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A flywheel is an energy storage device that uses its significant
moment of inertia to store energy by rotating.Flywheels have
long been used to generate or maintain power and are most
identified with the industrial age and the steam engine.In one
sense it can be thought of as a rechargeable battery that store
energy in the form of mechanical energy instead of electrochemical.

Flywheels have been gaining popularity as a possible replacement
for chemical batteries in vehicles, but until last year there
was no record of a flywheels being used to increase the efficiency
of a bicycle.
In 2011, Maxwell von Stein, a student at Cooper Union, added a
flywheel and a continuously variable transmission to his bike for
his senior project. He used a car flywheel he found that weighs
15 pounds. His idea won him the Nicholas Stefano Prize, which
is Cooper Union’s award for superior mechanical engineering
design. He also gained quite a bit of notoriety on various biking
websites and was featured in NPR’s weekly segment, “Science
Friday.”