Frame Flex on GCN

[cachagua]

If there is differential in downward force at the pedals, there will be torque applied to the BB, and the frame will be flexed...

We've got a couple of different torques here that we shouldn't confuse. One's the swinging of the BB that we'd see looking fore-to-aft along the bike, and the other's the rotation of the crank. The BB swinging isn't going to contribute to the bike's forward motion, but of course the rotation of the crank is exactly what makes the bike go forward.

However I think it there is another flex mode, not the BB swinging, that does effectively "shorten" the stays. The video we started with demonstrates it. If there were no strain released in that mode when the rider lets go of the rear brake -- no "re-lengthening" of the effective stay length -- then the wheel wouldn't move.

Would anyone like to try this at home? Duplicate the set-up in the video and do exactly the same thing, only let the rear brake off very gradually, so the wheel doesn't spin freely but just rotates a tiny, tiny bit when the frame relaxes. A degree? Two? It would be very little. But that's going to correspond to what we're calling "compression" of the stays.

[Kontact]

Originally Posted by Mark McM

Nope, you're wrong again. Let's consider one crank revolution, starting with the leg at the top of the stroke: As the rider pushes down, the frame flexes. Some of the rider's energy goes directly into the drivetrain, and some goes into the frame in the form of strain energy. At the bottom of the pedal stroke, the rider still exerts a significant downward force on the pedal, so the frame is still flexed. As the rider continues the crank rotation, he has to expend energy to raise his leg. But as the frame un-flexes, the energy released by the frame helps the rider to raise his leg, so the rider's net energy is the same as if the frame had not flexed. But none of the strain energy went into the drivetrain - all the energy that went into the drivetrain occured during the downstroke. If a rider began from a standing start and only made one crank revolution, then it is very clear that the strain energy restoring the rider's leg position made no contribution to drive the bicycle. And it is made particularly clear if there is a time delay between the rider pushing all the way to the bottom of the stroke, and re-starting the revolution to bring their leg back up.

The rider's legs, the way you are describing it, are part of the drivetrain.

No one is claiming that there is extra energy coming out of a flexy frame. I'm claiming that there is no real loss because the flex is simply used later in the pedal stroke, which is what the article you posted claims and it also seems to be what you are claiming, too.

The only conflict appears to be that you believe there is a difference between "helps the rider to raise the leg" and any other pro-forward motion input. I used the stays to illustrate another way stored energy could contribute to forward motion, but I don't think there is any real difference between "helps push the bike" or "helps turn the pedals" - those are the same thing. You are "pushing against the foot" and I'm saying they are the same thing.



But the author, you and I are all taking a very different stance from that typified by cachagua who believes that the release of strain energy drags the riders leg against pedaling motion, causing an anti-forward motion input, slowing the bike overall.

[cachagua]

And does that released strain energy, the energy that rotates the rear wheel in the video, contribute to your going forward, or does it not? That, I think, is our original, central question.

And we've been getting rather far afield.

[cachagua]

The author, you and I are all taking a very different stance from that typified by cachagua who believes that the release of strain energy drags the riders leg against pedaling motion, causing an anti-forward motion input, slowing the bike overall.

Yes thank you, exactly right, except that "slowing the bike" perhaps overemphasizes the effect I'm envisioning -- I'd call it "doesn't contribute to your going forward", which physics-wise is about the same thing, but more accurately reflects our experience of riding.

[Kontact]

Originally Posted by cachagua

We've got a couple of different torques here that we shouldn't confuse. One's the swinging of the BB that we'd see looking fore-to-aft along the bike, and the other's the rotation of the crank. The BB swinging isn't going to contribute to the bike's forward motion, but of course the rotation of the crank is exactly what makes the bike go forward.

However I think it there is another flex mode, not the BB swinging, that does effectively "shorten" the stays. The video we started with demonstrates it. If there were no strain released in that mode when the rider lets go of the rear brake -- no "re-lengthening" of the effective stay length -- then the wheel wouldn't move.

Would anyone like to try this at home? Duplicate the set-up in the video and do exactly the same thing, only let the rear brake off very gradually, so the wheel doesn't spin freely but just rotates a tiny, tiny bit when the frame relaxes. A degree? Two? It would be very little. But that's going to correspond to what we're calling "compression" of the stays.

The flex in the video comes from the tension between the pedal going down and the chain pulling back. The frame can't compress to the rear, so it ends up flexing into the lateral plane. And that seems to be what makes people's heads explode - this idea that flex energy could be stored outside the plane that we are comfortable thinking about.


If you let the brake out slowly the wheel will move. The fact that they let the wheel move enough to cause it to coast isn't really different than letting it move the amount of distance dictated by distance of the flex. Those are just two different illustrations of the same thing:

If you release the brake all at once, the resulting velocity of the wheel is a measure of released energy - the energy it took to accelerate the wheel from zero to whatever rotational speed it achieves. That demonstrates a work per second rate - Watts.

If you release the brake slowly, you are releasing a lot of that energy into the brake pads, so what you are demonstrating is no longer a transfer of Watts, but a transfer of work without reference to time - Joules. The drivetrain flexing will provide enough work to move the wheel from A to B. But since we took time out of it we are no longer talking about energy the way cyclists normally do - in Watts.


So you can demonstrate whichever you want, but they are demonstrations of different ways of understanding energy, not two separate things.

[Mark McM]

Originally Posted by Kontact

The rider's legs, the way you are describing it, are part of the drivetrain.

No one is claiming that there is extra energy coming out of a flexy frame. I'm claiming that there is no real loss because the flex is simply used later in the pedal stroke, which is what the article you posted claims and it also seems to be what you are claiming, too.

The only conflict appears to be that you believe there is a difference between "helps the rider to raise the leg" and any other pro-forward motion input. I used the stays to illustrate another way stored energy could contribute to forward motion, but I don't think there is any real difference between "helps push the bike" or "helps turn the pedals" - those are the same thing. You are "pushing against the foot" and I'm saying they are the same thing.

I don't think most people would define the rider (or their legs) as part of the drivetrain. I think most people would see it as the "engine". So most would separately classify the energy paths through them. But putting aside the definitions of energy paths, in the end there appears to be no net loss of energy in frame flex - the strain energy transferred into the frame is not lost, but is returned in a useful way (through one mechanism or another). Tests and rider performances appear to bear this out.

However .. I think the mechanism of the energy return actually is important at least in one regard. As we've been discussing, the changing force vectors on the pedals through the crank revolution will not only affect how much strain energy goes into the frame, but also at what points in the crank rotation that it goes in and comes back out. This in turn affects the mechanisms by which the returned energy is utilized. The reason this is important is because riders often vary their pedaling styles to match different situations (for example, I don't think anyone will claim that they pedal the same way between standing or seated, or between steady continuous effort and short full-power sprints). I think it is highly likely that riders will vary there pedaling styles (if only subtly) in response to frame stiffness, and with practice will adapt their pedaling to optimize their pedal motions/forces with the frame response.

[kramnnim]

I don't think the original GCN video is an accurate representation of what happens while riding because the trainer is flexing a lot.

But, even if it was accurate- yes, wheel will be propelled when the brake is released if the crank remains at 3 oclock.

I don't think the tension is released until around 6 oclock.

So to see what the effect would be, you would have to flex the frame+trainer at 3 oclock, hold it there while you allow the crank to move down to 6 oclock, and then release. I don't think the wheel would move nearly as much, If at all.

[Kontact]

Originally Posted by Mark McM

I don't think most people would define the rider (or their legs) as part of the drivetrain. I think most people would see it as the "engine". So most would separately classify the energy paths through them. But putting aside the definitions of energy paths, in the end there appears to be no net loss of energy in frame flex - the strain energy transferred into the frame is not lost, but is returned in a useful way (through one mechanism or another). Tests and rider performances appear to bear this out.

However .. I think the mechanism of the energy return actually is important at least in one regard. As we've been discussing, the changing force vectors on the pedals through the crank revolution will not only affect how much strain energy goes into the frame, but also at what points in the crank rotation that it goes in and comes back out. This in turn affects the mechanisms by which the returned energy is utilized. The reason this is important is because riders often vary their pedaling styles to match different situations (for example, I don't think anyone will claim that they pedal the same way between standing or seated, or between steady continuous effort and short full-power sprints). I think it is highly likely that riders will vary there pedaling styles (if only subtly) in response to frame stiffness, and with practice will adapt their pedaling to optimize their pedal motions/forces with the frame response.

I don't think a anyone who broke a driveshaft from too much torque on a muscle car would consider the motor separate from the drivetrain when looking for causes. If you are examining how power is distributed from leg muscles to road it is a bit arbitrary to say that flesh doesn't count.


As to adapting to how the energy goes in and comes out, that sounds like something your brain figures out in about 4 pedal strokes and is little different than the learning curve of walking with new shoes. Why would it be important?


It seems like the only important thing that these discussions really have to offer is convincing people that, despite how it might feel, you aren't actually throwing away energy if your frame flexes.

[Kontact]

Originally Posted by kramnnim

I don't think the original GCN video is an accurate representation of what happens while riding because the trainer is flexing a lot.

But, even if it was accurate- yes, wheel will be propelled when the brake is released if the crank remains at 3 oclock.

I don't think the tension is released until around 6 oclock.

So to see what the effect would be, you would have to flex the frame+trainer at 3 oclock, hold it there while you allow the crank to move down to 6 oclock, and then release. I don't think the wheel would move nearly as much, If at all.

The GCN thing isn't supposed to be an accurate model of riding. It is just supposed to show that lateral displacement of the BB is still "in line" with power transmission. It is a separate question how and when that energy gets to the rear wheel during actual pedaling.

[cachagua]

If you release the brake all at once, the resulting velocity of the wheel is a measure of released energy...

If you release the brake slowly, what you are demonstrating is a transfer of work without reference to time...

So you can demonstrate whichever you want, but they are demonstrations of different ways of understanding energy, not two separate things.

Could you go over what you thought I was trying to say? I'm not quite able to read the above as a response. Or if it's a response to something else -- my mistake.

It seems like the only important thing that these discussions really have to offer is convincing people that, despite how it might feel, you aren't actually throwing away energy if your frame flexes.

Naw, I think there's some other value besides that to this conversation -- coupla things, actually.

[Kontact]

Originally Posted by cachagua

Could you go over what you thought I was trying to say? I'm not quite able to read the above as a response. Or if it's a response to something else -- my mistake.

No, it was a response to your suggestion of letting off the brake very slowly. Both demonstrate the same trapped energy, but in different ways. I was illustrating why your suggestion is different than the GCN way - because it subtracts time so it doesn't show Watts of energy, just Joules of work.

Your suggestion is like the difference between slowly carrying a book upstairs and throwing it.

[cachagua]

No, it was a response to your suggestion of letting off the brake very slowly. Both demonstrate the same trapped energy, but in different ways. I was illustrating why your suggestion is different than the GCN way - because it subtracts time so it doesn't show Watts of energy, just Joules of work.

Your suggestion is like the difference between slowly carrying a book upstairs and throwing it.

I guess so, thanks. What I hoped to throw some light on is that there is a component of frame flex that puts the rear axle closer to the BB (since there had been some divergence of opinion on that). The distinction between letting the released strain energy spin the wheel, and using the brakes to keep it from freewheeling and only rotate as far as the chain actually pulls it, is (for my purpose) only useful to show how little the wheel rotates in direct response to the un-flexing.

And I wanted to highlight that because I think that's the component of flex, or the main one, that we've been trying to look at.

I'm totally curious about this but I don't have a trainer, or else I'd do the experiment myself. I'll bet the wheel moves just a degree or two at most. I could be wrong but I think it must be the tiniest, tiniest bit. But some. Definitely some.

[Kontact]

Originally Posted by cachagua

I guess so, thanks. What I hoped to throw some light on is that there is a component of frame flex that puts the rear axle closer to the BB (since there had been some divergence of opinion on that). The distinction between letting the released strain energy spin the wheel, and using the brakes to keep it from freewheeling and only rotate as far as the chain actually pulls it, is (for my purpose) only useful to show how little the wheel rotates in direct response to the un-flexing.

And I wanted to highlight that because I think that's the component of flex, or the main one, that we've been trying to look at.

I'm totally curious about this but I don't have a trainer, or else I'd do the experiment myself. I'll bet the wheel moves just a degree or two at most. I could be wrong but I think it must be the tiniest, tiniest bit. But some. Definitely some.

It isn't a great deal of energy. But if you told a Strava guy you were going to take away that amount of energy on every pedal stroke he'd blow a gasket.

[cachagua]

YAAAAH-ha-ha-ha-ha-ha! I would never, ever breathe a word of this to a Strava guy.

[cachagua]

There appears to be no net loss of energy in frame flex - the strain energy transferred into the frame is not lost, but is returned in a useful way (through one mechanism or another)...

Can you elaborate? Which mechanism returns strain energy in a useful way? I'm still waiting for an explanation of how a lesser force can overcome a greater one. Or if you're thinking about the release of BB sway "lifting" the pedal on its upward stroke -- that gets the pedal farther off the ground but I don't think it rotates the cranks forward. It tips the cranks back towards "vertical" (with respect to the bike) but it doesn't turn them.

Tests and rider performances appear to bear this out.

Not sure what you mean here. The tests we've been looking at, and rider performance (if we believe the riders) couldn't be more contradictory. The video and the FEA both claim you get all your pedaling energy back out of the frame (note however that they're both based on the same incorrect assumption) but riders generally agree that a stiffer frame wastes less energy, i.e. that frame flex is lost energy.

The changing force vectors on the pedals through the crank revolution will not only affect how much strain energy goes into the frame, but also at what points in the crank rotation that it goes in and comes back out. This in turn affects the mechanisms by which the returned energy is utilized. The reason this is important is because...

Now, you got to stop talking like that. Baffling bulls**t is what I do, it's my specialty and my trademark, and if you start sounding like that I call plagiarism!

I think it is highly likely that riders will vary their pedaling styles (if only subtly) in response to frame stiffness, and with practice will adapt their pedaling to optimize their pedal motions/forces with the frame response.

I guess you mean, in response to the different stiffnesses of different frames? Interesting... might that mean that stiffer frames of recent times aren't really faster than old ones, but today's riders think they are because they don't know how to ride the old ones? That's actually kind of tasty... but again, we'd better make sure not to say it to Strava-guy.