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%.
Here's an experiment many roadies could pull off. Put the front wheel in a trainer and nose it up against a wall. Put the pedal at the first click over 3 o'clock. Prop a ruler up on the back side of the pedal and set up your phone to film the ruler. Stand on the pedal. Your full weight is as much torque as you will ever give it. Each 1/8th inch of movement of the crank is one degree that it could be "giving back"
The focus on the amount of energy seems to be why this bothers people. So let's put it this way - if you push down on the crank hard enough to shorten the chainstay length by (let's say) 1mm during the highest torque portion, you will get that full 1mm back when torque drops sufficiently for the stays to unwind. Not 0.8mm, not 2mm - a full 1mm. And we know it is 1mm because all this happens within the frame's range of elastic deformation and frames don't change shape over time. In other words, no spring set occurs.
What did the rider lose by the frame flexing? 1mm of road at the moment of peak torque. What did the rider get back? 1mm of road distributed over several moments after peak torque.
Why might the rider want to move that 1mm from the moment it should have been expressed to where it actually happened? Because that means the peak momentary torque was lower, and the rider didn't have to contend with that wall. Instead peak torque is buffered by momentarily lowering the gearing by putting the chain's output somewhere other than the road.
That's all that's happening. So the math is Loss of Energy from Flex In + Loss of Energy from Flex Out - Savings of Energy in Rider From Not having to Overcome Peak Torque = Net gain/loss of energy. And since we are dealing with materials that make good springs, the first two losses are very low.