rotating weight: Weight (more correctly mass) that is rotating while the bike is moving, particularly the wheels. Mass near the outside edge of a wheel has about twice the stored energy of a similar non-rotating mass moving at the same speed. A bicycle wheel can be considered to be a good approximation of a hollow cylinder with most of its mass at or near the rim. The rotation of cranks, wheel hubs, and other parts are of less significance because both their radius and speed of rotation (angular velocity) are small. All mass resists changes in velocity (acceleration or deceleration) due to inertia. This resistance is noticeably greater where rotational inertia is also a significant component, so lighter wheel rims, spoke nipples, and tires will permit faster acceleration (or the same acceleration for less expenditure of energy). This effect is much reduced at lower speeds such as during hill climbing.