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Comment by tfolbrecht | Hacker News Reader

That is extremely weird considering the frame etc has so much more mass.

Do you have some math on that?

I just know the math indirectly, from observing that stopping distances scale linearly with rolling component mass but by the square root of the total mass of the vehicle. Relevant to modifying vehicles - if you put larger, heavier rims on your car, you must also upgrade the braking system or you're rendering it unsafe to drive by possibly doubling the stopping distances.

The number I've read is saving 1kg of rotating mass is equal to saving 4kg of chassis weight. I can't believe rotating components make up 1/5th of a car's mass though.

That's got to be averaged out over all rotation mass in the vehicle though right?

The shaft to a CV joint is going to have a helluva lot less angular momentum than a wheel.

It was a rule of thumb from a drag racing website so it's probably highly biased towards wheels. I think they did mention lightening the flywheel on the engine too, though - although that's best thought of as energy storage, rather than energy loss, since you reclaim the energy in the flywheel at the next gear change.

I would expect this to be true only below a certain speed given a certain vehicle mass.

It's obviously incorrect at 100 mile an hour with the engine idling and in neutral.

You seem to be ignoring the wheels. Lots of angular momentum there.

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