# Rotational weight vs dead weight

## Main Question or Discussion Point

I herd an argument about a motorcycle that said at a dead stop could a motorcycle balance while turning 15,000 RPM? I know while riding the object being in motion along with the tires rotating keep the motorcycle on 2 wheels. My question is could the engines turning force keep it balanced? The engine is mounted so it rotates parallel to the wheels so it does add some balance and it’s not trying to throw the bike to one side.

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Andrew Mason
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I herd an argument about a motorcycle that said at a dead stop could a motorcycle balance while turning 15,000 RPM? I know while riding the object being in motion along with the tires rotating keep the motorcycle on 2 wheels. My question is could the engines turning force keep it balanced? The engine is mounted so it rotates parallel to the wheels so it does add some balance and it’s not trying to throw the bike to one side.
Stability has to do with angular momentum. Spinning wheels have angular momentum. The bike will be vertically stable because tipping causes the angular momentum vector to tilt up. In order to conserve angular momentum, the bike has to rotate in the horizontal plane (about a vertical axis). But the friction of the two wheels on the road prevents a rotation of the bike about a vertical axis.

In order for the engine spin to make the bike stable, it would have to create angular momentum to produce a gyroscope effect. Since L = Iw, you need the engine to have significant moment of inertia or high speed or a combination of both. A crankshaft doesn't have much moment of inertia.

AM

Take the power of the engine (wattage) and divide this by pi*15,000 rpm.

If this value is greater then half the mass (kilograms) of the bike multiplied by the height of the bike's center of mass (meters) squared, then the bike has some chance of standing up due to gyroscopic force.

thanks for both of the replys. how you be able to figure out the power of the engine in watts?