Training wheels have long been the go-to accessory for teaching kids how to ride a bicycle. Now there is another option: the Gyrowheel, from Gyrobike in San Francisco. The Gyrowheel, which replaces the front wheel of a child's bike, rights the bike whenever it starts to tip. How? It contains a motorized spinning disk. In technical terms, the disk creates a force called gyroscopic precession. In practice, it makes a bike really stable at low speeds. Daniella Reichstetter founded Gyrobike in 2007, after licensing the technology from engineering students at Dartmouth College, where she was earning an M.B.A. The wheel has three stability levels; as riders improve, they go to the lower levels. It is available on the Gyrobike website (thegyrobike.com) for $99.95 and will appear in bike and toy shops this spring.