Motorcycle Shock Spring Rate Calculator
Include all riding gear (helmet, boots, suit).
Total weight with fuel and fluids.
Typically 45-55% depending on bike type.
Usually 30-35% for street/track, 33% standard.
Total vertical travel of the rear axle.
Total travel of the shock shaft itself.
Recommended Spring Stiffness
Metric Rate:
0.0 kg/mm
Imperial Rate:
0 lbs/in
Newton Rate:
0 N/mm
Calculated Leverage Ratio:
1:1
Understanding Motorcycle Spring Rates
Selecting the correct shock spring rate is the foundational step in motorcycle suspension tuning. The spring supports the combined weight of the motorcycle and the rider, determining how the suspension settles into its travel (sag) and how it reacts to bumps. If the spring is too soft, the bike will bottom out easily and wallow in corners. If it is too stiff, the ride will be harsh, and the tires will lose contact with the road surface.
The "Golden Rule" of Suspension: The spring rate holds the bike up; the damping controls how the spring moves. You cannot compensate for an incorrect spring rate using clicker (damping) adjustments.
How This Calculator Works
This calculator uses a geometric approach based on your motorcycle's physical dimensions and weight distribution. Here is the logic behind the results:
- Leverage Ratio: This is calculated by dividing the Rear Wheel Travel by the Shock Stroke. Most motorcycles use a linkage system that magnifies the force on the shock. A 2:1 ratio means for every 2mm the wheel moves, the shock moves 1mm.
- Static Load: We estimate the weight resting on the rear wheel by combining the bike's rear weight bias (typically around 52% for sportbikes) and the rider's weight. Note: Riders generally sit closer to the rear axle, so we apply a distribution factor to the rider's weight.
- Target Sag: This is the percentage of suspension travel used just by sitting on the bike. For most street and track applications, 30-35% is the ideal range.
Key Terminology
- kg/mm (Kilograms per Millimeter): The standard metric unit for spring rates. It represents how many kilograms of force are required to compress the spring by 1 millimeter.
- lbs/in (Pounds per Inch): The standard Imperial unit. It represents how many pounds of force are required to compress the spring by 1 inch.
- Free Sag: The amount the bike settles under its own weight (without the rider).
- Rider Sag (Race Sag): The amount the bike settles with the rider in full gear on board.
- Preload: The amount of tension applied to the spring before any external load is added. Preload is used to fine-tune sag height but does not change the spring's stiffness rate.
After Calculation: Verification
Mathematical models provide an excellent starting point, but real-world verification is necessary. Once you install the recommended spring:
- Set the Rider Sag to your target (e.g., 35mm on a sportbike).
- Measure the Free Sag (how much the bike sags under its own weight).
- If you have correct Rider Sag but zero Free Sag (or the suspension is topped out), your spring is too soft (you have used too much preload to hold the bike up).
- If you have correct Rider Sag but excessive Free Sag (more than 15-20mm), your spring is too stiff.