Frame Flex on GCN

[Mark McM]

Originally Posted by Kontact

What made you decide to ignore the mechanism I outlined for returning flex energy to the drivetrain via rear center elongation?

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.

[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.

Originally Posted by Kontact

The graph is for one pedal, and shows that the one pedal is actually pushing against the crank rotation at times. If the other pedal truly went to zero at any point where the first pedal was anti-rotation, the pedals would stop momentarily, the chain would get slack and there would be a clank as the pedals started to re-engage the freewheel pawls.

The reality is that, despite the highs and lows, we provide varying but continuous power to the rear wheel. The tension in the chain, on the spider and through your legs never goes to zero.

If you were to take that pedaling vector diagram and sum it with the other pedal you would find no spots where there isn't net pro-rotation force.

[Kontact]

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.

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.

[cachagua]

I follow your argument, I just don't see where you get this idea that we "ease off the pedals" when that isn't how pedaling works.

We do not apply constant, unvarying pressure on the pedals. We ease off, what else would you like to call it?

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.

[William]

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.

[Kontact]

Originally Posted by cachagua

We do not apply constant, unvarying pressure on the pedals. We ease off, what else would you like to call it?

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.

The point I was making, which you countered with the "ease off" thing, was that the power coming off the chainring is never zero and the motion is never zero. So there is no reason to think that the frame flex is just disappearing up our legs and we don't notice. We can feel it when we put that flex in, and we would feel it feeding back as it came out if it pushed against our legs.

[Mark McM]

Originally Posted by cachagua

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.

I'm not sure how you conclude that. Take, for example, the cases where the cranks are vertical (one pedal is at the top of the stroke, and one pedal is at the bottom). In this position, the differential between the right/left down forces on the pedals is maximum, which results in the maximum (torsional) frame flex. Due to the torsional rotation at the BB, the bottom pedal is at the maximum downward deflection, and the top pedal is at the maximum upward deflection. As the pedals continue to rotate through the next 180 degrees, the right/left force differential reverses, and the deflections also reverse. In other words, as the pedal rises at the back of the pedal circle, the reversal of the deflection acts to raise the pedal. Until the frame returns to its neutral, unflexed position, the stored energy in the frame is indeed helping to lift the rear leg.

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?

[Kontact]

Originally Posted by Mark McM

I'm not sure how you conclude that. Take, for example, the cases where the cranks are vertical (one pedal is at the top of the stroke, and one pedal is at the bottom). In this position, the differential between the right/left down forces on the pedals is maximum, which results in the maximum (torsional) frame flex. Due to the torsional rotation at the BB, the bottom pedal is at the maximum downward deflection, and the top pedal is at the maximum upward deflection. As the pedals continue to rotate through the next 180 degrees, the right/left force differential reverses, and the deflections also reverse. In other words, as the pedal rises at the back of the pedal circle, the reversal of the deflection acts to raise the pedal. Until the frame returns to its neutral, unflexed position, the stored energy in the frame is indeed helping to lift the rear leg.

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?

The problem is, this isn't true. The maximum frame deflection is during the power stroke, somewhere around 3 o'clock. The frame isn't flexing because we are stepping on it, it if flexing because we are trying to move the rear wheel so hard that tension in the chain is enough to pull the BB off center.

[kramnnim]

Originally Posted by Kontact

The frame isn't flexing because we are stepping on it, it if flexing because we are trying to move the rear wheel so hard that tension in the chain is enough to pull the BB off center.

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...

[Kontact]

Originally Posted by kramnnim

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...

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.

[kramnnim]

Originally Posted by Kontact

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?

Constant, even pressure is not applied to the pedals.

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.

[kramnnim]

Originally Posted by Kontact

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.

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.

[Kontact]

Originally Posted by kramnnim

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.

But the BB would flex in the same direction if you pulled up on the opposite crank. The flex isn't a result of a simple stepping on the crank, but because that's the only direction the BB can go when it is trapped between the power stroke and the chain.

If it was just downward force, the BB would flex most at 6 o'clock, not 3 o'clock.

[Kontact]

Originally Posted by kramnnim

Constant, even pressure is not applied to the pedals.

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.

I didn't say "even pressure". You inserted that.

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.

[kramnnim]

Originally Posted by Kontact

But the BB would flex in the same direction if you pulled up on the opposite crank. The flex isn't a result of a simple stepping on the crank, but because that's the only direction the BB can go when it is trapped between the power stroke and the chain.

If it was just downward force, the BB would flex most at 6 o'clock, not 3 o'clock.

It flexes the most at 3 oclock because that's when the leg is applying the most force