Frame Flex on GCN

[Kontact]

Originally Posted by kramnnim

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



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



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



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

The pedal is at 7 o'clock. It is moving up and back at a 30° angle in the plane of the crank. At the same time the bottom of the crank swings outboard and up. If the pedal is moving up from crank rotation and up from BB rotation, those two are additive, increasing the upward movement of the pedal.

[Mark McM]

Originally Posted by kramnnim

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.

This is just anecdote but ...

I have a 2002 Litespeed Ghisallo titanium frame. At the time, this was advertised as the worlds lightest production frame. At 865 grams, it is still very light today. (For comparison, the Moots RSL titanium frame, their lightest frame, is advertised at 1165 grams, or 35% more). Not coincidentally the Ghisallo is also one of the flexiest production frames ever made. It very visibly flexes not just under pedal force, but also even under rapid steering maneuvers. The frame tubes are so thin, that you can see and feel them compress when you squeeze them with your fingers. I have several other frames, but needless to say the Ghisallo is the flaxiest.

Riding up steep hills, I climb measurably faster on the Ghisallo than on my other bikes*. This bike happens to be built with a variety of other ultralight (and flexy) parts, so it weighs about 5 lb. less than my other bikes, and reduces the total weight of bike + rider by about 3%. Also not coincidentally, 3% is also about how much faster I climb on this bike.

If this super flexy frame is wasting energy, it isn't by enough to matter.


(My bike club has a weekly hill training ride which I attend regularly, and experience shows I consistently perform better on the Ghisallo than when I bring other bikes.)

[kramnnim]

Originally Posted by Kontact

The pedal is at 7 o'clock. It is moving up and back at a 30° angle in the plane of the crank. At the same time the bottom of the crank swings outboard and up. If the pedal is moving up from crank rotation and up from BB rotation, those two are additive, increasing the upward movement of the pedal.

Upward movement of the pedal does not equal crank rotation. The pedal is moved up because the crank arm is moved up because the BB shell is moved up. No rotational force is being added.

[cachagua]

Upward movement of the pedal does not equal crank rotation.

Right, that's part of what I was trying to separate up there with the two modes of frame flex. The upward movement of the pedal is indeed the sum of the crank rotation and the BB swing, but they don't sum to more crank rotation, only more upward pedal movement.

[cachagua]

(This is parenthetical to the current discussion, but I think it's where the current discussion will eventually end up:

Originally Posted by Mark McM

I have a 2002 Litespeed Ghisallo titanium frame. If this super flexy frame is wasting energy, it isn't by enough to matter...

That illustrates perfectly what I think is the real bottom-line result of our original question: if frames waste energy, as I believe they do, it's nevertheless such a small amount that it doesn't matter -- and therefore, stiffer frames aren't advantageous.)

[kramnnim]

Originally Posted by Mark McM

This is just anecdote but ...

I have a 2002 Litespeed Ghisallo titanium frame. At the time, this was advertised as the worlds lightest production frame. At 865 grams, it is still very light today. (For comparison, the Moots RSL titanium frame, their lightest frame, is advertised at 1165 grams, or 35% more). Not coincidentally the Ghisallo is also one of the flexiest production frames ever made. It very visibly flexes not just under pedal force, but also even under rapid steering maneuvers. The frame tubes are so thin, that you can see and feel them compress when you squeeze them with your fingers. I have several other frames, but needless to say the Ghisallo is the flaxiest.

Riding up steep hills, I climb measurably faster on the Ghisallo than on my other bikes*. This bike happens to be built with a variety of other ultralight (and flexy) parts, so it weighs about 5 lb. less than my other bikes, and reduces the total weight of bike + rider by about 3%. Also not coincidentally, 3% is also about how much faster I climb on this bike.

If this super flexy frame is wasting energy, it isn't by enough to matter.


(My bike club has a weekly hill training ride which I attend regularly, and experience shows I consistently perform better on the Ghisallo than when I bring other bikes.)

Thanks for this...I have been wanting to buy a Ghisallo for several years. I've ignored the "it's a noodle" claims, but it sounds like it may not be a good fit for me...

Curious what your times would be on a stiff carbon frame of equal weight.

[Kontact]

Originally Posted by kramnnim

Upward movement of the pedal does not equal crank rotation. The pedal is moved up because the crank arm is moved up because the BB shell is moved up. No rotational force is being added.

Any movement added in a net pro-rotation direction contributes to rotational force.

If not, the piston action of our legs isn't adding rotational force, either. It is a simple vector problem.

[kramnnim]

My complaint with my flexiest frame is that it feels sluggish when taking off from a stoplight. It is vertically compliant and is great over rough roads, but feels dead compared to other frames I have. On the other hand, the Cervelo R5 I had was very stiff, felt like a rocket. But it was too harsh and beat me up.

The Ghisallo might be the same weight as some of my carbon frames, but is less stiff. Would love to compare them. Maybe I'd get used to the flex...

[kramnnim]

Originally Posted by Kontact

Any movement added in a net pro-rotation direction contributes to rotational force.

If not, the piston action of our legs isn't adding rotational force, either. It is a simple vector problem.

Disagree. The piston action of your leg is rotating the crank arm around the fulcrum point. The upward flex of the BB shell is moving the fulcrum point.

[cachagua]

Originally Posted by kramnnim

The piston action of your leg is rotating the crank arm around the fulcrum point. The upward flex of the BB shell is moving the fulcrum point.

Nice, that's exactly the distinction. Wish I'd found as elegant a way of putting it.

[Kontact]

Originally Posted by kramnnim

Disagree. The piston action of your leg is rotating the crank arm around the fulcrum point. The upward flex of the BB shell is moving the fulcrum point.

And in moving the fulcrum moves the pedal. If there was no chain the pedal wouldn't be moved, but since the chain prevents the crank from backpedaling, the net movement raises the pedal.

[kramnnim]

Originally Posted by Kontact

And in moving the fulcrum moves the pedal. If there was no chain the pedal wouldn't be moved, but since the chain prevents the crank from backpedaling, the net movement raises the pedal.

What? How does the chain prevent backpedaling?

And pedal movement is still not the same as crank rotation.

[Kontact]

Originally Posted by kramnnim

What? How does the chain prevent backpedaling?

And pedal movement is still not the same as crank rotation.

Because we know that the chain isn't going slack - it has tension on it the whole time.

Pedal movement in the direction of pedaling IS crank rotation.

[kramnnim]

Originally Posted by Kontact

Because we know that the chain isn't going slack - it has tension on it the whole time.

Pedal movement in the direction of pedaling IS crank rotation.

Tension from pressure on the other crank arm at 1oclock...(which is playing catch up because the BB is flexing away from it...)

Pedal movement in the direction of pedaling is not crank rotation if the fulcrum point is also moving in the same direction at the same speed.

[kramnnim]

Originally Posted by Kontact

And in moving the fulcrum moves the pedal. If there was no chain the pedal wouldn't be moved, but since the chain prevents the crank from backpedaling, the net movement raises the pedal.

The chain doesn't prevent backpedaling. The other crank arm does. If the crank arms were not connected (aka Powercranks), the chain would go slack unless the other crank arm (at 1oclock) started to apply force, until the leg switched from pushing down to pulling up. (between 7 and 8oclock)