Quote:
Originally Posted by kramnnim
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.
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The two diagrams are not in phase. If they were in phase and you added them, you would see that there is always force applied to the chainring.
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.