#106
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What's getting left out of your analysis is that there are actually two ways for the frame to store some pedaling energy - by lowering the chainring (right pedal load), or raising the chainring (left pedal load). Moving between them is when the stored energy is pushed down the stays. All the various static examples are demonstrating is that the flex is still in line with the drivetrain, and isn't just flying out into space. Then you take that understanding and apply it to the fact that the tension on the chain never goes to zero and the BB flex automatically alternates from right-to-neutral-to-left. |
#107
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But what about the crank arm and pedal spindle? Wouldn't they have also flexed or deflected and contributed in some way?
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#108
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To me, it looks like the trainer is flexing more than either frame.
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#109
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I don't know how I can make this simpler. If you and I push our hands together and push equally hard, our hands will stay still. If you relax a little and push softer, and I keep on pushing just as hard, will our hands move toward me as a result? Quote:
Tension on the chain never goes to zero -- doesn't need to; not part of my account. BB flex alternates from right to neutral to left -- okay, if it goes back to neutral in the middle then manifestly there's nothing carried over to the next pedal stroke. |
#110
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It isn't like a spring that is simply being allowed to uncompress. Pedaling actively untwists the drivetrain, putting the work used to flex the frame to use. And that would be true even if the frame had a pivoting BB that required no real force to "flex". If the distance from chainring to cassette varied throughout the pedal stroke because the crank pivoted laterally on a bearing, then you'd still get the effect of force going through the chain from pedaling through neutral. |
#111
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So hypothetically the twisting of the chainstays could be isolated and provide forward motion by themselves? A jig could be made to illustrate that movement and demonstrate how it could propel a bike or at the very least rotate the rear wheel.
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#112
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#113
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#114
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Again, this isn't because chain tension has relaxed, it is because the lateral displacement of the BB has relaxed into a position that requires more chain. And even if the frame didn't want to relax to that position, the other half of your pedal stroke doesn't give it any choice. Right pedal, neutral, left pedal, neutral. Short stays, long stays, short stays, long stays. Less chain, more chain, less chain, more chain. Last edited by Kontact; 02-12-2018 at 03:09 AM. |
#115
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That movement wouldn't be enough to advance the bike even one click of the freehub even on the finest of engagement mechanisms. The bike would just be sitting there flexing in both directions. If the bike were a fixed gear, it would still just rock back forth. It's not just energy output, it's energy over time and there's no way you could propel a bike with that tiny of a movement by itself. The movement of the BB is over the time of a riders cadence so it's too gradual.
A spring can launch something in the air, but if you slow down the release of that energy (energy over time) you could have a car spring that couldn't even launch a penny. Seems more like trying to row a boat leaving the oars in the water the whole time. Last edited by ergott; 02-12-2018 at 06:10 AM. |
#116
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This is the million dollar question. Where exactly is the spring's energy going? Like your penny example, slowing down the spring will transfer the energy to the device that is slowing down the spring and not the penny. With hub crank and pedal based power meters it would not be too hard to do some empirical testing with frames of various stiffness.
__________________
Cheers...Daryl Life is too important to be taken seriously |
#117
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I'm willing to bet that anything going in here is way under the accuracy tolerances of the best power meters available (±1%).
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#118
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Using a simpler approach (strain energy), he also mentions in the podcast that he calculated about 40W for power which goes into flexing a frame under an all-out sprint (I think it was for a 1500-2000W effort). This was meant as an upper-bound value, but even if it is true, it is about 2% of the power input, which is more or less the accuracy of most strain-gage based power meters on the market today. |
#119
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That's the only real point here. Either the energy stays in the drivetrain either way, or there is no energy difference. |
#120
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??
Me wonders how much unrecoverable energy has been spent on this subject.
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