Frame Flex on GCN

[Kontact]

Originally Posted by cachagua

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.



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.

At no point when you are pedaling do you remove all tension from the pedals. If you believe something different something different there's nothing to discuss.

[Black Dog]

Originally Posted by Kontact

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.

Ok. Let me try to be more articulate. I am not saying that muscles are not contracting, they are. I am saying that the contractions are not generating movement of the limbs because they can not overcome the resistance. I am speculating that a frame with some flex could absorb some of the force that would otherwise be wasted not turning the cranks due to excessive resistance and then return it to the system. This could also reduce peak muscle loads and reduce fatigue. I hope this is clearer than my previous post.

[Kontact]

Originally Posted by Black Dog

Ok. Let me try to be more articulate. I am not saying that muscles are not contracting, they are. I am saying that the contractions are not generating movement of the limbs because they can not overcome the resistance. I am speculating that a frame with some flex could absorb some of the force that would otherwise be wasted not turning the cranks due to excessive resistance and then return it to the system. This could also reduce peak muscle loads and reduce fatigue. I hope this is clearer than my previous post.

I agree. The flex reduces the peaks and delivers that force later.

[cachagua]

At no point when you are pedaling do you remove all tension from the pedals...

Of course not. You don't have to reduce your pedaling force to zero, you just have to reduce it, for what I describe to happen. When you're putting forth maximum force, it's equal to the riding inertia you're overcoming plus the frame flex, and when you back off even the tiniest bit the frame flex springs back correspondingly. Back to the bike on the trainer: if 150 pounds will push the pedal down onto the block, then 75 would let the pedal back up part way -- but still would not turn the back wheel, if the brake remained locked.

The flex reduces the peaks and delivers that force later.

Yes, please say more. Reduces the peaks by how much and in what way, and delivers the force where and how?

[Kontact]

Originally Posted by cachagua

Of course not. You don't have to reduce your pedaling force to zero, you just have to reduce it, for what I describe to happen. When you're putting forth maximum force, it's equal to the riding inertia you're overcoming plus the frame flex, and when you back off even the tiniest bit the frame flex springs back correspondingly. Back to the bike on the trainer: if 150 pounds will push the pedal down onto the block, then 75 would let the pedal back up part way -- but still would not turn the back wheel, if the brake remained locked.



Yes, please say more. Reduces the peaks by how much and in what way, and delivers the force where and how?

In much the same way that Biopace does - by diverting some of the force into storage then expending it later in the pedal stroke. A small part of peak force gets turned into the frame flexing, and when the shift is made to the opposite crank arm, it is expended as forward motion of the bicycle as the chainstays extend back to normal length.

[msl819]

This may have been asked but if the bike in the video didn’t have a chain would you get the same results of rear tire spin? Is the energy being stored in the frame and returned through the rear wheel or is the chain being flexed under pressure and once the rear brake is released it turns the wheel over.

[Kontact]

Originally Posted by msl819

This may have been asked but if the bike in the video didn’t have a chain would you get the same results of rear tire spin? Is the energy being stored in the frame and returned through the rear wheel or is the chain being flexed under pressure and once the rear brake is released it turns the wheel over.

The resistance of stepping down on the pedal is from the chain in tension. No chain, no resistance. No chain, nothing to pull the wheel when the brake is released.

[martl]

I see it that way: it only recently has been widely acknowledged that "frame response" actually plays a part in how a bike *feels*. Whether it plays a part in how the bike *performs* and if so, in what way, is still very much up to debate - as this thread shows.

So, the GCN vid is to be welcomed as a contribution to the debate. A definite truth it won't and can't tell.

[msl819]

Originally Posted by Kontact

The resistance of stepping down on the pedal is from the chain in tension. No chain, no resistance. No chain, nothing to pull the wheel when the brake is released.

Right, what I am asking is if the energy that is stored and transferred is more in the chain or frame. Obviously, no frame flex no stored energy in the chain and vice versa. But when we are loading our frame in a real world situation and the rear wheel isn’t locked to build the tension do the physics function in the same way given different variables. Maybe the question I am asking is does the chain function the same way when it isn’t held in tension with the rear brake as it does in the video and if not does that alter the experiment?

[andrewsuzuki]

Originally Posted by msl819

what I am asking is if the energy that is stored and transferred is more in the chain or frame

The FEA suggests ~1% of rider power goes towards flexing the frame (a steel frame of decent quality). Not sure about chain elasticity however.

Originally Posted by martl

I see it that way: it only recently has been widely acknowledged that "frame response" actually plays a part in how a bike *feels*.

Well, people looking for performance steel bikes have long known to look at tubing specs. Reynolds 531? must be a good bike! Triple-butted? must be a good bike! Whether or not they actually reasoned out why 531 or triple-butted tubes tend to make a good bike.

[Kontact]

Originally Posted by msl819

Right, what I am asking is if the energy that is stored and transferred is more in the chain or frame. Obviously, no frame flex no stored energy in the chain and vice versa. But when we are loading our frame in a real world situation and the rear wheel isn’t locked to build the tension do the physics function in the same way given different variables. Maybe the question I am asking is does the chain function the same way when it isn’t held in tension with the rear brake as it does in the video and if not does that alter the experiment?

While everything stretches or compresses to an extent, the tension shown in the video is from the frame changing shape - specifically it is twisting. Does that frame twist that much while riding? Yes, but mainly when sprinting or climbing. But we can easily see frames twisting under load, and that twisting is going to decrease the distance from the rear cog to the chainring.

[Mark McM]

Originally Posted by Kontact

In much the same way that Biopace does - by diverting some of the force into storage then expending it later in the pedal stroke. A small part of peak force gets turned into the frame flexing, and when the shift is made to the opposite crank arm, it is expended as forward motion of the bicycle as the chainstays extend back to normal length.

While there may be some energy storage/return in the frame, the two mechanism described above don't work in that regard:

1) Biopace chainrings don't store energy. They merely change effective leverage ratio (gear ratio) of the drivetrain.

2) Chainstays are far too stiff in tension/compression to store any meaningful energy in compression, so their is essentially no energy "expended as forward motion of the bicycle as the chainstays extend back to normal length". Instead, the primary mode of chainstay flex is bending/torsion. This produces a frame deflections (and deflections at the crank) which are orthogonal to the direction of chain travel. This is the root of the whole quandary about how lateral/torsional elastic deflection can result in forward motion of the bicycle.

[Kontact]

Originally Posted by Mark McM

While there may be some energy storage/return in the frame, the two mechanism described above don't work in that regard:

1) Biopace chainrings don't store energy. They merely change effective leverage ratio (gear ratio) of the drivetrain.

2) Chainstays are far too stiff in tension/compression to store any meaningful energy in compression, so their is essentially no energy "expended as forward motion of the bicycle as the chainstays extend back to normal length". Instead, the primary mode of chainstay flex is bending/torsion. This produces a frame deflections (and deflections at the crank) which are orthogonal to the direction of chain travel. This is the root of the whole quandary about how lateral/torsional elastic deflection can result in forward motion of the bicycle.

Biopace rings "store" energy by keeping crank speed higher going into the dead zone in exchange for lower input force at 3 o'clock.


I didn't say the chainstay is compressing. I said that the twisting of the chainstays compress the chainstay distance. This is just like what a coil spring does - the change in deflection of the coils 'compresses' the overall length of the spring.

[Mark McM]

Originally Posted by Kontact

Biopace rings "store" energy by keeping crank speed higher going into the dead zone in exchange for lower input force at 3 o'clock.

If anything, just the opposite occurs. The asymmetry of Biopace chainrings is orthogonal to most non-round chainrings, and the cranks slow down during the "dead zone", and speed up during the power stroke.

Originally Posted by Kontact

I didn't say the chainstay is compressing. I said that the twisting of the chainstays compress the chainstay distance. This is just like what a coil spring does - the change in deflection of the coils 'compresses' the overall length of the spring.

Chainstays are not anything like a coil spring. Chainstays are not coiled. Chainstays are oriented in-line with the drive force whereas in a coil spring (or any other torsion spring) the flexural member is oriented orthogonally to the force. If chainstays are suppose to act like coil springs, they are completely mis-oriented.

[Kontact]

Originally Posted by Mark McM

If anything, just the opposite occurs. The asymmetry of Biopace chainrings is orthogonal to most non-round chainrings, and the cranks slow down during the "dead zone", and speed up during the power stroke.

Correct. They speed up in the power stroke because less energy is tapped off to make bicycle velocity, and they slow down in the dead zone as that 'stored' crank velocity is used up making power later. You are not disagreeing with me.

Chainstays are not anything like a coil spring. Chainstays are not coiled. Chainstays are oriented in-line with the drive force whereas in a coil spring (or any other torsion spring) the flexural member is oriented orthogonally to the force. If chainstays are suppose to act like coil springs, they are completely mis-oriented.

They aren't "supposed to" act like anything but structural members, but we are talking about what they actually do on bicycles, and what they do is twist around their shared centerline. And when they twist, the distance between the BB and the rear hub decreases. When the energy used to twist them is released, the rear center elongates. This elongation could be said to push the bike forward from the rear hub, or it could be said to pull the chain forward, driving the wheel. These aren't actually any different - just "feet push down on the earth, the earth pushes up on your feet" kind of semantics.

Each individual chainstay is not compressing, but the 'rear center' assembly is compressing. And I used a coil spring as a comparison because the wire composing the spring does not compress, but the spring as a whole does because the wires flex. This is all just an explanation for something that was implied by MSL819's question - that the chain might stretch or the individual chainstay might compress. The chainstays change shape, which decreases their length like a bow gets shorter when you pull the string.