#196
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Yes, there is a component of frame flex that puts the rear axle closer to the BB. But of all the flex components, this is one of the smallest. Since strain energy is inversely proportional to stiffness, this flex component will be a very small part of the total strain energy in the frame. While the GCN video may not be an entirely realistic simulation of pedalling, it clearly shows that the majority of the strain energy in the frame is from the horizontal twisting of the BB, not from the (linear) compression of the rear triangle. Anyone who wants to claim otherwise, will need to come up with direct evidence to support their argument.
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#197
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As you know, as the rider presses down on the pedal during the down stroke, the frame flexes (by twisting at the BB), and both the pedal and the BB drop in response. The peak downward force in fact exceeds the bicycle drag force during this phase. Towards the end of the down stroke, the rider may ease off on the down force. This will allow the frame to unflex, because the flex force then exceeds to the pedal force. The pedal's downward speed may also decelerate, as the BB starts to rise - but as long as enough force is applied to the pedal so the pedal doesn't rise as fast as the BB, the the upward motion of the BB will cause the crank to rotate forward. And the energy that caused this extra crank rotation (and thus extra energy applied to the drivetrain) came from the strain energy stored in the frame. The drivetrain force due to the release of the strain force may not exceed the drag force, but it is results in the strain energy enterring the drivetrain. Quote:
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#198
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I think the thing is rider placebo is a valuable effect. If a rider feels more efficient on a stiff frame, that is good for them, if they feel better on a flexier frame, that's good for them. No different than superstitions in baseball. |
#199
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__________________
Cheers...Daryl Life is too important to be taken seriously |
#200
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It takes energy to flex a frame. And it isn't hard to measure how much energy. If that isn't a measurable loss and the energy at the wheel is the same, doesn't that prove that the flex returned its energy? What else could have made up for the loss? |
#201
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#202
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I realize I'm repeating myself, but why do these analyses alway attempt to reduce a complex three dimensional movement to just what is observed perpendicular to the plane of the chainline? Is that even vaguely realistic when much of what is happening to the bike is like the bending of a bow from the middle of downtube all the way to the rear hub? |
#203
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But if it (a certain degree of flex or lack of flex) makes you feel better, ride it. |
#204
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In the second diagram, the maximum R/L pedal force differential occurs at about 4 o'clock, so the BB begins its rise after 4 o'clock - when the rising BB does help with crank rotation. |
#205
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The conclusions of the analysis can easily be confirmed by eye - the twisting of the frame/BB around the longitudinal axis is large and pronounced, and can be easily seen. At the same time, any shortening of the distance between the axle and BB can not be seen by the naked eye. Why do you keep concentrating on something which is so obviously inconsequential? |
#206
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And I'm not really advocating for flex, just somewhat against the marketing of drive train stiffness - especially when it leads us to sacrifice ergonomics like Q or ankle clearance to provide a quality that we don't benefit from. Dealing with problematically weirdo stuff like BBRight for several years will make you want to talk about the Emperor's Clothes. My personal experience has been that stiff bikes are fun to ride, and don't necessarily have to be unpleasant. But I did still see with those bikes that they were on the edge of ideal tire traction at times and could "waste" energy doing what they were supposed to be ideal for - sprinting up hills. I have also had the experience of riding a flexible aluminum Vitus that just seemed slow - as if the relatively poor spring qualities of narrow aluminum tubing was maybe turning energy into the work hardening that more naturally springy materials didn't suffer from. So it is not hard to see why people love stiffness if they have had similar experiences. So my interest isn't obsessive advocacy for one design philosophy or another, but just getting cyclists to see that the problems of frame design are actually subtle and even counter-intuitive because pedaling is not nearly as simple as a 2D textbook illustration would suggest. It sounds like enough tests have been performed to suggest that relatively average BB flex is not less efficient, and the reasonable discussion at this point is really just about the mechanism of efficiency, rather then whether it is true or not. It is a fun and interesting conversation. |
#207
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Like I said earlier, there are also two separate flex issues - where the strain energy is stored, and how its release actually feeds back into the drivetrain. |
#208
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#209
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If there is constant, positive force going through the drivetrain at all times that you are pedaling, when would you not benefit from it?
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#210
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Sigh. Take your bike outside and climb out of the saddle at 60rpm, rocking the bike with your arms. Take note of how much rotational force is going to the chain at 6 oclock.
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