MX Suspension Spring Rate Calculator
Calculate the ideal fork and shock spring stiffness for your weight and riding style.
lbs
kg
Important: Weigh yourself wearing full riding gear (helmet, boots, braces). This usually adds 15-20 lbs (7-9 kg).
450cc 4-Stroke
350cc 4-Stroke
250cc 4-Stroke
250cc 2-Stroke
125cc/150cc 2-Stroke
85cc
Motocross (Standard)
Supercross (Stiffer)
Enduro/Woods (Softer)
Desert/GP (Mixed)
Beginner / Novice
Amateur / Intermediate
Expert / Pro
Vet Rider (30+)
Recommended Suspension Setup
Fork Spring Rate:
Shock Spring Rate:
Recommended Race Sag:
Recommended Static Sag:
Note: Results show rates in kg/mm (common for Showa/KYB) and N/mm (common for WP/KTM). Always verify with your owner's manual or local suspension tuner.
Why Correct Spring Rate is Critical
The spring rate of your motocross bike determines how the suspension supports the combined weight of the bike and rider. If your springs are too soft, the bike will sit too low in the stroke (choppered out), causing poor turning and frequent bottoming out on jumps. If the springs are too stiff, the suspension won't compress enough over small bumps, resulting in a harsh ride, headshake, and lack of traction.
How to Interpret the Results
This calculator estimates the ideal spring rate based on physics and general manufacturer data. Here is what the numbers mean:
- kg/mm (Kilograms per Millimeter): The standard unit for Japanese suspension components (Showa, KYB). A 0.50 kg/mm spring requires 0.5kg of force to compress it 1mm.
- N/mm (Newtons per Millimeter): The standard unit for European bikes (KTM, Husqvarna, GasGas using WP suspension). To convert kg/mm to N/mm, multiply by roughly 9.8.
- Race Sag: The amount the suspension compresses with the rider on the bike in full gear. This is the most critical tuning variable.
- Static (Free) Sag: How much the bike sags under its own weight without a rider. This confirms if your spring rate is correct for your weight.
The Golden Rule of Sag
For full-size bikes (125cc and up), the industry standard for Race Sag is typically 100mm to 105mm. Once you set your race sag, you must check your Static Sag:
- Static Sag between 30mm – 40mm: Correct Spring Rate.
- Static Sag less than 30mm: Spring is too soft (you had to add too much preload to get race sag).
- Static Sag more than 40mm: Spring is too stiff (you barely have any preload on the spring).
Riding Styles & Adjustments
Your discipline changes your needs. Supercross requires much stiffer springs to handle steep jump faces and massive landings. Enduro/Woods riders prefer softer springs to absorb rocks, roots, and maintain traction at lower speeds. This calculator automatically adjusts the recommended rate based on the discipline you select.