How do you calculate motorcycle spring rate?

Spring rate calculation requires total geared rider weight (body weight plus approximately 15–20 lbs for helmet, armored jacket, pants, and boots), vehicle curb weight, rear swingarm linkage leverage ratio (typically 2.6:1 to 3.1:1 on rising-rate linkages), and target dynamic sag percentage (typically 28% to 33% of total suspension travel).

What is the difference between static sag and rider sag?

Static sag (also called free sag) is the distance the motorcycle suspension compresses under its own wet vehicle weight with zero rider on board. Rider sag (dynamic sag) is the distance the suspension compresses with the fully-geared rider seated in riding position. Checking both reveals whether your spring rate is correct: if you achieve proper rider sag but have zero static sag, your spring is too soft and over-preloaded.

How do you convert between N/mm, kg/mm, and lbs/in spring rates?

To convert N/mm to kg/mm, divide by 9.80665 (e.g., 9.5 N/mm ÷ 9.80665 = 0.969 kg/mm). To convert N/mm to lbs/in, multiply by 5.71015 (e.g., 85 N/mm × 5.71015 = 485.4 lbs/in). To convert lbs/in to N/mm, divide by 5.71015.

Chassis & Dynamics Calibration

Motorcycle Suspension Spring Rate & Rider Sag Calculator

Calculate recommended front fork spring rates (N/mm & kg/mm) and rear shock spring rates (N/mm & lbs/in) calibrated to your geared rider weight, passenger load, linkage geometry, and riding discipline. Includes static sag sanity checks and preload guidelines.

1. Rider, Passenger & Luggage Mass

Specify rider weight dressed in full riding gear (helmet, jacket, armor, and boots add approx. 18 lbs / 8 kg).

Geared Rider Weight185 lbs

100 lbs350 lbs

Passenger Weight (Optional)0 lbs

Solo (0)250 lbs

Luggage / Saddlebags0 lbs

None (0)110 lbs

2. Riding Discipline & Chassis Architecture

Suspension velocity and travel vary drastically between race tracks, highway baggers, and motocross tracks.

Front Suspension

Recommended Fork Spring Rate

0.92N/mm(0.09 kg/mm · 5.3 lbs/in)

TOTAL FORK TRAVEL125 mm

TARGET RIDER SAG35 – 40 mm (28 – 32%)

STATIC (FREE) SAG TOLERANCE25 – 30 mm (Bike on ground, unladen)

Tuning Guideline: If rider sag is correct but static sag is less than 15 mm, the spring is too soft and heavily compressed by preload. If static sag exceeds 35 mm, the spring is too stiff.

Rear Monoshock

Recommended Shock Spring Rate

82.0N/mm(468 lbs/in · 8.36 kg/mm)

REAR SHOCK TRAVEL130 mm

TARGET RIDER SAG28 – 33 mm (22 – 25%)

STATIC (FREE) SAG TOLERANCE10 – 15 mm (Unladen bike weight)

Preload Tip: Always set spring preload collar so the spring has a minimum of 4–6 mm of initial installed compression before adjusting damping clickers.

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Suspension Spring Rate & Sag Setup Principles

1. Geared Weight vs. Street Clothes Weight

Never calculate spring rates using your bathroom scale weight in socks. Riding gear—including an ECE/Snell helmet (3.5 lbs), CE Level 2 leather jacket or MX roost armor (7–9 lbs), armored riding pants, and reinforced steel-shank boots (5–6 lbs)—adds between 15 and 22 lbs (7 to 10 kg) of sprung mass that directly compresses your springs.

2. The 30% Dynamic Sag Rule

Motorcycle suspension must be able to extend into road depressions as well as compress over bumps. By setting dynamic rider sag to approximately 30% of total travel, you reserve 70% of stroke for bump absorption and 30% of "negative travel" so the wheels stay in continuous contact with the ground when traversing dips in the pavement.

3. The Static Sag Sanity Check

After setting proper rider sag, step off the bike and measure static sag. If static sag is 0 to 5 mm, your spring is too soft and you had to apply excessive preload just to hold yourself up. If static sag is over 20 mm on a sportbike (or over 45 mm on an MX bike), your spring is too stiff.

Read the Full Sag Calibration Guide

Detailed 3-point caliper measurements, stiction averaging, and damping clicker baselines.

Read Full Suspension Guide →