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



Crankyfeet

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artemidorus said:
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.
Fair enough. That was an assumption I made. I stand corrected. I'm still waiting for a response on the point that energy loss is proportional to the "noodliness" or deflection of the frame squared. And likewise proportional to the square of the pedal force applied.
 

Crankyfeet

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artemidorus said:
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.
Manufacturers test results I take it you mean. John told us that his calculations were all theoretical based on provided coefficients ("known values of stiffness").
 

Crankyfeet

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John, if you could answer this for me please. For the life of me, I can't conceptually work out how the torque applied by the "spring" (in this case the torsioning of the frame) is that different from the torque applied by the crank arm in your model. If you are assumimg that the pedal force is transferred to the frame at the BB, the moment arm on the BB perpendicular to the frame is not that different from the crank arm itself. I am stumped on how the 0.5 kx^2 only comes out at 5/10,000's of the energy applied to the crank.

If the F = kx of the frame "spring" is equivalent to the pedal force times the lateral crank displacement, how do we end up with the frame energy absorption being only 1/2000 of the crank energy?

I realise I may be making a big error here somewhere... :confused:
 

Phill P

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Thank god JOHN is able to put some theory behind his opinin, and doesn't just berate people who dissagree.

JOHN - please correct me if I'm wrong, but your study only looks at the energy absorbed by the frame (and can be applied to components) due to flex, and assumes that the rest is returned in the form or torque.

I'm not surprised the amount of energy absorbed is so low, but my arguement is that energy returned as the frame releases its stored energy does not get returned as effiecently as torque generated at the cranks which produces tension in the chain, and hence propel the rider and bike. I am suggesting the returned force is later in the rotation of the cranks compared to when peak torque from pedalling is generated and is therefore not converted to forward motion.

Is the amount lost going to be a lot again.....not in most cases. If you are spinning a high cadence the frame is not going to flex much. If you are the sort of rider who can get out of the saddle and get the chain to rub the deraillor or flex the chain stay and wheels so they rub the blocks, then maybe you are going to stuffer some losses (and not just from rubbing the brake pads).

The arguement I can't put a figure on is how much.

But if we are going to ignore all the small iterative advantages that have been developed over the years then why not go back to wood rims and plumbing pipe for tubing??

A comment on some of the arguements above....just because I'm not willing to fund any testing doesn't make me wrong. Doesn't make you correct either. Why don't you fund the testing to prove yourself correct? An arguement based on well thought out theory or a good test (like John often produces) is going to sway more people than critisising the person for what ever seems to be annoying you.

PS I've been an automotive engineer for many years and got A grades in uni doing material science, computational solids, and mechanics of vibrations (working out the natural frequencies of bodies using the integration etc of hypobolic sine functions and solving mutliple lines of functions and unknowns using boundry conditions). In fact I came top of that subject for my year.

Keep talking down to people and throwing your big words around, people will certianly respect you more and it helps prove your points. Oh yeah I forgot, you don't care what people think of you, just so long as you are right.

ok now I'll cut the cattiness now.....
 

Crankyfeet

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Don't get your underpants twisted too much by alienator, Phill. He actually has some good advice to give on biking when he adopts a more respectful manner. He just can't help being a pretentious patronizing ***** at times... :rolleyes::D
 

TheDarkLord

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Crankyfeet said:
Don't get your underpants twisted too much by alienator, Phill. He actually has some good advice to give on biking when he adopts a more respectful manner. He just can't help being a pretentious patronizing ***** at times... :rolleyes::D
Amen to that! If you really want to have a debate alienator, put aside your condescending attitude, and put forth in straight terms what analysis you did if any. But if you keep coming up with arguments starting with "lack of data" and then switching to "I did all the analysis", etc. you are just making clear that you are just spewing lies and BS to intimidate the hell out of people. Now that may work with the general public, and maybe you do it on a regular basis in your real life; but it will not work in a forum where there are some people who are scientists. Now, OTOH, I have seen conferences where some bigshots use the same argument that you use here to browbeat people who challenge their theories. I don't understand how a scientist can follow such methods...
 

artemidorus

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Crankyfeet said:
. I'm still waiting for a response on the point that energy loss is proportional to the "noodliness" or deflection of the frame squared. And likewise poportional to the square of the pedal force applied.
John's model used a formula for stored energy that was, indeed, based on the square of the deflection of the frame (0.5kx^2, "^2" means squared). Because deflection of a spring is proportional to applied force, then energy is also proportional to the applied force squared. So you are right on both points, but John has already told us that.
 

artemidorus

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Phill P said:
I'm not surprised the amount of energy absorbed is so low, but my arguement is that energy returned as the frame releases its stored energy does not get returned as effiecently as torque generated at the cranks which produces tension in the chain, and hence propel the rider and bike. I am suggesting the returned force is later in the rotation of the cranks compared to when peak torque from pedalling is generated and is therefore not converted to forward motion.
John's model agrees with you - he assumes for clarity that all the stored spring energy is lost - this is his 0.05% power wastage.
 

Crankyfeet

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artemidorus said:
John's model used a formula for stored energy that was, indeed, based on the square of the deflection of the frame (0.5kx^2, "^2" means squared). Because deflection of a spring is proportional to applied force, then energy is also proportional to the applied force squared. So you are right on both points, but John has already told us that.
John told us that the loss effect was 0.05% and was linear over a range of wattages. My point is that the stored energy is proportional to the SQUARE of the force applied at the crank (assuming a linear relationship between force and power ie. constant cadence). Refer to post #153 for an explanation. Since k is constant (in the F = kx equation), the pedal force is proportional to the deflection (x), which is squared to determine the energy stored (E = 0.5 kx^2).

Please correct me if/where I am wrong so I can save wasting time on a fallacious point...

Thanks for explaining things BTW.
 

TheDarkLord

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Here is one more consideration: when people talk of output power as measured by a power meter, is it only the power applied to the cranks, or does it relate somehow to the power output by the rider as a whole? If you look at the final sprint in the race, the riders are standing up, and they are waving their bike one way and the other for balance as they pump the pedals. So, if you talk about the total power output of the rider, some of it is going to be spent in the flexing of the frame during this action as opposed to being applied to the cranks. I doubt that a simple model will take this into account, and this could be important in the pro and Cat 1 type races, where riders win by a fraction of a second. Of course, I am not implying that this is a huge difference that will be felt by a typical rider...
 

thoughtforfood

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alienator said:
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.
Sweetie, I wasn't demeaned. You demeaned someone else. I COULDN'T care less if you demean me, you are a prickly blowhard.

As to the experiment, you have chosen to show no data, why should I? It appears clear to me that you all are discussing the spring effect of the frame solely. I am discussing the known FACT that the alteration of the path of a wheel and tire cause loss of energy. See, we have known that for a long time now. Now, that loss of energy is because of an increase in friction, and it isn't returned like the energy of the frame (acting as a spring). There is a measurable loss of energy, period. Doesn't matter how much.

At one point you say there isn't any loss, then you say that there is, but it is lost in the noise, then you say you don't have data, but everyone else needs some. Then you say you have done the experiment, but YOU have no data.

I realize now what is going on here. You don’t know what you are talking about, and you just like to belittle people to make yourself feel intelligent. I guess if that is how you get your kicks, go right ahead; but please, change your avatar from Mr. Pop to someone else. You don't really carry his gestalt very well.

Oh, and when you DO care less (you know, since you COULD care less) let us know.
 

Felt_Rider

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popcorn.gif
 

AngryPenguin

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Isnt it nice that with the advent of electronic and internet communications you can watch people bitching at each other whatever time in the morning it is :D
 

Crankyfeet

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I'm also pretty convinced that the losses will be different for the same power at different cadences. The frame will only bend to the extent that the crank (drive train) provides resistance to the pedal thrust (force). At high cadence (lower gear) there is less resistance in the drivetrain, hence less transference to torsional bending in the frame. At higher gears and greater pedal force for the same power input/output (ie. a lower gear), there is more force applied to the crank and BB and hence more torsional and lateral deflection in the frame and therefore more energy absorption.

In fact, if you can push a high enough gear hypothetically, you may be able to put all your weight and thrust into the pedal, and not transmit one watt of power through to the road. You would effectively be bouncing on the pedal, at an incredibly high gear ratio, and your losses due to all elasticities in the system (frame, crank, chainring, chain, rear cassette, wheel, and tire etc.) would be 100% of input force.

There seems to be a myriad of complexities in this model that don't seem to be acccounted for, but again, correct me if I am wrong in this cursory analysis.
 

Phill P

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Crankyfeet said:
Don't get your underpants twisted too much by alienator, Phill. He actually has some good advice to give on biking when he adopts a more respectful manner. He just can't help being a pretentious patronizing ***** at times... :rolleyes::D
I agree... I'm a tad irritable atm though ;) .
 

mikesbytes

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AngryPenguin said:
Isnt it nice that with the advent of electronic and internet communications you can watch people bitching at each other whatever time in the morning it is :D
Yeh, no need to watch Days of our Lives
 

sogood

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Crankyfeet said:
Don't get your underpants twisted too much by alienator, Phill. He actually has some good advice to give on biking when he adopts a more respectful manner. He just can't help being a pretentious patronizing ***** at times... :rolleyes::D
That's entertainment value! :D
 

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
From my calculations, the deflection of the frame is proportional to the force (hence torque) applied to the BB. It is not proportional to power or wattage as power incorporates cadence.

Hence, for the same power value, you can have differing force values on the pedals (with corresponding differing cadences) which produce differing deflections and differing energy absorptions in the frame (proportional to the square of the deflection) and hence you can have DIFFERING EFFICIENCIES AT THE SAME POWER INPUTS.

Also by similar math, you can have the same absolute amount of energy energy lost at two different power inputs.

Also, the system appears to be a highly complex one, that involves multiple springlike deflections from multiple degrees of freedom (eg. torsional deflection of the frame, lateral deflection of the frame, vertical deflection of the frame, deflection of the crank, deflection of the chainring, deflection of the chain, deflection of the rear cassette, deflection of the hub/cassette, lateral deflection of the wheel, longitudinal deflection of the tire, lateral deflection of the tire). The amount of energy that the frame absorbs will be dependant on the interaction of all these points of "give" in the system. The determination of proportional energy absorptions (ie. losses - in the cases where the energy can't be retransferred back) would require, it seems, an iterative process of redistributing residuals and deflections throughout the system. It is enough though to realise that these various degrees of freedom influence/affect each other IMO.

It is also a factor that the crank torque is a varying quantity in the pedal stroke. If a significant portion of the high torque zone of the pedal stroke is taken up hypothetically stretching a very elastic chain, then force won't be absorbed in the frame until the chain deflection (stretch) reaches a point that it counteracts the force applied to the chain by the crank torque. The more deflection, the more that valuable displacement is lost in the high torque zone of the pedal stroke (around 3 o'clock), and hence less available to displace the frame. If springback occurs in the low torque zone of the stroke (in the case of the chain springing back) then it will interrupt the cadence rhythm.

Only when all paths of lesser resistance are loaded up, will the tire push on the road. The more give in the system, the less of the power arc of the stroke goes into propelling the bike. The system's components are interconnected by the fact that only after all lesser resistances are loaded up, will the frame deflect. Hence energy loss in the frame is dependant on energy losses (and elasticities) in the other degrees of freedom.
 

JohnO

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Crankyfeet said:
Don't get your underpants twisted too much by alienator, Phill. He actually has some good advice to give on biking when he adopts a more respectful manner. He just can't help being a pretentious patronizing ***** at times... :rolleyes::D

It's the ghost of Boudreaux... well, he never took it to a personal level, but that guy sure knew his stuff.
 

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.
Another typical demeaning type "I'm superior" post from the cyclingforums science legend. Well put argument... not.

Here's the point. John makes claims of his model producing linear efficiencies as a function of "power". But the amount of deflection in the frame is not proportional to power. It is proportional to the torque which is dependant on resistance to one's pedal thrust. You can't load up the frme much if you take the chain off and stomp down on the pedal. But try putting a 300 tooth chainring on the front and then stomp down from a stationary start and see how much force is lost at the tire/road connection.

So if the model theory/results are invalidated, then they aren't worth much in my book. Please don't take offense from this Joihn. Firstly, I may be wrong. Secondly, it is the model as the basis of your conlusions that I am judging, not you as a person, to whom I know nothing about.

I might be wrong, but please show me my error in thinking. I'm prepared to accept that I might have made an error. Can't see it myself though. Otherwise I will have to assume there is no answer to my points of contention and they invalidate the model.

Science is cool because it is egoless and searches for the truth as its primary value. Hypothesis, theory, testing, data, and peer review are all part of this process, as you should know alienator, assuming your claims of being a scientist are true (given your other snake oil BS though... I'm not convinced).

So please don't belittle Phill, the qualified mechanical engineer, with personal condescension. Stick to the points in the debate. Which so far I see that your points are baseless, other than some claim that you have done analysis and looked at all the research on this topic, and stand behind John's results blindly it seems because they support your own opinion.
 

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