What Is a Flywheel?

A flywheel is a rotating disc connected to an engine’s crankshaft. Its main job is to:

• Store rotational energy
• Smooth engine pulses
• Improve idle stability
• Control torsional vibration

When your engine fires, power comes in pulses. The flywheel smooths those pulses into steady rotation.

Without it, engines would feel rough and unstable.

What Is a Flywheel Torque Calculator?

A flywheel torque calculator is a tool that calculates:

• Moment of inertia
• Kinetic energy storage
• Torque capacity
• Hoop stress
• Maximum safe RPM
• Torsional vibration risk

It takes inputs such as:

• Flywheel mass
• Outer diameter
• Inner diameter (hub bore)
• Maximum operating speed (RPM)
• Material type
• Number of engine cylinders
• Safety factor

Then it performs physics-based calculations to give you real engineering values.

Key Inputs Explained

Understanding each input helps you get accurate results.

1. Flywheel Mass

Mass directly affects inertia.

Heavier flywheels:

• Store more energy
• Smooth idle better
• Reduce stalling
• Rev slower

Lighter flywheels:

• Improve throttle response
• Reduce rotational drag
• Rev faster
• Provide less smoothing

Mass can be entered in kilograms (kg) or pounds (lbs).

2. Outer Diameter

Diameter has a major effect on inertia because rotational energy increases with radius squared.

A small increase in diameter can greatly increase:

• Moment of inertia
• Energy storage
• Torque capacity

Units supported:

• mm
• cm
• inches

3. Inner Diameter (Hub Bore)

If your flywheel has a center hole, it reduces mass distribution.

• Enter 0 for a solid disc
• Larger inner diameters reduce inertia

This affects the radius of gyration, which influences torque storage.

4. Maximum Operating Speed (RPM)

RPM determines:

• Angular velocity
• Kinetic energy
• Hoop stress
• Torsional frequency

Higher RPM dramatically increases stress because stress increases with speed squared.

The calculator allows:

• RPM
• rad/s

5. Material Type

Material determines:

• Density
• Ultimate tensile strength (UTS)
• Maximum safe stress

Common materials include:

Stronger materials allow higher RPM and lower failure risk.

6. Engine Cylinders and Stroke Type

This is used for torsional vibration analysis.

The calculator evaluates:

• Firing frequency
• Natural frequency
• Resonance risk

If the firing frequency matches the natural frequency, vibration increases sharply.

That’s when failures happen.

7. Safety Factor

A safety factor protects against:

• Material defects
• Fatigue
• Thermal cycling
• Manufacturing tolerances

Typical values:

• 2.5 for street vehicles
• 3.0 or more for racing

Core Calculations Explained

Here is what the flywheel torque calculator computes behind the scenes.

1. Moment of Inertia

Formula:

I = m × k

Where:

• m = mass (kg)
• k = radius factor based on outer and inner radius

Inertia determines resistance to acceleration.

Higher inertia means:

• More stored energy
• Slower acceleration

2. Kinetic Energy Storage

Formula:

KE = ½ I ω²

Where:

• I = moment of inertia
• ω = angular velocity

Results are displayed in:

• kJ (kilojoules)
• Wh (watt-hours)

This tells you how much energy your flywheel stores at max RPM.

3. Torque Capacity

Torque is calculated using:

Torque = I × angular acceleration

This shows how much torque the flywheel can resist during acceleration.

Displayed in:

• N·m
• lb-ft

4. Hoop Stress

Hoop stress determines structural safety at high RPM.

Formula concept:

Stress ∝ density × (speed × radius)²

If hoop stress exceeds allowable stress, the flywheel can fail.

The calculator shows:

• Hoop stress (MPa)
• Maximum safe RPM
• Stress ratio

If stress ratio exceeds 1.0, it’s unsafe.

5. Torsional Vibration Analysis

The tool calculates:

• Engine firing frequency
• Flywheel natural frequency
• Frequency ratio

If ratio falls between 0.8 and 1.2, resonance risk increases.

Resonance can cause:

• Crankshaft fatigue
• Gear noise
• Component failure

Understanding the Results Section

The calculator provides several outputs.

Moment of Inertia (kg·m²)

This defines how resistant the flywheel is to acceleration.

Example classifications:

• < 0.05 → Motorcycle / compact
• 0.05–0.2 → Sport compact
• 0.2–0.5 → Performance street
• 0.5–1.0 → Heavy street / truck
• 1.0 → Industrial

Kinetic Energy (kJ and Wh)

Shows stored rotational energy at maximum RPM.

Higher values mean:

• Better idle smoothing
• Slower throttle response

Torque Capacity (N·m)

Indicates resistance to sudden acceleration.

Important for:

• Launch control
• Clutch engagement
• Racing setups

Hoop Stress (MPa)

If stress approaches material limit:

• Reduce RPM
• Increase safety factor
• Choose stronger material

Torsional Frequency Warning

If the calculator shows:

“Operating near torsional resonance”

You should:

• Add a harmonic damper
• Modify flywheel inertia
• Adjust RPM range

Real-World Example

Suppose you enter:

• 25 kg mass
• 300 mm diameter
• 8000 RPM
• Steel material
• 4-cylinder 4-stroke

The calculator may show:

• Moderate inertia
• Good energy storage
• Safe stress ratio
• Balanced torsional frequency

This would suit a performance street car.

If RPM is increased to 12,000, hoop stress may exceed safe limits. That signals danger.

Why Flywheel Torque Calculation Matters

A flywheel is not just a heavy disc. It affects:

• Engine response
• Driveability
• Durability
• Safety

Incorrect sizing can lead to:

• Poor acceleration
• Rough idle
• Structural failure
• Catastrophic breakup at high RPM

Using a flywheel torque calculator removes guesswork.

Street vs Racing Flywheel Selection

Street Use

• Higher inertia
• Better smoothness
• Larger safety factor

Racing Use

• Lower inertia
• Faster revving
• High-strength alloy materials
• Higher safety factor

The calculator helps balance response and reliability.

Common Mistakes to Avoid

1. Ignoring safety factor
2. Exceeding safe RPM
3. Choosing material based only on weight
4. Overlooking torsional resonance
5. Using incorrect units

Always double-check inputs.