Select the category that best matches your motorcycle.
Street / Road Use (Standard)
Track Day / Racing (Stiffer)
Touring / Comfort (Softer)
Affects the stiffness calculation for performance vs comfort.
Recommended Fork Spring Rate
9.5N/mm
Shock Spring Rate
95 N/mm
Target Static Sag (F)
25-30 mm
Target Rider Sag (F)
35-40 mm
Calculation Logic: Based on a standard 75kg rider baseline.
Fork rate adjusts by approx 0.05 N/mm per kg deviation.
Shock rate adjusts by approx 0.5 N/mm per kg deviation.
Track usage adds ~5% stiffness; Comfort removes ~5%.
Spring Rate vs. Rider Weight Projection
Chart shows estimated Fork Spring Rate (N/mm) as rider weight increases for the selected bike type.
Recommended Sag Settings Reference
Discipline
Static Sag (Front)
Rider Sag (Front)
Rider Sag (Rear)
Mastering Suspension: The K-Tech Spring Weight Calculator Guide
Suspension is arguably the most critical component of motorcycle performance, yet it is often the most overlooked. Whether you are shaving seconds off lap times at the track or seeking a plush ride for cross-country touring, the foundation of your setup is the spring rate. This k-tech spring weight calculator guide is designed to help you understand, calculate, and select the correct springs for your specific weight and riding style.
What is a K-Tech Spring Weight Calculator?
A spring weight calculator (often referred to as a spring rate calculator) is a tool used to determine the ideal stiffness of the coil springs inside your motorcycle's forks and rear shock. "K-Tech" refers to one of the premier manufacturers of high-performance suspension components, known for their precision engineering in racing series worldwide.
The calculator takes your rider weight—including all protective gear—and your motorcycle's characteristics to recommend a spring rate measured in Newtons per millimeter (N/mm) or Kilograms per millimeter (kg/mm). Using the correct spring rate ensures the suspension operates in the middle of its stroke, providing maximum traction, stability, and comfort.
Who should use this?
Track Riders: Who need precise handling and resistance to bottoming out under heavy braking.
Street Riders: Who find their stock suspension too harsh or too soft.
Adventure Riders: Who carry heavy luggage and need to compensate for the extra load.
Spring Rate Formula and Mathematical Explanation
Suspension springs follow Hooke's Law, which states that the force ($F$) needed to extend or compress a spring by some distance ($x$) scales linearly with that distance.
The formula is: F = k * x
Where:
F = Force applied (Weight of rider + bike)
k = Spring Rate (Stiffness constant)
x = Displacement (Sag)
Variables Table
Variable
Meaning
Unit
Typical Range (Sportbike)
Spring Rate (Fork)
Stiffness of front springs
N/mm
8.0 – 11.0 N/mm
Spring Rate (Shock)
Stiffness of rear spring
N/mm
80 – 120 N/mm
Rider Sag
Compression with rider on bike
mm
30 – 40 mm
Static Sag
Compression under bike weight only
mm
10 – 30 mm
Practical Examples (Real-World Use Cases)
Example 1: The Track Day Enthusiast
Scenario: Mark rides a Yamaha R6. He weighs 85kg and wears a full leather suit (approx. 8kg). Total weight: 93kg. He rides exclusively on the track.
Input: 93kg, Sport Bike, Track Discipline.
Stock Spring: Usually setup for a 75kg rider (~9.0 N/mm).
Calculation: The extra weight requires a stiffer spring to prevent excessive dive during braking.
Result: The calculator recommends a 10.0 N/mm fork spring and a 105 N/mm rear shock spring. This keeps the bike stable under high G-forces.
Example 2: The Lightweight Commuter
Scenario: Sarah rides a Kawasaki Ninja 400. She weighs 55kg with gear. She finds the ride harsh and the bike doesn't settle into corners.
Input: 55kg, Sport/Street, Comfort Discipline.
Stock Spring: Often too stiff for light riders on entry-level bikes.
Calculation: The calculator identifies that she is significantly under the standard weight.
Result: A softer 7.5 N/mm fork spring is recommended. This allows the suspension to compress properly over bumps, improving grip and comfort.
How to Use This K-Tech Spring Weight Calculator
Weigh Yourself Correctly: Do not guess. Put on your helmet, jacket, boots, and gloves. Stand on a scale. This is your "Rider Weight."
Select Your Bike Type: Different bikes have different weight distributions. A motocross bike is lighter than an adventure bike, requiring different baseline calculations.
Choose Your Discipline:
Select Street for a balance of performance and pothole absorption.
Select Track for stiffer support during aggressive braking and cornering.
Select Comfort if you prioritize a plush ride over cornering speed.
Analyze the Results: Look at the recommended N/mm value. Compare this to your stock springs (check your owner's manual). If the difference is more than 0.5 N/mm, a spring swap is highly recommended.
Key Factors That Affect Spring Weight Results
While rider weight is the primary variable, several other factors influence the final decision on spring rates:
1. Motorcycle Geometry and Linkage
The rear shock usually connects to the swingarm via a linkage system. This creates a leverage ratio. A bike with a linear linkage might need a different spring rate compared to one with a progressive linkage, even for the same rider weight.
2. Unsprung Weight
Heavy wheels or tires increase unsprung mass. While the spring supports the sprung mass (chassis + rider), the ratio between sprung and unsprung mass affects how the suspension reacts to bumps.
3. Riding Pace
Faster riders generate higher G-forces. A professional racer brakes harder than a novice, transferring more weight to the front wheel. Consequently, faster riders generally require stiffer springs than slower riders of the same weight.
4. Oil Level (Air Gap)
In forks, the air gap above the oil acts as a secondary air spring. Reducing the air gap (adding oil) increases bottoming resistance at the end of the stroke, which can fine-tune a spring that is slightly too soft.
5. Preload Adjustments
Preload does not change the spring rate (stiffness); it only changes the ride height and initial sag. If you have to max out your preload to get the correct sag, your spring is too soft. If you have zero preload and still have too little sag, your spring is too stiff.
6. Friction (Stiction)
Old seals or bushings create friction, which can mask incorrect spring rates by preventing the suspension from moving freely. Always ensure your suspension is serviced before measuring sag.
Frequently Asked Questions (FAQ)
What is the difference between N/mm and kg/mm?
They are units of stiffness. 1 kg/mm is approximately equal to 9.8 N/mm. European manufacturers (like K-Tech, Öhlins) often use N/mm, while Japanese manufacturers often use kg/mm.
Can I just increase preload instead of changing springs?
No. Preload only adjusts the starting position of the suspension (sag). It does not make the spring stiffer. If your spring is too soft, adding preload might get the ride height correct, but the bike will still dive excessively under braking.
How do I know if my spring is too hard?
If you cannot achieve the target "Rider Sag" (e.g., 35mm) without removing all preload, or if the bike feels harsh and chatters over small bumps, the spring is likely too stiff.
Does this calculator work for progressive springs?
This calculator provides a rate for linear springs, which are preferred for track and high-performance riding. Progressive springs change rate as they compress and are harder to calculate a single value for.
What is "Free Sag" or "Static Sag"?
This is how much the bike settles under its own weight without the rider. It is a crucial diagnostic tool. If you have correct Rider Sag but zero Free Sag, your spring is too soft (and you've over-compensated with preload).
Should I change fork and shock springs together?
Ideally, yes. A balanced bike is essential. Having a stiff front and soft rear (or vice versa) will cause the bike to "pitch" unpredictably during transitions.
How accurate is this calculator?
This tool provides a high-confidence estimation based on standard linkage ratios. For professional racing, on-track data acquisition and specific linkage analysis are required for the final 1% of tuning.
Does fork oil weight affect spring rate?
No. Fork oil viscosity affects damping (how fast the spring compresses and rebounds), not the spring rate itself.