What is the truth behind bike weight? Does it really help THAT much?



alienator

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thoughtforfood said:
Actually, even without a larger contact patch, you still lose energy because of the manner in which a round object reacts to altering its pitch in relation to its line of travel. You are gonna lose energy, no way around it.

The point is, what amount of energy is lost? Is it significant to the system in question? Is the energy lost greater in magnitude than the noise in the system?

The idea that something has to be lost is a bit too simplistic. Of course, by the second law of thermodynamics, something has to be lost; however just because something is lost doesn't mean that the system responds in any noticeable way to that loss.

John's point and the point of all the analysis ever done on this, uhm, point, is that the losses are statistically insignificant and are lost in the noise. Moreover, if you create a model wherein the system is perturbed so much that there are significant losses, then that perturbation will exist even for stiff systems. Example: if you model a 350lb rider with a cadence so bad that the force applied to the pedals is applied at a 60 degree angle to the bike plane's normal, the energy applied to BB, normal to the bike's plane will be the same whether the bike is made of pot metal, CF, or Inconel X. The frequencies at which the frames respond might be different, but the energies won't change.

Measuring frame deflection doesn't really tell you anything instructive. It'll only verify that stiffer frames, i.e. frames with higher effective spring constants, deflect less for a given force input. It'll also only show that a given energy, U, put into frame one is the same as put into frame two. That doesn't change. Differing spring constants don't limit the amount of energy put into a system.
 

artemidorus

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Crankyfeet said:
How do you know what proportion is put into this "spring system" and what proportion of the force is transferred to the wheel. This is the essence of efficiency.
He is assuming for purposes of simplicity that ALL the energy going into deforming the frame is lost, ie that it does work heating the frame. In fact, and this has already been discussed, it is likely that some/most of the stored energy in the deformed frame goes back into doing the work of turning the cranks, so John's model calculates worst case losses.
One feature of frame deformation that was not included in the model, however, is the effect of a turning bias on the rear wheel. Whether this could do extra waste work in a slight sideways scrubbing of the tyre, or whether subtle matching turns of the front wheel prevent this wastage, I don't know.
 

alienator

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thoughtforfood said:
Actually, even without a larger contact patch, you still lose energy because of the manner in which a round object reacts to altering its pitch in relation to its line of travel. You are gonna lose energy, no way around it.


Again, that's a given. The point is that the energy lost is very small. Of course energy is lost: that's the whole point of S=k*Ln(ω). Τhere is no such thing as a free lunch. The energy losses in bike systems are very small over all. Logically, then, the losses when examined individually, are much smaller. It's difficult for people to even reliably sense the big losses in bike systems: aero losses, losses due to rolling resistance, and etc.
 

artemidorus

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TheDarkLord said:
I would include a damping term though, as there is bound to be some energy that will be absobed by the bike frame. The damping term could dominate the losses in the system; but I wouldn't be surprised if any loss is very small.
His entire model assumes that all the frame deforming energy is "damped" rather than being returned to the cranks in contributing to useful work.
 

thoughtforfood

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alienator said:
Again, that's a given. The point is that the energy lost is very small. Of course energy is lost: that's the whole point of S=k*Ln(ω). Τhere is no such thing as a free lunch. The energy losses in bike systems are very small over all. Logically, then, the losses when examined individually, are much smaller. It's difficult for people to even reliably sense the big losses in bike systems: aero losses, losses due to rolling resistance, and etc.
Then, taking my point into account, altering the pitch of the tires to a greater extent will cause a greater loss of energy. The more perpendicular to the line of travel, the greater the loss of energy. Is it just noise? If you were cycling 10 miles in a sprint it might make a measurable difference, otherwise, probably very little. However, that was not the assertion. Your assertion was that "evil losses due to frame noodlism just ain't there." Well, maybe we can determine what constitutes "evil," but it sounds to me as though you were suggesting that it is non-existent. Then you say that it is there.

I would also suggest that two equal sprinters, one on a stiff bike, and one on a noodle, would necssitate an outcome that favored the rider on the stiff bike every time. That is measurable.

Maybe making a clear point, and not demeaning someone in doing so would be a better option for you.
 

alienator

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thoughtforfood said:
Then, taking my point into account, altering the pitch of the tires to a greater extent will cause a greater loss of energy. The more perpendicular to the line of travel, the greater the loss of energy. Is it just noise? If you were cycling 10 miles in a sprint it might make a measurable difference, otherwise, probably very little. However, that was not the assertion. Your assertion was that "evil losses due to frame noodlism just ain't there." Well, maybe we can determine what constitutes "evil," but it sounds to me as though you were suggesting that it is non-existent. Then you say that it is there.

I would also suggest that two equal sprinters, one on a stiff bike, and one on a noodle, would necssitate an outcome that favored the rider on the stiff bike every time. That is measurable.

Maybe making a clear point, and not demeaning someone in doing so would be a better option for you.

It ain't there=it's lost in the noise and is of so very little consequence that your ass can't measure it, that your ass can't reliably say that it has any effect at all, that by being lost in the noise, it's not worth thinking about.

The point was clear. Whether you felt demeaned or not really doesn't matter to me. I mean, I could really care less.

Your sprinter experiment....well, have at it Toodles, and let us know what results your experimental accumen comes up with. I'll bet big money that again, the difference is lost in the noise. I'll bet by the time you get around to quantifying--if you're even capable of it--all of the different variables that will influence your data, that you'll find you won't be able to discern a measureable difference.

Have at it.

Let us know when you publish your paper.
 

Crankyfeet

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alienator said:
John's point and the point of all the analysis ever done on this, uhm, point, is that the losses are statistically insignificant and are lost in the noise.
So now you are privy to all the proprietary analysis ever done on this? That's a very bold statement to make. I'd like to know who has undertaken tests, apart from John here. With no disrespect to John intended, I am not convinced that the "x" values given by manufacturer's are the "x' values for the frame material and truss given the forces being applied to the bottom bracket. Torsional deflection of a truss has a lot to do with the truss and joint design IMO as well as the material. But if you think that the deflection, or material is irrelevant, then go for a sprint on a PVC tube frame, and see if you perform the same.

So where is your data, since you cite here that you have seen "all the analysis ever done"? Let's face it alienator: you start with a mindset opinion (eg. Expensive frames are over-hyped), then you just BS around, trying to sound academic, dropping a completely irrelevant equation every now and then, telling us that you "done did the analysis" and the above quote of having seen all the analyses ever done, asking for people to show their data when it doesn't agree with your viewpoint, then seeming to lie that you have data yourself (where is it??), and not showing us anything (picture: alienator feverishly private messaging John Swanson to send him some of his calcs).

I'm prepared to admit I'm wrong. I don't have any data, other than trying to work out the system in my head and the opinion of pro cyclists (and amateurs as well) garnered from their experience. But I have never claimed to have any data. You have. If I could just be convinced by your big talk about the data, and that you do have any credible data (after all, data is only as good as the test or model that produces it in any case).

I also know that energy is lost and that it is not necessarily redelivered back into the system with "springback" (if it springs back when the pedals at 12 and 6 o'clock, then there will be nada return of energy to the drivetrain). The question here is magnitude (however any amount of loss is a concern, and it always goes in one direction, unlike noise which is random). So far we have just been feasted with platitudes to back up your thesis that stiffness is a non-factor to the performance of a sprinter at the money end of a race.
 

Crankyfeet

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In case anyone's wondering why alienator doesn't want to respond to my arguments directed at him. About a week or so ago, alienator got called on a ******** stunt he tried to pull on someone (or two?) in the bike cafe, by a couple of people who could see through his BS. Now I think he is in a tiff... :p

What's the matter "cupcake"? You sulking? You can answer if you want. This is just a discussion forum after all. I promise I'll respect your validated responses. Afterall, I am deeply interested in getting to the bottom of this issue of stiffness. And you have told us you have seen all the analyses ever done, and even "done did the analysis" yourself. Can you help me out? I sooo want to be showered by your infinite wisdom and rigorous analysis and research.
 

alienator

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I'll reply to you just once: you, crankyfeet, are a non-entity in my book. Therefore, you don't get a response. This is the only one.
 

Crankyfeet

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alienator said:
I'll reply to you just once: you, crankyfeet, are a non-entity in my book. Therefore, you don't get a response. This is the only one.
Oh isn't that convenient. I called you on a couple of lies it appears... and you don't want to answer me (they may not be lies... though it is a stretch for me to imagine you have seen all the analysis ever done on this subject, and as yet we haven't seen any of your own analysis that you have claimed to have done).

Actually, I agree with most of your points usually (even some of the ones you have made in this thread). Not that I know anything. I'm not even saying that you are wrong, just asking you to pony up on your seemingly vaccuous claims. I just dislike it when you try to bully people with ******** to support your opinions... the same behavior in which you normally delight in exposing in your many adversaries here in cycling forums.

I'm prepared to accept I'm wrong in my "hunch" if you can produce all this analysis and data that you have researched. Afterall... you made the claim... and it is you that normally asks others to produce data.. even when they haven't claimed to have any.
 

Crankyfeet

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ScienceIsCool said:
Oh yeah. And the efficiency is the same whether you're putting out 12 or 1200 Watts. The system is incredibly linear over a wide range of inputs.

John Swanson
www.bikephysics.com
John, by my calculations, the energy stored (or lost if you assume it is released in the dead zone of the pedal stroke and therefore wasted) is proportional to the square of the deflection in the frame ("x" or the spring deflection).

Hence if a more flexible frame has a stiffness modulus ("k" - lateral at the BB, or torsional stiffness coefficient) that is half that of another, more stiff frame, then the energy absorbed (lost) will be four times that of the stiff frame. If the noodly frame deflected three times the amount of the stiff frame (ie. the stiffness modulus "k" was 1/3, and therefore "x" is 3 times for F = kx to stay constant) then the loss of energy would be nine times the amount.

I might be wrong, but it seems like the loss of energy is proportional to the square of the amount it deflects (Its noodliness). Given that the moment applied to the BB (the force applied to the pedal, since the moment arm is constant) is the same in both stiff and noodly scenarios in our hypothetical test.

This suggests an exponenential (squared) relationship between force applied to the pedal, and the energy lost to the system (because F = kx, deflection increases linearly with force but energy increases by the square, E = 0.5 kx^2). Which makes the loss scenarios more significant for the Boonen types thrusting with huge forces in the sprint end to a race (assuming these are significant - however we all agree that they exist).

Put in other terms, if Boonen for example can generate 1400 watts in a sprint (I have read this figure from a magazine but have no idea as to its accuracy), the power lost due to the spring effect of the frame will be (1400/400)^2 = 12.25 times the power lost at 400 watts. This assumes that pedal force is linearly proportional to power generated.

The energy loss doesn't take into account possible energy losses as the rear wheel deflects due to frame twisting, and perhaps traces a less than straight path as a result. However the torsional twisting may actuially mitigate some loss at the wheel/road interface, because the effect of torsional deflection would offset the effects of lateral deflection due to the moment applied to the system by the pedal stoke.
 

Phill P

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Boy this thread has taken off..........off topic anyway!

The long standgin arguement in this thread that nobody has any data to support stiffer is better is true. But that doesn't mean you can pick your own result without any data either.
There is no proof to be provided here at stiffness has no factor. You neither prove or dis prove the hythopsis or the null hypothesis. There are no scientfic facts to support anbodies arguements, and no point getting personal with people when you don't have a leg to stand on either based off your very own arguement.


Every material has a co effiecient of restetution (spelling??). What it refers to is the amount of energy absorbed compared tot he energy put in. Similar to damping....

Steel return more energy than Al, maybe that i why steel is often thought to feel more lively or springy even though it flexes more (in general).

So I don't feel the amount of energy lost in the material through flex actually relates to the amount of flex, it relates to the material properties and how much energy is absorbed and hence not returned.

However I do believe the point made many times prior that the engery is put into the frame at the point of max torque :2-5pm and the direction of deflection is created because of the direction of the forces from pedeling, and the frame geometry.
But unfortunately when the frames starts to flex back and return the energy, the pedals are no longer in the peak torque zone, so the force coming back from the frame is less effeciently delivered back to the rider and hence does not generate the same torque on the cranks compared to when the frame flex was generated.

I just can't ee that the energy put into the frame from flex is delivered to produce torque on the cranks and hence propel the bike and rider.

I would like to see some theory to back up the hypothesis that flex does not effect power output, not "you are wrong because you have no facts". If no theory lets see a test that proves flex has no effect.
 

alienator

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ScienceIsCool said:
I didn't perform an experiment. I used the known values of stiffness and computed the theoretical losses. But yes, you made one of my points for me. The losses are generally too small to be measured... :)

This is how I calculated the losses:

- Assume the stiffness values (expressed as k = N/mm) represent a linear, spring relationship. And it should because there are no non-linear elements.
- The energy stored in a spring (and subsequently lost) is equal to 0.5kx^2 where x is the displacement.
- The amount of displacement can be estimated from F = kx and P = Tw, where P is power, T is torque, and w is the rotational speed of the cranks.
- For a given power output and cadence, you can figure the average torque on the crank.
- Assuming the instantaneous peak torque is ~ 4 times average (that's generous) and using a force diagram, you can calculate the force that goes into bending the frame (i.e., pushing on our model of a spring)
- This force gives you the energy stored in the spring as described above: 0.5kx^2 and all that.
- This energy loss happens twice every rotation of the cranks, which happens 2cadence/60 seconds.
- Therefore, the power lost is equal to Energyx30/cadence Watts.
- Divide that by the original power input and there's your efficiency.

I found that using this model, which should be accurate by way better than an order of magnitude, the efficiency of a bike frame is ~99.95%. That might change by ~0.025% depending on whether you have a super stiff or super flexible frame.

John Swanson
www.bikephysics.com

Good stuff, Swanson, as usual. Your analysis likely overestimates the bending moment applied to a frame from pedaling, but then you know that. The reality is that maximum torque not only has to act to bend the frame, it will also act to bend a crankarm, bend a BB axle, and if someone wants to be really anal about such things, bend chainrings. Then, where that max torque occurs will impact the maximum force and how it couples to frame bending. Your analysis points out exactly why first order analyses are valuable. Even if your first order figure is off and a frame is only 99.9% efficient, that would mean that a whopping 1.5 Watts would be lost to a frame when a rider is putting out 1500 W. Wow. That's so huge. 1.5 Watts. Heck, that's less than all those Watts that FSA says ceramic bearings will save you!
 

alienator

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Phill P said:
I would like to see some theory to back up the hypothesis that flex does not effect power output, not "you are wrong because you have no facts". If no theory lets see a test that proves flex has no effect.

That is not the case. The energy lost to frame flex is so damned small that it's not worth thinking about. That is how it's been all along. The idea, though, that all that energy lost to frame flex must really drag you down, well, that idea rests on nothing. Zippo.

Are you putting up money to do tests? Yes? The theory is right there. Whether John Swanson writes it out in simple terms so that the non-technical folks can understand or whether it's stated otherwise: it's there. If you can't believe it, or someone else can't believe it, then that's your problem. There are, after all, still people that think the world is flat and that men never landed on the moon. There are entire religions ****** off about the Second Law of Thermodynamics. And every year there's another boatload of misguided souls who believe that they've discovered how to make a perpetual motion machine or how to assign fractional principle quantum numbers to electrons. Not understanding, though, doesn't change the physical realities that govern how matter and energy interact on a daily basis. Sorry.

Now iffin' you wanna throw your eggs in the "energy lost to my frame is a real drag" basket, well, have at it. You may as well, at the same time, start calculatin' all the time yer missin' as a result of the Lorentz time transformation and how yer speed, whatever yer doin', scales compared to the speed of light.......'cuz, well, they're both big wastes of time, given the very small magnitudes of the values concerned.
 

Crankyfeet

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alienator said:
Good stuff, Swanson, as usual. Your analysis likely overestimates the bending moment applied to a frame from pedaling, but then you know that. The reality is that maximum torque not only has to act to bend the frame, it will also act to bend a crankarm, bend a BB axle, and if someone wants to be really anal about such things, bend chainrings. Then, where that max torque occurs will impact the maximum force and how it couples to frame bending. Your analysis points out exactly why first order analyses are valuable. Even if your first order figure is off and a frame is only 99.9% efficient, that would mean that a whopping 1.5 Watts would be lost to a frame when a rider is putting out 1500 W. Wow. That's so huge. 1.5 Watts. Heck, that's less than all those Watts that FSA says ceramic bearings will save you!
Yes... and even assuming that the stiffness modulus John used was actually the torsional stiffness modulus of the frame truss and not the stiffness modulus of the actual material itself, let's see... 1.5 watts over 1500 watts equates to 1.5 feet over 1500 feet...Hmmm...could be the difference between winning and losing. But then if you were the DS of a pro team with a $9,000,000 annual budget, you'd give up 1.5 feet in the last 1500 feet wouldn't you. It's just "noise".

Or put in a different way, 230m over a 230km race. That's 230m you've got to make up just to break even.... Hmmm....

So is this the "I've done did the analysis" you were referring to. What a joke... :p
 

artemidorus

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Crankyfeet said:
Yes... and even assuming that the stiffness modulus John used was actually the torsional stiffness modulus of the frame truss and not the stiffness modulus of the actual material itself
John used test results for stiffness of the assembled frame on a jig. He is far from being silly enough to give us the material modulus.
 

artemidorus

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Crankyfeet said:
.. 1.5 watts over 1500 watts equates to 1.5 feet over 1500 feet...
Not true. There is certainly not a linear speed response to wattage. You can count on much less than than a .1% speed loss from a .1% wattage loss.
 

Crankyfeet

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alienator said:
That is not the case. The energy lost to frame flex is so damned small that it's not worth thinking about. That is how it's been all along. The idea, though, that all that energy lost to frame flex must really drag you down, well, that idea rests on nothing. Zippo.

Are you putting up money to do tests? Yes? The theory is right there. Whether John Swanson writes it out in simple terms so that the non-technical folks can understand or whether it's stated otherwise: it's there. If you can't believe it, or someone else can't believe it, then that's your problem. There are, after all, still people that think the world is flat and that men never landed on the moon. There are entire religions ****** off about the Second Law of Thermodynamics. And every year there's another boatload of misguided souls who believe that they've discovered how to make a perpetual motion machine or how to assign fractional principle quantum numbers to electrons. Not understanding, though, doesn't change the physical realities that govern how matter and energy interact on a daily basis. Sorry.

Now iffin' you wanna throw your eggs in the "energy lost to my frame is a real drag" basket, well, have at it. You may as well, at the same time, start calculatin' all the time yer missin' as a result of the Lorentz time transformation and how yer speed, whatever yer doin', scales compared to the speed of light.......'cuz, well, they're both big wastes of time, given the very small magnitudes of the values concerned.
I haven't seen John spell out his model in detail yet. Even he admits that he would have to go back a long time to dig up his figures and coefficients. So John's word that the number's good cause he says so is good for you. It comes down to trust. John sounds like a nice guy to me. But in science, we all dissociate ourselves from personal considerations, in analysing a model, theory or experiment. But you "done did" your judgment analysis on John's 99.95% figure generated already.

John, you're probably right, and I would be pleased if you were, but my personal "jury's" still out until I can get some data off manufacturers, or perhaps get some analysis done myself. I'd actually be happy to prove you right, notwithstanding however that my own prowess in this analysis may pale in comparison to yours.

As alienator said, the model is quite complex in reality as there are many degrees of freedom in the system which will absorb energy depending on each individual F = kx equation for each component.

Notwithstanding my point above (post #153) which postulates that energy loss is proportional to the square of the "noodliness", and the square of the pedal force.
 

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