Originally Posted by james_swift
I read somewhere that 1lb of rotating weight is the equivalent of 2lbs on the bike (you can ask the roadie forum).
Who needs the roadies?
The energy required to raise a mass M to a speed V is the familiar:
Et=0.5*M*V^2
The energy required to rotate an object with moment of inertia I to a rotaional rate omega is:
Er=0.5*I*omega^2
If the mass M is distributed at the radius R, the the moment of inertia is:
I=M*R^2
For a wheel moving forward at speed V, the rotation rate is:
omega=V/R
so Er=0.5*M*R^2*V^2/R^2=0.5*M*V^2
So a non-rotating mass requires energy
Et=0.5*M*V^2
While a rotating mass requires both the translational energy and the rotational energy to get up to speed.
Et+Er=0.5*M*V^2+0.5*M*V^2 = M*V^2; twice the non-rotating energy.
But, who cares? The energy isn't lost. It's preserved in the angular momentum of the wheel. You might take more time to spin up, but having spun up, the higher angular momentum will give the bike a more stable feel. Unless you are a racer for whom the ability to accerate quickly is critical, I wouldn't fuss overly much about rotating versus non-rotating mass.