Frame Flex on GCN

[kramnnim]

And so we agree on the 7oclock.

You also said this:

The frame doesn't flex down at 6 o'clock. That's not part of what happens during pedaling and has no place in the discussion.

My Pioneer shows very clearly that this is not true. (whenpesalingoutofsaddleatlowrpmwhilerockingthebik ebackandforth)

[cachagua]

Originally Posted by Kontact

You never propose what you do think...

Originally Posted by cachagua

Strain energy in the frame cannot "spring back" as acceleration.

That's from post #16. I've continued to develop that idea as we've gone along, and if you want I can recap some of what I've posted.

[Kontact]

Originally Posted by cachagua

That's from post #16. I've continued to develop that idea as we've gone along, and if you want I can recap some of what I've posted.

I'm sorry to have to explain this, but saying what doesn't happen is not the same as saying what does.


You have NEVER explained where you think this energy is going and by what mechanism.

[Kontact]

Originally Posted by kramnnim

I think a flexy frame wastes energy under certain pedaling styles.

By what mechanism? Where does it go and how is it isolated from the driveline that created it?

[kramnnim]

Originally Posted by Kontact

By what mechanism? Where does it go and how is it isolated from the driveline that created it?

Downward pressure from the rider flexes the frame rather than providing radial force to the crank arm to make the crank rotate. Frame flexes back when the now tangential force from the rider lessens and does not provide any rotation to the crank arm.

[Kontact]

Originally Posted by kramnnim

Downward pressure from the rider flexes the frame rather than providing radial force to the crank arm to make the crank rotate. Frame flexes back when the now tangential force from the rider lessens and does not provide any rotation to the crank arm.

If the frame flexes back after 6 o'clock, that means the pedal is being raised. If the pedal is being raised while it is already traveling upward, how is that not helping the rider pedal?

[kramnnim]

Originally Posted by Kontact

If the frame flexes back after 6 o'clock, that means the pedal is being raised. If the pedal is being raised while it is already traveling upward, how is that not helping the rider pedal?

The upward and lateral movement of the BB (including the entire crankset as well as the pedal) does not result in crank rotation.

[Kontact]

Originally Posted by kramnnim

The upward and lateral movement of the BB (including the entire crankset as well as the pedal) does not result in crank rotation.

The pedal is on the end of the crank and it is rotating upwards between 6 and 12 o'clock. So if the end of the pedal is lifting as it swings up from the BB, and your foot is at 7 o'clock when that happens, it is most certainly helping move your foot in the direction of pedaling.

[kramnnim]

Originally Posted by Kontact

The pedal is on the end of the crank and it is rotating upwards between 6 and 12 o'clock. So if the end of the pedal is lifting as it swings up from the BB, and your foot is at 7 o'clock when that happens, it is most certainly helping move your foot in the direction of pedaling.

Does the frame unflexing lift the entire BB and crankset, or just the pedal?

When the frame unflexes, does the pedal rise a greater distance than the center of the BB?

Does "moving in the direction of pedaling" result in rotational force to the crank arm?

How far can you propel a bike without rotating the crank arms?

[Mark McM]

Originally Posted by kramnnim

Does the frame unflexing lift the entire BB and crankset, or just the pedal?

When the frame unflexes, does the pedal rise a greater distance than the center of the BB?

Does "moving in the direction of pedaling" result in rotational force to the crank arm?

How far can you propel a bike without rotating the crank arms?

I think this discussion has come down to arguing around in circles.

The key question to the current point of contention appears to be: In the bigger picture, is there a difference between the stored flex energy driving the bike forward directly, and the stored flex energy making it easier for the rider to drive the bike forward?

In the end, it comes down to the total energy the rider expends, and the total energy that propels the bike. All the evidence seem to indicate that the flex in the frame has little affect on either of them. Between the moment the frame begins to flex, and the moment the frame has un-flexed, the amount of energy expended by the rider, and the amount of energy that propels the bike, will be the same as if the frame hadn't flexed at all.

[Kontact]

Originally Posted by kramnnim

Does the frame unflexing lift the entire BB and crankset, or just the pedal?

When the frame unflexes, does the pedal rise a greater distance than the center of the BB?

Does "moving in the direction of pedaling" result in rotational force to the crank arm?

How far can you propel a bike without rotating the crank arms?

Your first question is easiest for you to answer by looking at your bike and all the images showing how BBs flex down and over.

As the frame unflexes, the pedal moves more than the BB if it is above or below the line that the BB is flexing around. So if the pedal is a 7 o'clock, it is moving more than the center of the BB. It's a lever.

Yes, because it adds to the movement that is already happening.

Depends what you mean. A 4 o'clock the GCN video shows how the bike would be moved. At 6 o'clock the question doesn't make a lot of sense - you don't pedal a bike by stepping straight down on a static pedal, and you can't use the motion of a static pedal if it isn't allowed to continue to rotate. It sounds like you are trying to "prove" that flex can't help pedaling, but the only way you can do that is to stop the pedaling.

[kramnnim]

Originally Posted by Mark McM

I think this discussion has come down to arguing around in circles.

The key question to the current point of contention appears to be: In the bigger picture, is there a difference between the stored flex energy driving the bike forward directly, and the stored flex energy making it easier for the rider to drive the bike forward?

In the end, it comes down to the total energy the rider expends, and the total energy that propels the bike. All the evidence seem to indicate that the flex in the frame has little affect on either of them. Between the moment the frame begins to flex, and the moment the frame has un-flexed, the amount of energy expended by the rider, and the amount of energy that propels the bike, will be the same as if the frame hadn't flexed at all.

Yes, it is going in circles.

I would agree that in most modern carbon frames, there isn't enough flex to affect the amount of energy expended.

If the frame is considerably flexier than that...many riders may notice the flex, and the bike may feel sluggish. I have yet to see proof that such flexy frames are not wasting energy.

[cachagua]

Originally Posted by Mark McM

In the bigger picture, is there a difference between the stored flex energy driving the bike forward directly, and the stored flex energy making it easier for the rider to drive the bike forward?

I think there's a significant difference between two components or modes of flex, which I began to try to distinguish a few pages ago but didn't fully develop the idea then. Let me see if I can sketch it out here:

What I think you're calling "the energy that makes it easier for the rider" -- we've talked about it lifting the rider's foot on the pedal -- that energy comes from forces the rider puts on the frame which would never drive the bike forward, no matter how the frame behaves. The example is when the bike's on the trainer, the pedal is at 6 o'clock, and you press down and "sway" the BB to the side. You could do that on the road, instead of on the trainer, and bounce up and down and flex and unflex the frame all day, and never go anywhere.

So on one hand there's effort, or force, that wouldn't ever move you forward, and frame flex that, when it returns its strain energy, ...won't move you forward.

Then on the other hand there's force which, if it didn't get borrowed by flex, would move you up the road. And the flex that's a response to this force is best regarded as a different mode of flex from the first. We used the example of the chainstays' bowing sideways for this, and the thing that separates this flex from the first mode is that it reduces the effective distance between the rear cog and the chainring. Another way to say it is that this flex shortens the "free chain" between the crank and the rear wheel.

Originally Posted by Mark McM

Between the moment the frame begins to flex, and the moment the frame has un-flexed, the amount of energy expended by the rider, and the amount of energy that propels the bike, will be the same as if the frame hadn't flexed at all.

I still have some doubt about that, and I have a scenario I want to sketch out to frame my question -- however I've run out of time just now. But, have I clarified what I mean there? Does it look reasonable to isolate those two modes of flex, even though I haven't yet said what it'll be useful for?

[kramnnim]

Originally Posted by Kontact

Your first question is easiest for you to answer by looking at your bike and all the images showing how BBs flex down and over.

So we are agreed that the entire BB, with crank arms and pedals, flexes.

As the frame unflexes, the pedal moves more than the BB if it is above or below the line that the BB is flexing around. So if the pedal is a 7 o'clock, it is moving more than the center of the BB. It's a lever.

Yes. But does flex result in or assist with crank rotation?

Yes, because it adds to the movement that is already happening.

Does it add rotational force? In what way? Please explain.

Depends what you mean. A 4 o'clock the GCN video shows how the bike would be moved. At 6 o'clock the question doesn't make a lot of sense - you don't pedal a bike by stepping straight down on a static pedal, and you can't use the motion of a static pedal if it isn't allowed to continue to rotate. It sounds like you are trying to "prove" that flex can't help pedaling, but the only way you can do that is to stop the pedaling.

You've yet to explain how the frame flexing back can help with crank rotation. How else can it help to propel the bike?

[kramnnim]

Originally Posted by cachagua

What I think you're calling "the energy that makes it easier for the rider" -- we've talked about it lifting the rider's foot on the pedal -- that energy comes from forces the rider puts on the frame which would never drive the bike forward, no matter how the frame behaves. The example is when the bike's on the trainer, the pedal is at 6 o'clock, and you press down and "sway" the BB to the side. You could do that on the road, instead of on the trainer, and bounce up and down and flex and unflex the frame all day, and never go anywhere.

This is the flex that I believe is wasting energy. And it sure seems like the frame is being flexed like this when out of the saddle, rocking the bike.