#181
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However I think it there is another flex mode, not the BB swinging, that does effectively "shorten" the stays. The video we started with demonstrates it. If there were no strain released in that mode when the rider lets go of the rear brake -- no "re-lengthening" of the effective stay length -- then the wheel wouldn't move. Would anyone like to try this at home? Duplicate the set-up in the video and do exactly the same thing, only let the rear brake off very gradually, so the wheel doesn't spin freely but just rotates a tiny, tiny bit when the frame relaxes. A degree? Two? It would be very little. But that's going to correspond to what we're calling "compression" of the stays. |
#182
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No one is claiming that there is extra energy coming out of a flexy frame. I'm claiming that there is no real loss because the flex is simply used later in the pedal stroke, which is what the article you posted claims and it also seems to be what you are claiming, too. The only conflict appears to be that you believe there is a difference between "helps the rider to raise the leg" and any other pro-forward motion input. I used the stays to illustrate another way stored energy could contribute to forward motion, but I don't think there is any real difference between "helps push the bike" or "helps turn the pedals" - those are the same thing. You are "pushing against the foot" and I'm saying they are the same thing. But the author, you and I are all taking a very different stance from that typified by cachagua who believes that the release of strain energy drags the riders leg against pedaling motion, causing an anti-forward motion input, slowing the bike overall. |
#183
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And does that released strain energy, the energy that rotates the rear wheel in the video, contribute to your going forward, or does it not? That, I think, is our original, central question.
And we've been getting rather far afield. |
#184
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#185
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If you let the brake out slowly the wheel will move. The fact that they let the wheel move enough to cause it to coast isn't really different than letting it move the amount of distance dictated by distance of the flex. Those are just two different illustrations of the same thing: If you release the brake all at once, the resulting velocity of the wheel is a measure of released energy - the energy it took to accelerate the wheel from zero to whatever rotational speed it achieves. That demonstrates a work per second rate - Watts. If you release the brake slowly, you are releasing a lot of that energy into the brake pads, so what you are demonstrating is no longer a transfer of Watts, but a transfer of work without reference to time - Joules. The drivetrain flexing will provide enough work to move the wheel from A to B. But since we took time out of it we are no longer talking about energy the way cyclists normally do - in Watts. So you can demonstrate whichever you want, but they are demonstrations of different ways of understanding energy, not two separate things. Last edited by Kontact; 02-15-2018 at 12:13 PM. |
#186
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However .. I think the mechanism of the energy return actually is important at least in one regard. As we've been discussing, the changing force vectors on the pedals through the crank revolution will not only affect how much strain energy goes into the frame, but also at what points in the crank rotation that it goes in and comes back out. This in turn affects the mechanisms by which the returned energy is utilized. The reason this is important is because riders often vary their pedaling styles to match different situations (for example, I don't think anyone will claim that they pedal the same way between standing or seated, or between steady continuous effort and short full-power sprints). I think it is highly likely that riders will vary there pedaling styles (if only subtly) in response to frame stiffness, and with practice will adapt their pedaling to optimize their pedal motions/forces with the frame response. |
#187
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I don't think the original GCN video is an accurate representation of what happens while riding because the trainer is flexing a lot.
But, even if it was accurate- yes, wheel will be propelled when the brake is released if the crank remains at 3 oclock. I don't think the tension is released until around 6 oclock. So to see what the effect would be, you would have to flex the frame+trainer at 3 oclock, hold it there while you allow the crank to move down to 6 oclock, and then release. I don't think the wheel would move nearly as much, If at all. |
#188
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As to adapting to how the energy goes in and comes out, that sounds like something your brain figures out in about 4 pedal strokes and is little different than the learning curve of walking with new shoes. Why would it be important? It seems like the only important thing that these discussions really have to offer is convincing people that, despite how it might feel, you aren't actually throwing away energy if your frame flexes. |
#189
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#190
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#191
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Your suggestion is like the difference between slowly carrying a book upstairs and throwing it. |
#192
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And I wanted to highlight that because I think that's the component of flex, or the main one, that we've been trying to look at. I'm totally curious about this but I don't have a trainer, or else I'd do the experiment myself. I'll bet the wheel moves just a degree or two at most. I could be wrong but I think it must be the tiniest, tiniest bit. But some. Definitely some. |
#193
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Last edited by Kontact; 02-15-2018 at 11:26 PM. |
#194
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YAAAAH-ha-ha-ha-ha-ha! I would never, ever breathe a word of this to a Strava guy.
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