MIT’s Copenhagen Wheel … the Wheel can store energy every time the rider puts on the brakes, and then give that power back to provide a boost when riding uphill or to add a burst of speed in traffic.
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MIT’s Copenhagen Wheel

… the Wheel can store energy every time the rider puts on the brakes, and then give that power back to provide a boost when riding uphill or to add a burst of speed in traffic.

Source: web.mit.edu