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Fidanza Flywheel (less than 8 pounds)
The Fidanza wheel is made of super-hard 6061 machined aluminum billet stock. The friction surface is of special heat-treated 1050 plate steel for handling the heat and pressure, and is attached to the wheel using a computer-designed pattern of super-high-strength aerospace fasteners. Unlike most all-aluminum flywheels, the steel Fidanza friction surface can be machined.

Freeing up so much rotating mass from the motor makes it very responsive. It will rev up much more quickly, and low-speed acceleration is improved. (Note: This DOES mean low-speed driveability, like rolling at 5 mph in a parking lot, is worse. There's no free lunch.)