Where are the numbers relating stiffness to speed or power?
#551
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#552
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I don't see how that equation tells you anything about how much energy is stored by a frame. If you model it as a spring, changing the spring constant will just cause a corresponding adjustment in deflection if input is the same. The same energy is stored because the same energy is put in.
As has been mentioned already, you don't model it as a spring though, its a spring and damper. Tell me the values to use to model the dampening and we can say if the energy lost in the dampener is "significant", or "insignificant". How big is the shock absorber in this system?
As has been mentioned already, you don't model it as a spring though, its a spring and damper. Tell me the values to use to model the dampening and we can say if the energy lost in the dampener is "significant", or "insignificant". How big is the shock absorber in this system?
#553
Professional Fuss-Budget
My apologies. “Heavily damped was a poorly chosen phrase. (I work with very high-Q systems, so heavily damped may mean something different to me.) I was not implying that the system is over-damped or even-critically damped, only that the damping is non-zero and measureable.
And to continue to be clear, I don't think anyone is saying that frame deflection is has zero loss -- I certainly am not. (E.g. I've been referring to "friction" rather than "damping.") The claim is that the losses are insignificant, or at least far smaller than typically assumed. As noted, that depends on points of comparison, and the context.
I do go farther than some, in that I assert that the certainty of that belief is unjustified, given the lack of empirical evidence. Or at least, people are taking what may be a reasonable assumption ("a traditional steel frame transfers less efficiently than a new CF frame") and applying it without justification and/or evidence to other situations ("therefore, this late-model bicycle that deflects at 70nm/deg at the BB is faster than this other late-model bicycle, which deflects at 60 nm/deg at the BB").
#554
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I don't see how that equation tells you anything about how much energy is stored by a frame. If you model it as a spring, changing the spring constant will just cause a corresponding adjustment in deflection if input is the same. The same energy is stored because the same energy is put in
As has been mentioned already, you don't model it as a spring though, its a spring and damper. Tell me the values to use to model the dampening and we can say if the energy lost in the dampener is "significant", or "insignificant". How big is the shock absorber in this system?
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I think they're 50-100 hz sample rate, maybe too slow for the bottom bracket. Which is why I've been meaning to buy one for an arduino project ... yet, if all you want to know is the frequency and some data on the damping you might not need more than direction and magnitude.
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Changing the spring constant directly changes the potential energy stored in the spring, assuming the same force is applied.
No one who has posted on the thread knows the values of the spring constant or the damping constant, so we don't know if the effect is significant. I think that's why the O.P. wants to see some data.
No one who has posted on the thread knows the values of the spring constant or the damping constant, so we don't know if the effect is significant. I think that's why the O.P. wants to see some data.
#557
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Thanks for the clarification.
And to continue to be clear, I don't think anyone is saying that frame deflection is has zero loss -- I certainly am not. (E.g. I've been referring to "friction" rather than "damping.") The claim is that the losses are insignificant, or at least far smaller than typically assumed. As noted, that depends on points of comparison, and the context.
I do go farther than some, in that I assert that the certainty of that belief is unjustified, given the lack of empirical evidence. Or at least, people are taking what may be a reasonable assumption ("a traditional steel frame transfers less efficiently than a new CF frame") and applying it without justification and/or evidence to other situations ("therefore, this late-model bicycle that deflects at 70nm/deg at the BB is faster than this other late-model bicycle, which deflects at 60 nm/deg at the BB").
And to continue to be clear, I don't think anyone is saying that frame deflection is has zero loss -- I certainly am not. (E.g. I've been referring to "friction" rather than "damping.") The claim is that the losses are insignificant, or at least far smaller than typically assumed. As noted, that depends on points of comparison, and the context.
I do go farther than some, in that I assert that the certainty of that belief is unjustified, given the lack of empirical evidence. Or at least, people are taking what may be a reasonable assumption ("a traditional steel frame transfers less efficiently than a new CF frame") and applying it without justification and/or evidence to other situations ("therefore, this late-model bicycle that deflects at 70nm/deg at the BB is faster than this other late-model bicycle, which deflects at 60 nm/deg at the BB").
#558
Descends like a rock
Changing the spring constant directly changes the potential energy stored in the spring, assuming the same force is applied.
No one who has posted on the thread knows the values of the spring constant or the damping constant, so we don't know if the effect is significant. I think that's why the O.P. wants to see some data.
No one who has posted on the thread knows the values of the spring constant or the damping constant, so we don't know if the effect is significant. I think that's why the O.P. wants to see some data.
So, the question remains, how much dampening does a bicycle frame have? The spring part doesn't matter.
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If the same force is applied, changing the spring constant changes the amount of deflection. Think about sitting on a car. When you sit on a Caddy with a cushy suspension, it will deflect a decent amount. Go sit on a sports car that has stiff spring (same force, you) and it will deflect less. Both will rebound and the energy stored will come back. The one that loses the most energy will be the one with the most dampening, not the one with the stiffest spring.
So, the question remains, how much dampening does a bicycle frame have? The spring part doesn't matter.
So, the question remains, how much dampening does a bicycle frame have? The spring part doesn't matter.
No offense pallen but maybe we need a sticky for that, or at least OP edit the original post and include a footnote.
#560
Making a kilometer blurry
If the same force is applied, changing the spring constant changes the amount of deflection. Think about sitting on a car. When you sit on a Caddy with a cushy suspension, it will deflect a decent amount. Go sit on a sports car that has stiff spring (same force, you) and it will deflect less. Both will rebound and the energy stored will come back. The one that loses the most energy will be the one with the most dampening, not the one with the stiffest spring.
Yep.
#561
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Hooke's Law may not apply to the carbon fiber used in bike frames.
https://link.springer.com/article/10....0366347#page-1
AKA Non-hookean
F=kx is an approximation.
I think the use strain energy would be of appropriate here.
The strain between the molecules will be released as heat and kinetic energy. Of the kinetic energy only some would be useful to propel the bike forward.
https://link.springer.com/article/10....0366347#page-1
AKA Non-hookean
F=kx is an approximation.
I think the use strain energy would be of appropriate here.
The strain between the molecules will be released as heat and kinetic energy. Of the kinetic energy only some would be useful to propel the bike forward.
#562
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If the same force is applied, changing the spring constant changes the amount of deflection. Think about sitting on a car. When you sit on a Caddy with a cushy suspension, it will deflect a decent amount. Go sit on a sports car that has stiff spring (same force, you) and it will deflect less. Both will rebound and the energy stored will come back. The one that loses the most energy will be the one with the most dampening, not the one with the stiffest spring.
So, the question remains, how much dampening does a bicycle frame have? The spring part doesn't matter.
So, the question remains, how much dampening does a bicycle frame have? The spring part doesn't matter.
(Sorry, WPHamilton already explained this perfectly.)
#563
Descends like a rock
Damping isn't everything - the spring constant still matters. An extremely stiff spring with high damping can still be less lossy than a soft spring with low damping. Very little potential energy may be stored in the stiff spring, so very little can be lost to damping. In the extreme case of an infinitely stiff spring, no energy can be lost to damping.
#564
Descends like a rock
I get that, but the argument here is about how much of the energy do you get back. For example, you store more energy in a soft spring, but you get 99.99% of it back vs storing a small amount of energy in a stiff spring and getting 99.99% back. The difference would be insignificant if there is not a lot of dampening in the system. If there is a lot of energy lost in "dampening", then the spring constant matters more. We still cant answer "significant" vs "insignificant" without actual numbers.
Last edited by pallen; 07-25-13 at 03:32 PM.
#565
Making a kilometer blurry
I get that, but the argument here is about how much of the energy do you get back. For example, you store more energy in a soft spring, but you get 99.99% of it back vs storing a small amount of energy in a stiff spring and getting 99.99% back. The difference would be insignificant if there is not a lot of dampening in the system. If there is a lot of energy lost in "dampening", then the spring constant matters more. We still cant answer "significant" vs "insignificant" without actual numbers.
I think that if a rider is going for every possible advantage at any cost, then it makes sense to upgrade every year. For someone like me who just wants to have fun and win a few Cat 3 or Masters races every year, that can be accomplished on any reasonable race bike from the last 15 years.
#566
Descends like a rock
I would guess that its not very significant, but I really have no idea. Maybe a stiff frame just feels good to some riders - I'm good with that.
#567
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I think that your position is almost certainly correct for the "average" cyclist, but it may not hold for serious racers. The major difference between an average cyclist and a top pro is that the pro can produce a lot more power. Since the potential energy in a spring scales with the square of the applied force, the frame losses will also scale as the square of the applied force. Top sprinters may produce 3-4x more force at the pedals than an average cyclist, so frame losses could become significant for them. Given that pros are also interested in gaining every (tiny) advantage that they can, a stiff frame may mean a lot to them.
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#569
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I agree that if you hit a frame with a hammer it will vibrate for a long time. The vibrational modes that are excited by the hammer, however, are not important in this discussion. Your hammer probably has exciting "guitar string" vibrations in the main tubes. The mode of interest involves side-to-side motion of the bottom bracket.
You can get some qualitative data about the "bottom bracket" damping from a simple experiment. Stand next to your bike and lean it away from you. (Choose your softest frame to make the experiment easier.) Place your foot on the bottom bracket (or pedal) and push sideways until you induce an observable amount of flex. Take your foot quickly off the bottom bracket and see what happens. If you started with several millimeters of flex, it should be pretty easy to see if the bottom bracket oscillates back and forth.
#571
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Data from Champoux's study of bicycle dynamics agrees with me rather than you: he has a damping factor of about 1.3% for the first bending mode: PDF here
#572
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Sean Kelly said that he was told his saddle was too low. His words, not mine.
A flexy fork means the bike will drift after the apex in high speed corners. The fork determines the tracking of the bicycle on high speed descents.
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To relate this back to your original argument, you appeared to try to counter the rebound argument by saying that a softer frame would lose energy because it absorbed more energy when flexed and lost much of it on rebound because of heavy internal damping. I said I agreed with the first but not the second part of this argument.
I mentioned the hammer thing because the loss angle for metals is more or less constant with frequency, so the ringing is evidence that it isn't heavily damped. And because I like hitting frames with hammers.
Last edited by Mark Kelly; 07-25-13 at 07:49 PM.
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Oh hey, I took all those classes, and another one called Mathematical Modeling of Mechanical Systems. They don't really help this discussion without specific numbers though. I mean, the concepts are useful in understanding the dynamics of what could be going on, but, If I understand correctly, the crux of this cluster of a thread is that everyone acknowledges that there is some energy lost in frame flex. Some people say its significant, some say its insignificant. Until we have a way to quantify it, all we have is subjective perceptions and experience. As you have alluded to, it could even be significant for some and insignificant for others.
#575
Blast from the Past
Note to self, don't let Mark "The Hammer" Kelly near my bike.
Last edited by Voodoo76; 07-25-13 at 05:42 PM.