Frame Flex on GCN

[cachagua]

See 4:35 to 6:00 in the GCN video...

I didn't want to respond to the video here in our discussion because the ideas in it are embarrassingly ill-thought-out.

Here's what's happening in the video: he pushes the pedal down to flex the frame, using the rear brake to simulate the resistance to acceleration. Then he maintains that pressure on the pedal, while releasing the brake -- and snap, the frame relaxes, and the wheel spins.

This would correspond to your maintaining a perfectly unvarying pressure on the pedals through all 360 degrees, and your bike becoming easier to accelerate for a tiny moment at the bottom of each pedal stroke. But the former, while of course we try, isn't possible, and as for the latter -- I'm afraid Isaac Newton outlawed it a long time ago. The demonstration in the video is not an accurate model of riding your bike.

The bike-and-rider system's resistance to acceleration does not ever decrease. When strain energy comes out of the frame -- when you're actually riding, not dickin' around with a trainer -- there will always be something easier for it to push than you, forward.

[Kontact]

Originally Posted by cachagua

The bike-and-rider system's resistance to acceleration does not ever decrease. When strain energy comes out of the frame -- when you're actually riding, not dickin' around with a trainer -- there will always be something easier for it to push than you, forward.

And that would be what?

[cachagua]

And that would be what?

Two answers to that: first, it doesn't matter -- not to the point I'm making, which is that frame flex does not rebound in a way that contributes to moving you up the road.

But second, let me offer this guess: as the frame releases its strain energy, rather than increasing your forward motion, it slows your pedaling.

Here's how I imagine that happening: when you push on the pedals of a hypothetical 100% efficient bike, the resistance your legs encounter is the inertia of making you go forward. When you push against a real bike, some of your push makes you go, and some flexes the bike. All the time you're pushing, the bike is pushing back -- the energy in its flex wants to come back out, you might say. And when you push a little softer, between the peaks in your output, your legs will go a little shorter distance around the pedal circle as the frame straightens.

What other way could the system behave? Frames flex when you pedal, none of us questions that. When you ease up on the pedals, and the frame springs back, does the energy go to where it couldn't go before, i.e. your inertia? It's still just as hard to move you up the road as it's been all this time. Or instead, does it go toward where it's just become easier for it to go, now that you're not pushing so hard?

That's how I picture things.

[Kontact]

Originally Posted by cachagua

Two answers to that: first, it doesn't matter -- not to the point I'm making, which is that frame flex does not rebound in a way that contributes to moving you up the road.

But second, let me offer this guess: as the frame releases its strain energy, rather than increasing your forward motion, it slows your pedaling.

Here's how I imagine that happening: when you push on the pedals of a hypothetical 100% efficient bike, the resistance your legs encounter is the inertia of making you go forward. When you push against a real bike, some of your push makes you go, and some flexes the bike. All the time you're pushing, the bike is pushing back -- the energy in its flex wants to come back out, you might say. And when you push a little softer, between the peaks in your output, your legs will go a little shorter distance around the pedal circle as the frame straightens.

What other way could the system behave? Frames flex when you pedal, none of us questions that. When you ease up on the pedals, and the frame springs back, does the energy go to where it couldn't go before, i.e. your inertia? It's still just as hard to move you up the road as it's been all this time. Or instead, does it go toward where it's just become easier for it to go, now that you're not pushing so hard?

That's how I picture things.

I don't know why pushing against the bike would a variable thing where your legs do it sometimes but not others. Why would imposing a spring in the transmission of energy cause your legs to work differently than they do when the transmission is rigid?

That sounds very arbitrary. Why would your legs "know" to absorb feedback from stored spring energy, but not absorb feedback from fighting inertia?
Both are feeding back into your legs.

[Kontact]

I think this animation of a handcar might be instructive:



Imagine that the blue drive shaft is a piston with a spring in it. The spring can store energy by compressing or stretching. When Mr. Lefty pushes down the spring shaft compresses, storing energy. When Mr. Righty starts to push down, the motion is reversed, so his effort is pulling the compressed shaft up. Before that motion can stretch the spring, it first gets a free boost from Mr. Left's spring compression, so he has an easier time getting his lever moving. Compression gets traded for stretch, and when it is Lefty's turn again the drive shaft is preloaded to assist the start of his push.

Both men are keeping tension on their levers, so the stored spring energy is never going to just sproing away as loss. As long as the spring is efficient (and real springs are quite efficient), loss of energy from elastic deformation is low. And that small loss may be balance out (or exceeded) by the increased efficiency from a decrease in input amplitude.

[Mark McM]

Originally Posted by Kontact

I think this animation of a handcar might be instructive:



Imagine that the blue drive shaft is a piston with a spring in it. The spring can store energy by compressing or stretching. When Mr. Lefty pushes down the spring shaft compresses, storing energy. When Mr. Righty starts to push down, the motion is reversed, so his effort is pulling the compressed shaft up. Before that motion can stretch the spring, it first gets a free boost from Mr. Left's spring compression, so he has an easier time getting his lever moving. Compression gets traded for stretch, and when it is Lefty's turn again the drive shaft is preloaded to assist the start of his push.

Both men are keeping tension on their levers, so the stored spring energy is never going to just sproing away as loss. As long as the spring is efficient (and real springs are quite efficient), loss of energy from elastic deformation is low. And that small loss may be balance out (or exceeded) by the increased efficiency from a decrease in input amplitude.

Ah, that explains why all reciprocating crank drive systems (such as internal combustion piston engines) use elastic connecting rods!

Oh wait ... they don't.

If the connecting compresses when Mr. Lefty pushes down, Mr. Left has to exert more energy, because he has to push the lever further than if the rod were rigid (the extra energy is stored in the rod). Mr. Righty might get a little bit boost as the rod rebounds, but then he has to re-exert the same amount of energy to stretch the rod as he pushes down. In the end, there is no net energy gained or lost as power is transmitted through the crank. But there very well might be a loss as power is applied to the lever, since both Mr. Lefty and Mr. Righty have to move the lever further in both directions.

As shown in the spring drive crank above, there have been several attempts to add 'energy storage' into the drive system. All have failed to produce more efficient drive systems.

[andrewsuzuki]

Finally found an archive to the site describing how the strain energy is stored and returned to the drivetrain, with a finite element analysis.

https://web.archive.org/web/20060214.../Frameflex.htm

Additionally a graph of power going in and out of the frame

https://web.archive.org/web/20060214...wer_output.htm

[fa63]

From the study:

"Conclusion:

Having concluded that frame flex does not waste energy, I do not believe that frame stiffness is irrelevant. You could say that a stiff frame feels more responsive. A stiffer frame can give the rider more confidence especially in a sprint."

[William]

When I started racing I was on a frame that flexed quite a bit under pressure in the standing sprint, on an uphill sprint I could actually get it to ghost shift when really laying down power. That said I won a lot of races on that bike and honestly, I don't think the flex really added power directly back to the drive train. What I think was happening was the flex actually caused it to "snap" back fractions faster than stiffer frames I was on later. That would get the frame back faster for the top of the stroke on the other side and vice versa. Translating into a faster sprint rhythm and quicker pedal stroke....harmonics. Probably only fractions of a second on each side but that adds up over the length of the sprint.

I've often wondered why subsequent frames, production and custom didn't quite have the feel of that frame in the sprint, but then I was under the impression that I needed to go stiffer due to my height and strength. Too much flex isn't good either though...

Sprints...
https://www.youtube.com/watch?v=nqWANNRlqpo

Science of the Sprint (sharing for the overhead of the sprint bike throw from side to side)...
https://www.youtube.com/watch?v=SIruLOp-PrM


Keep in mind I'm not a scientist and I don't play one on TV either.






William

[Kontact]

Originally Posted by Mark McM

Ah, that explains why all reciprocating crank drive systems (such as internal combustion piston engines) use elastic connecting rods!

Oh wait ... they don't.

If the connecting compresses when Mr. Lefty pushes down, Mr. Left has to exert more energy, because he has to push the lever further than if the rod were rigid (the extra energy is stored in the rod). Mr. Righty might get a little bit boost as the rod rebounds, but then he has to re-exert the same amount of energy to stretch the rod as he pushes down. In the end, there is no net energy gained or lost as power is transmitted through the crank. But there very well might be a loss as power is applied to the lever, since both Mr. Lefty and Mr. Righty have to move the lever further in both directions.

As shown in the spring drive crank above, there have been several attempts to add 'energy storage' into the drive system. All have failed to produce more efficient drive systems.

There is no net energy loss. Correct.

How are Lefty and Righty going to move the crank further in both directions? The crank is a fixed length.

But on a bicycle with BB flex, the load path of the crank arm isn't a perfect circle because the flex will make it slightly ovalized.


No one is saying springy-ness adds any energy to the system. The point is that sprung parts don't actually give energy away - it is still trapped in the drive train between the motor and the road, and it can't go out side that or back upstream.

[Mark McM]

Originally Posted by Kontact

There is no net energy loss. Correct.

No net energy loss in the connecting rod or crank. But due to the extra travel of the lever, there may be extra energy loss at the lever.

Originally Posted by Kontact

How are Lefty and Righty going to move the crank further in both directions? The crank is a fixed length.

I didn't say the crank moves further, I said the lever (operated by Messrs. Lefty and Righty) moves further. Since the lever moves the connecting rod, if the connecting rod contracts/expands, the lever will have to move an extra distance corresponding to the change in length of the connecting rod. And that means that Messrs. Lefty and Right have to move their arms further to move the lever further, likely requiring them to expend more energy.

Originally Posted by Kontact

No one is saying springy-ness adds any energy to the system. The point is that sprung parts don't actually give energy away - it is still trapped in the drive train between the motor and the road, and it can't go out side that or back upstream.

There may not be energy lost in the cranks/frame, but if the rider has move their legs through a longer distance, there may be energy lost in the rider's legs.

[Kontact]

Originally Posted by Mark McM

No net energy loss in the connecting rod or crank. But due to the extra travel of the lever, there may be extra energy loss at the lever.


There may not be energy lost in the cranks/frame, but if the rider has move their legs through a longer distance, there may be energy lost in the rider's legs.

Why does more distance become more energy? Are you saying long crank arms waste energy somehow?

[Black Dog]

I think part of the issue that we are missing here is the energy expended by a rider that does not generate muscle contraction and limb motion. Think of pushing against a wall with as much force as you can generate. Since the wall does not move there is no work being done. However, you are burning calories and exhausting muscles. This is why we have gears on bikes so that we can generate movement when we apply force to the peddles under a wide range of conditions (wind, hills etc..). To illustrate: try starting from a dead stop on a hill in a 53-11.

If a frame has some flex and the energy is returned then it is less likely that a rider would be generating none motive force and thus would be more efficient.

[Kontact]

Originally Posted by Black Dog

I think part of the issue that we are missing here is the energy expended by a rider that does not generate muscle contraction and limb motion. Think of pushing against a wall with as much force as you can generate. Since the wall does not move there is no work being done. However, you are burning calories and exhausting muscles. This is why we have gears on bikes so that we can generate movement when we apply force to the peddles under a wide range of conditions (wind, hills etc..). To illustrate: try starting from a dead stop on a hill in a 53-11.

If a frame has some flex and the energy is returned then it is less likely that a rider would be generating none motive force and thus would be more efficient.

I honestly don't understand any of this, and I think your last sentence must be missing some words. But if you push against a wall your muscles are definitely contracting.

[cachagua]

Both men are keeping tension on their levers...

If in that diagram the men are maintaining unvarying pressure on their levers, that's exactly what doesn't happen on a bike. If it did, we'd be having a very different conversation, but it does not -- I don't think you're seriously trying to assert that.

Here's another way of thinking about it. Remember that video? The rider pushes the pedal down onto the block, the frame flexes, he lets the brake loose and the energy stored in the frame's flex turns the wheel.

Again, for clarity: the release of the frame flex turns the rear wheel when what's been resisting its rotation stops resisting, and the thing that's been pushing it -- the rider's foot on the pedal -- keeps pushing just as hard.

But this is not what happens when you ride. What happens when you ride would be illustrated by the rider's pushing down the pedal, flexing the frame, and then NOT releasing the brake but instead easing up on the pedal. Bike + rider's inertia stays the same, pedaling force decreases.

What would happen then? The pedal would rise back up to where it started, pushed by the strain energy being released from the frame.

Compression gets traded for stretch...

In other words, after the frame flexes from one leg's pedal stroke, that flex, when released, adds its energy to the next stroke? I don't think it does.

Let's look at the bike on the trainer again. You keep the brake squeezed, but ease up on the cranks, the right pedal rises up from the block -- but at the same time, the left pedal also makes a counter-rotating motion. The release of the frame flex does not advance the other pedal but moves it backwards, and thus doesn't increase the force of the next stroke, but decreases it, too.