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

alienator

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

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 bullshit 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.

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Crankyfeet

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.

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Phill P

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.

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alienator

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

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 pissed 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

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...

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artemidorus

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.

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artemidorus

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.

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Crankyfeet

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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Crankyfeet

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.

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Crankyfeet

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").

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Crankyfeet

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...

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Phill P

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.....

__________________
******BLATANT ADVERTISING ALERT******
I have ceramic bearings for:
Campy/Fulcrum high end hubs/wheels
Shimano hubs and pedals
check out my ebay items here