#136
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Didn't we already go over this? There is no meaningful change in the rear center distance (the GCN video didn't show any, and they didn't claim any either). If there is no significant deflection, there is no significant energy storage. You're grasping for straws.
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#137
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No. The graph shows both pedals. Marked L and R.
When the crank arms are vertical, there is, at most, only slight rotational pressure being applied to the pedals. Not enough pressure to flex a frame. Also, very little of the flex is stored in the drivetrain, ready to be put in to rotational force. This whole discussion is based around the frame flexing, the BB area moving left/right in relation to the rest of the frame. That force truly does go to zero as your legs alternate pressure. Quote:
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#138
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And you are right that there isn't force enough to flex the pedals when the cranks are vertical, but that flex is the product of previous pedal input and the resistance of the rear wheel through the chain. That tug of war doesn't stop when the pedals are vertical, it just decreases to the point that the BB straightens. The question is: If force through the cranks and against the resistance of the chain deflects the BB to the side, where does that deflection force go when the deflection is taken out? The frame doesn't SPROING! back to center, so it isn't just released. It comes out about at the same rate it went in, and if constant pressure is applied to the pedals, where can it go? Some folks seem to think that the BB flex is separate from the force applied to the chain, but it comes from the tension between the chain, pedal and spindle. Otherwise it wouldn't occur at 3 o'clock. |
#139
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Not stop, not go backwards, not apply zero pressure, but apply a pressure less than the maximum we apply in some other o'clock of the pedal rotation. In the lower right, compare the four o'clock position and the five o'clock position. Four is maybe about the maximum, in that pedal stroke, and five is -- less. That's all I'm talking about when I say "easing off on the pedals", and according to those strain gauges, at least, it is indeed how pedaling works. Mark has a suggestion that, as the frame un-flexes, it can help the opposite leg through the back part of its stroke, but given that the un-flexing retards the motion of the leg we're looking at, and the BB spindle connects the two cranks, I think we have to rule that out too. |
#140
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So the forces in a sprint and a standing climb never have the bike completely vertical through the pedal stroke. If you look at the attached video of Cavendish sprinting (and not even going full out) the frame is always at an angle as the forces are applied, this seems to me that it would promote more possible flex to the BB area? More so than a bike sitting upright in a trainer as referenced in the original video.
https://www.youtube.com/watch?v=Kb67p8Cb7v0 William PS: Apologies If I missed it in the running commentary. |
#141
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#142
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Curiously, the torsional flex at the BB has an interesting affect on the shape of the pedal 'circle'. Instead of moving in a perfect circle, the upward and downward deflections cause the shape to be stretched into an ellipse. I wonder what affect this has on muscle utilization? |
#143
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#144
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Seems like most of the flex is the BB area moving left/right in relation to the centerline of the bike/frame because of the downward force of the pedals...not because of the rotational force...
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#145
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Those are the same forces at 3 o'clock. But the BB flexes most under the most drivetrain force - which is during the power stroke, not at 6 o'clock.
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#146
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I don't think the frame would spring back at the same rate because the force against the pedals varies throughout the pedal stroke. You can stretch a rubber band quickly, and release it slowly... Maybe the peak force on the pedal is at 3 oclock, maybe it is 100 newton meters, and maybe the frame flexes 6mm at 100nm. At the bottom of the pedal stroke, the pressure would have gradually dropped from 100 back to 0-10nm, and the frame would gradually flex back to straight before flexing the opposite direction as the opposite leg begins to apply pressure. The frame isn't flexing enough to be all that noticeable. It's not like bobbing around on a full suspension bike with no lockout. |
#147
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The BB is flexing because of the downward pressure from the rider, not the rotational force. If you stuck a motor in the seatpost, the BB would not flex left/right very much, if any.
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#148
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If it was just downward force, the BB would flex most at 6 o'clock, not 3 o'clock. |
#149
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If you watch this video of what the BB is doing under heavy load, you'll see that the most BB sway is right during the power stroke and is largely gone by the time the pedal is at the bottom. If this was just an issue of putting weight on the pedals, the flex would be at the bottom to match Mark's graphic. Instead, that's when it is going away. |
#150
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