How to use this calculator
- Choose shape. Use solid disk or annular disk.
- Enter mass and radius. Use the rotating part mass and effective radii.
- Enter RPM and gearing. Set engine speed, overall ratio and tire radius.
- Read inertia. Review stored energy and equivalent vehicle mass.
How it works
The calculator treats the flywheel as a uniform disk or annular disk, then calculates stored kinetic energy: E = 0.5 x I x omega^2. Reflected mass uses m_eq = I x (gear / tire radius)^2.
Use the clutch torque capacity calculator and RPM drop calculator for the surrounding drivetrain setup.
Worked example
Verified against the live calculator
An 8 kg solid flywheel with 140 mm effective radius has about 0.078 kg*m^2 of inertia. At 6,500 rpm it stores roughly 18 kJ before ratio and tire effects.
Frequently asked questions
How do you calculate flywheel inertia?
For a solid disk, inertia is one half times mass times radius squared. For an annular disk, use one half times mass times the sum of outer and inner radius squared.
What is equivalent vehicle mass?
It is the flywheel inertia reflected through the selected gear ratio and tire radius as an equivalent mass at the vehicle.
Why does equivalent mass change by gear?
Reflected inertia scales with gear ratio squared, so flywheel inertia feels larger in lower gears.
Is this exact for a real flywheel?
No. Real flywheels have ring gears, hubs, pockets and uneven mass distribution. This is a geometry screen.
Method & assumptions
- Uniform disk geometry is a simplification; CAD mass properties are better when available.
- Equivalent vehicle mass is gear-specific and ignores other rotating inertia in the drivetrain and tires.