Frame Flex on GCN

[Kontact]

Originally Posted by johnniecakes

Me wonders how much unrecoverable energy has been spent on this subject.

The forum should probably be closed so no energy is wasted on bike discussion any more.

[cachagua]

When the sway goes through the center point... either the crank is going to stop your legs or the chain will pull the wheel.

The former is close to what happens. The crank doesn't stop your legs entirely, but retards their motion. As I've mentioned, I'm not talking about the cranks stopping, or your legs' force dropping to zero -- instead of a hiccup, your legs are slowed down at the same gradual rate at which you reduce your pedaling force.

Right pedal, neutral, left pedal, neutral.
Short stays, long stays, short stays, long stays.
Less chain, more chain, less chain, more chain.

Good. Second and fourth in each of those sequences, the strain energy is emptied out of the frame -- however fast or slow it happens, wherever in your pedal stroke, whichever zodiac sign you happen to be in -- and nothing remains which would help the next stroke. You say, "the energy stays in the drivetrain" -- and that's exactly right: it cannot ever get out of the rear wheel to become forward motion.

How much unrecoverable energy has been spent on this subject...

Are you kidding? There is no more thrilling item in the forum just now! The energy may be unrecoverable, but when was the last time a couple of blind men stood around arguing whether an elephant is like a rope, or like a tree, for the entertainment of the onlookers? Uh, since the election, that is.

Nine pages, look at us go!

[Kontact]

Originally Posted by cachagua

Good. Second and fourth in each of those sequences, the strain energy is emptied out of the frame -- however fast or slow it happens, wherever in your pedal stroke, whichever zodiac sign you happen to be in -- and nothing remains which would help the next stroke. You say, "the energy stays in the drivetrain" -- and that's exactly right: it cannot ever get out of the rear wheel to become forward motion.

It has nothing to do with "helping the next stroke". I'm saying that the energy not used in the power stroke that is stored as BB sway is used later in that stroke, not the next one. And that's why I referenced Biopace eariler, which is another way that a drivetrain can delay the transmission of force to the rear wheel while still delivering all of it.

[cachagua]

The energy not used in the power stroke that is stored as BB sway is used later in that stroke, not the next one....

Nope, we've covered that too. That's the lesser-force-can't-overcome-the-greater-one thing. You ease off on the pedals, but the rear wheel doesn't ease off on the inertia.

Try this: if I offer you $5000 for your car, and someone else offers you $10,000, who's gonna end up with your car?

[Kontact]

Originally Posted by cachagua

Nope, we've covered that too. That's the lesser-force-can't-overcome-the-greater-one thing. You ease off on the pedals, but the rear wheel doesn't ease off on the inertia.

Try this: if I offer you $5000 for your car, and someone else offers you $10,000, who's gonna end up with your car?

We've covered this - the mass of the bike is not greater than your ability to pedal. That's why we don't come to a stop on steep climbs just because you pedal through the dead zone.

[cachagua]

I think you know that's not what I'm talking about.

[Kontact]

Originally Posted by cachagua

I think you know that's not what I'm talking about.

Despite you keep saying "we've covered this", I don't know what you're talking about. When you pedal the drivetrain, even a flexy drivetrain, does not feed back upstream. Even in the dead spots, which aren't really dead.

[cachagua]

I don't know what you're talking about.

I think you're right.

And maybe this is the most telling difference between our positions. I know what you're talking about, I just think it's incorrect.

However if I can't get any picture of what I'm thinking about across to you, then our fellow Paceliners will have to look elsewhere for entertainment, won't they? But we've had a good run. I thank you. And I wish you safe passage over those damned trolley tracks.

[kramnnim]

To me, it seems obvious that the energy stored in the flexy frame would simply result in the frame bending back and forth as the force to the pedal relaxes and switches over to the opposite pedal.

[Mark McM]

Originally Posted by kramnnim

To me, it seems obvious that the energy stored in the flexy frame would simply result in the frame bending back and forth as the force to the pedal relaxes and switches over to the opposite pedal.

As pointed out earlier, the energy that goes into frame flex is just a small percentage of the total energy transmitted through the drive train. The frame is highly elastic and has virtually no damping, so the energy is returned in some form when the frame unflexes - but as cachagua has pointed out, there is no mechanism for this energy to be transferred into the drivetrain to directly drive the bike forward. Instead, I think the most likely action is that the small amount of energy stored in the frame during the downward push of the forward leg is returned by giving a little help raising the rear leg on the back of the pedal stroke.

It is also interesting to take note of how the rider applies forces to the pedals. Instrumented pedals have shown that rider typically maintain large downward forces on the pedals all the way to the bottom of the pedal circle, and don't unweight the pedals until the pedal is rising. Here's a typical example of the pedal force vectors:



Energy stored in the frame from the downward pedal force can not be returned until the force decreases. Since this happens at the back of the back of the pedal circle (as the pedal is rising), it can't be transferred into the drivetrain to drive the bike forward. Instead, the action of the returned energy is simply to raise the rear leg. Of course, this is useful in its own right, as it decreases the energy the rider needs to expend to raise their leg.

So in the end, energy expended by the rider to flex the frame on the down stroke may be returned to the rider on the up stroke, so the net energy expended by the rider may be same with or without frame flex. But it is clear that energy stored in the frame does not directly drive the bike forward.

[Kontact]

Originally Posted by cachagua

I think you're right.

And maybe this is the most telling difference between our positions. I know what you're talking about, I just think it's incorrect.

However if I can't get any picture of what I'm thinking about across to you, then our fellow Paceliners will have to look elsewhere for entertainment, won't they? But we've had a good run. I thank you. And I wish you safe passage over those damned trolley tracks.

Don't be insulting. 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.

[Kontact]

Originally Posted by Mark McM

As pointed out earlier, the energy that goes into frame flex is just a small percentage of the total energy transmitted through the drive train. The frame is highly elastic and has virtually no damping, so the energy is returned in some form when the frame unflexes - but as cachagua has pointed out, there is no mechanism for this energy to be transferred into the drivetrain to directly drive the bike forward. Instead, I think the most likely action is that the small amount of energy stored in the frame during the downward push of the forward leg is returned by giving a little help raising the rear leg on the back of the pedal stroke.

It is also interesting to take note of how the rider applies forces to the pedals. Instrumented pedals have shown that rider typically maintain large downward forces on the pedals all the way to the bottom of the pedal circle, and don't unweight the pedals until the pedal is rising. Here's a typical example of the pedal force vectors:



Energy stored in the frame from the downward pedal force can not be returned until the force decreases. Since this happens at the back of the back of the pedal circle (as the pedal is rising), it can't be transferred into the drivetrain to drive the bike forward. Instead, the action of the returned energy is simply to raise the rear leg. Of course, this is useful in its own right, as it decreases the energy the rider needs to expend to raise their leg.

So in the end, energy expended by the rider to flex the frame on the down stroke may be returned to the rider on the up stroke, so the net energy expended by the rider may be same with or without frame flex. But it is clear that energy stored in the frame does not directly drive the bike forward.

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

[dave thompson]

Tranquilós amigos, tranquiló.

[kramnnim]

Originally Posted by Kontact

Don't be insulting. 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.

But it is how it works, as seen in the screenshot from the pedaling efficiency graph from a Pioneer power meter.

[Kontact]

Originally Posted by kramnnim

But it is how it works, as seen in the screenshot from the pedaling efficiency graph from a Pioneer power meter.

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.