That was my expectation from the title as well. I thought it would look like an over-sized mainspring from a watch (https://en.wikipedia.org/wiki/Mainspring htt

That was my expectation from the title as well. I thought it would look like an over-sized mainspring from a watch (https://en.wikipedia.org/wiki/Mainspring https://en.wikipedia.org/wiki/Mainspring). There was a small company back in the 90's or 00's that was developing very high rpm flywheels to store energy but they weren't able to work out a licensing deal with the car manufacturers.