real-world measurement and preload adjustment.
Common setup mistakes
- Using bodyweight only and forgetting gear weight.
- Guessing linkage ratio incorrectly.
- Trying to fix wrong spring rate with excessive preload.
- Ignoring front/rear balance after changing rear spring.
Final note
This calculator is for educational and planning use. Use it to narrow your spring choice, then validate with measured static sag, rider sag, damping setup, and test rides. If your bike is highly modified or used in competition, a professional suspension tuner is still the best route.