Originally Posted by Darth Lefty

All right. Here is my criticism of the concept. In order to store energy, movement is required. A bike frame - even a noodly one - is stiff. Now that we have a bowyer in the conversation, imagine a bike frame strung like a bow. How far could you pull it back? How far would the arrow go? Not far, because the bike frame cannot store very much energy, because it's stiff. A spring that barely moves is storing barely any energy. When you pedal a bike you are putting in a force similar in magnitude to pulling a bowstring. This is enough to sometimes bend some bikes enough the wheel rubs the brake pad. That's like a millimeter, or a fraction of a degree. A bow, which is actually flexible and does return a lot of energy, lets the arrow slide about two feet. A spring moves in proportion to the force and stores energy in proportion to the movement squared. A spring that is 2x stiffer stores a quarter of the energy for the same force. A bike frame that is 100x stiffer than the bow stores 0.001% of the energy. It's just not enough to change the resistance profile enough to account for a 4% much less 12%.