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  1. #26
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    Oy vey.........
    Now the same old crap gets trotted out. This thread was started with science, and now ends with internet science.

  2. #27
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    Quote Originally Posted by skepticman View Post
    It's very unfortunate that so many bike manufacturers and bike shops rely on perpetuating this myth for much of their business.
    Perpetuating such a myth is at the very core of all businesses wishing to make a lot of money selling luxury goods. Good bicycles can easily last 20 years, so the manufacturer must find a way to separate the buyer from his bicycle long before those 20 years are up. Suggesting that his 2-year old bicycle is out-of-date and that he therefore needs a new one is about the only way to do that.

  3. #28
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    Quote Originally Posted by goodboyr View Post
    Oy vey.........
    Now the same old crap gets trotted out. This thread was started with science, and now ends with internet science.
    Specifically what "old crap" are you referring to, and what makes it "crap" in your mind ?
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  4. #29
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    I can't recall see any marketing saying a stiffer bike makes you faster. They just tout stiffness from what I've seen.
    I don't know anyone who sprints who doesn't like the feel of a stiff BB area.
    I haven't read the whole thread but not sure what the hub bub is about. People like stiff bikes and companies make them. That's about it for this topic as far as I can see.

  5. #30
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    Quote Originally Posted by TmB123 View Post
    How much power would be lost by a rubbing chain on the FD cage because of a flexy bottom bracket. Anything at all measurable? I know it isn't power loss by the frame itself, but still an effect of it if at all significant.
    I stopped riding my custom steel frame (built for racing crits) for this reason. It was built for me when I raced at 145 pounds. As I got older and fatter, I could see the chain rings nutating under power. I could readily cause the ring to move a couple mm - enough to rub both sides of the derailleur cage.

    I know a steel frame can be made differently and built with a more appropriate tube set for my current weight and power. However, that frame flexed enough to see and to cause the chain to rub the derailleur. The bike is my favorite bike, but I don't ride it anymore.

    Currently, I ride a venge - which is actually a bit more 'relaxed' in geometry than the steel bike. The venge is pretty stiff in the BB and has very little vertical compliance. While "science" may say there is no difference between the power efficiency of my old steel frame and my venge, a quick ride on a bumpy road or an all out sprint will tell you something different.

    At the end of the day, it is about feel and personal preference. But, I am not buying that there is no discernible difference in frame flex between a Rivendell and a Tarmac. Since you can feel a difference, the difference matters.

  6. #31
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    Quote Originally Posted by goodboyr View Post
    This thread was started with science, and now ends with internet science.
    The thread did not start with science*.

    The thread started with a citation to a bicycle maker selling an idea that flexible frames are the way to go.

    From a marketing standpoint, how is that different than other manufacturers selling an idea that stiff frames are the way to go?


    *A "double blind" test comprising the author and two other individual's performance impressions of two frames of different relative stiffness is not quite conclusive evidence.

  7. #32
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    Thanks, ibericb, for your input, and those references! I appreciate your help in getting this debate back on a rational footing, which, as this thread shows, is harder than I expected. Frankly, I have a bit of trouble understanding the obvious emotional attachment of some of the posters to this silly notion of the superiority of the (laterally) stiff frame.

    At the end of the day it's about the total performance of bike + rider that matters most. That includes a riders ability to develop power as well as the bikes response to that input. Both of those will probably vary between different riders as well as riders with different objectives at varied times.
    Couldn't have said it any better.

  8. #33
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    Quote Originally Posted by crit_boy View Post
    The thread did not start with science*.

    The thread started with a citation to a bicycle maker selling an idea that flexible frames are the way to go.
    That was just meant as an illustration. The main point is indeed that science does not provide any support for the idea of the benefit of reduced frame flex, at least within a realistic range of "serious" road bikes from within the last 20 years or so.

    Quote Originally Posted by crit_boy View Post
    A "double blind" test comprising the author and two other individual's performance impressions of two frames of different relative stiffness is not quite conclusive evidence.
    Absolutely, I agree, it's not conclusive at all. But, it's a lot more evidence than anyone else in this debate has ever offered.
    Last edited by Pirx; 09-14-2015 at 06:48 AM.

  9. #34
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    K, thought experiment: what if a rider's foot were coupled to the pedal through a coil spring?

  10. #35
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    Quote Originally Posted by ibericb View Post
    Specifically what "old crap" are you referring to, and what makes it "crap" in your mind ?
    The concept of "planing" and the attempt to take make an experiment and scientific conclusion out of something where there are a large number of other independent and dependent factors that are not isolated or controlled.

  11. #36
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    Quote Originally Posted by Teuthis View Post
    K, thought experiment: what if a rider's foot were coupled to the pedal through a coil spring?
    Excellent analogy. Now make sure that coil spring has a maximum deformation of perhaps a tenth of an inch, and ask how things will change if you replace that spring with one that deforms only a twentieth of an inch. Perform the experiment and let us know.

  12. #37
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    Quote Originally Posted by goodboyr View Post
    The concept of "planing" and the attempt to take make an experiment and scientific conclusion out of something where there are a large number of other independent and dependent factors that are not isolated or controlled.
    I spent over 40 years of my life designing and doing scientific experiments in research labs. I've written and reviewed hundreds of published peer reviewed research papers, grant proposals, etc. I have yet to see even a single experiment that controls ALL of the variables in a complex system. That doesn't make the work or attendant conclusions any less valid, so long as it is recorded and reported accurately.

    When you have some real measurable results to present, or can provide another source with at least comparable design and control, then please bring it. Until then your attempt to dismiss some reasonable work is nothing more than ungrounded drivel.
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  13. #38
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    Quote Originally Posted by goodboyr View Post
    The concept of "planing" and the attempt to take make an experiment and scientific conclusion out of something where there are a large number of other independent and dependent factors that are not isolated or controlled.
    Look at my first response. The machine I described would work because it eliminates any human interaction.
    The pressure driving the cylinders could be adjusted and measured and would be consistent. It's impossible for any human to put consistent force into anything.

    I agree that human "perception" is scientifically useless. People can ignore reality and delude themselves into believing anything. Hence this thread.

    The goal of a bike is to propel itself (and the rider) forward. Any energy that does not do exactly that is wasted. Only that pedal force that is transferred to driving the rear wheel is useful.
    Any force used to flex the BB is at a 90 degree angle to what drives the wheel. It does not contribute to driving the bike forward; it can't, the bike can't convert energy in one direction into force in another. Doesn't matter how much of that force is eventually returned, it's in the wrong direction.

    So it's simple. Describe exactly how the side-to-side movement of the BB contributes to driving the bike forward.
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  14. #39
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    Quote Originally Posted by Pirx View Post
    That test has been done, with some 1983 steel steed (Pinarello) versus a modern carbon bike (2009 Lapierre). Result, in a nutshell: No difference.



    The difference was all in your head, at least the difference caused by increased stiffness. Stiffness has no measurable effect on acceleration, and no effect on average speed. Your Cervelo is lighter, hence the better acceleration, and much more aerodynamic, thus higher average speed. Nothing to do with stiffness.
    It's actually much less aerodynamic with its huge 'squoval' tubes. Stiffer, yes. More aero, no way.
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  15. #40
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    this is all gut feeling in my head logic. Nothing that has to do with anything resembling science. With that said, there has got to be some......floor or fit for purpose here. Meaning, there must be a level of flex in which an further reduction in flex there would be no performance increase yet, increased flex would show degradation of performance.

    crazy idea?

  16. #41
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    Who won?

    OK, here it is. If your frame flexes, it does limit the acceleration. This is why...
    Acceleration is force acting on mass. If your frame flexes, when you hit the maximum force, the frame flexes limiting the force to some level less than that maximum. Therefore your maximum acceleration is less.
    This force is returned to the system when the frame unflexes, but at that point the output level may not be there to allow the use of the force.
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  17. #42
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    Quote Originally Posted by Pirx View Post
    Excellent analogy. Now make sure that coil spring has a maximum deformation of perhaps a tenth of an inch, and ask how things will change if you replace that spring with one that deforms only a twentieth of an inch. Perform the experiment and let us know.
    Considering the springyfoot suggests many questions, most of which ibericb has addressed already. Most importantly, not "Is there any flex," but "How much flex under what conditions, and does it matter anyway?"

    I got to wondering about time: if the rider compresses his springyfoot, but the spring then extends before 6 o'clock crank position, is any power actually lost (not counting negligible heat in the spring) or just displaced a bit in time? The result of which would be a "feel" issue, as noted by ibericb?

    And maybe a little flexy is desirable, aka, compliance? And whattabout super lightweight bikes that are so spindley that stiffness becomes a bigger issue? What if the flex stored in the frame rebounds vertically against gravity, rather than rotationally in the crank?

    The springyfoot analogy got me thinking all this. And indeed, Pirx, seems some real science could be done (or shared if already done) to clarify this. But contemplating the experiment yields a pile of variables: rider weight, power, cadence, subjective feel, frame size and type, wheels, tires, inflation, blah, blah.

    I have no idea myself, of course, and I'd like a clue so my next bike purchase can be more informed than "Stiffer is better! Buy now!" But thinking about it seems to indicate that there's plenty to think about.

  18. #43
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    Quote Originally Posted by Randy99CL View Post
    Look at my first response. The machine I described would work because it eliminates any human interaction.
    Notice that I had already answered this one. The mechanics in your experiment is simple enough to be fully understood. The answer is, there is exactly zero difference. Take it from someone who has published many, many research papers in the general field of theoretical mechanics.

    Quote Originally Posted by Randy99CL View Post
    So it's simple. Describe exactly how the side-to-side movement of the BB contributes to driving the bike forward.
    That's trivial, in principle: Imagine the pedal fixed in space, held in place by a foot that's not moving. Now have the BB rotate (obviously, it's not just a side-to-side motion) as it will when the frame un-flexes. Result: there will be an incremental rotation of the crank axle, which can generate a propulsive force. Of course, in reality such motions will be superimposed on the "normal" crank rotations, and relative phases matter. Here's the thing: For someone who actually understands the physics of this, these details are irrelevant. Moreover, it is understood that the exact details you are asking for are extremely complex, and that common-sense intuition is mostly useless in an attempt to assess the energy gains or losses.

    Quote Originally Posted by duriel View Post
    Acceleration is force acting on mass.
    No, acceleration is acceleration, not a force acting on anything. Acceleration results from a net force acting on a mass. Words have meanings, a nd meaning is important.

    Quote Originally Posted by duriel View Post
    If your frame flexes, when you hit the maximum force, the frame flexes limiting the force to some level less than that maximum.
    No, frame flex has exactly nothing to do with limiting the force. The force is exactly the same.

    Quote Originally Posted by duriel View Post
    Therefore your maximum acceleration is less.
    No, it isn't.

    Quote Originally Posted by duriel View Post
    This force is returned to the system when the frame unflexes, but at that point the output level may not be there to allow the use of the force.
    Huh? I cannot parse this into anything meaningful.

    Bottom line: Don't even try to understand the mechanics of this process based on some vague remnants of your high-school physics, if any. You either understand energy conservation, in which case you can make some general statements that are precise, or you don't, in which case you'd be forcing yourself into either experiments, or very detailed computer simulations. Both of these will typically be far beyond your reach, and grasp.

  19. #44
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    Quote Originally Posted by Randy99CL View Post
    ...So it's simple. Describe exactly how the side-to-side movement of the BB contributes to driving the bike forward.
    The determination that a frame's lateral flex does not lead to power loss is not new. The BQ work relied on a new FEA to ascertain or confirm what was explained well in 1974 by Calspan in a research report done for Schwinn. In that report you will find both the mathematical evolution and proof of the concept , as well as some limited practical test results to determine the extent of lateral and vertical deflection on different Schwinn frames of the era.

    Quoting from that report:

    "For all pedal force deflection curves which go through zero at the deflection extremities, any energy which is lost in frame deformation at a particular instant is subsequently recovered fully in the form of propulsive energy, since the instantaneous spring energy, and therefore the cumulative energy lost in propulsion, returns to zero at every half cycle of the rider's force deflection curve. The only qualifications to this conclusion arise from conditions previously stated, specifically:

    • that the forces between rider and bike are as shown in Figure 2c

    • that energy lost in the form of heat in the frame material is neglected."

    An explanation of the physical mechanisms that follows the preceding proof will be found on page 13 of the report, which you can read for yourself.

    There is a very important caveat to note, that is well explained in that report, and that is the elastic return of propulsive force is dependent upon the force applied to the cranks vs. crank position (the first qualification noted above). That result is predicated upon zero force application with the pedals in their 12 and 6 o'clock positions. That is the fairly normal and typical seated force profile, or very near it. A departure from the efficient return of propulsive force would be expected when the rider applies appreciable force when the pedals are near vertical. That might typically be expected in an out-of-saddle extreme pedal force situation, such as climbing or sprinting. In those situations then some small fraction of the force applied by the rider to the pedals at those positions should be expected to be lost by spring work being done directly on the rider himself in a way that cannot be converted into propulsive force. However, force being applied at those positions in likely from supportng the rider's weight, and would not be constructive in propulsion to any significant degree anyway.
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  20. #45
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    Quote Originally Posted by Randy99CL View Post
    Describe exactly how the side-to-side movement of the BB contributes to driving the bike forward.
    I'm no expert on physics, but I think the side-to-to side movement of the BB is irrelevant to forward propulsion. The force going down on the pedals turns the crank around, driving the chain and thus the rear wheel. Whether or not the sideways force applied by the rider flexes the frame or not would not change the downward force. I don't think you can steal the sideways force and make it down force just by making the bike stiffer. The sideways force would remain the same, just not flex the frame sideways as much if the frame was made stiffer. The sideways force always remains sideways force.

    Very crude analogy: (Pirx, correct me if I'm wrong and the analogy makes no sense)
    If you apply a sideways force to a lightweight ball rolling down a hill, it will go sideways, but continue downhill at same rate as it was. Apply same sideways force on a much heavier ball rolling down the hill, it, too, will continue down the hill at its same rate, but won't go sideways as easily. The sideways force will not slow either ball from its downward roll.

    So, I'd agree with Pirx that frame stiffness is marketing hype. Especially since they have now been stiff enough for years that mortals can't really flex them, anyway. Recently I read a paper by a wheel company (Zipp maybe? Can't recall atm) saying they have to purposefully build flex into their wheels, because too stiff a wheel causes the rim to rub the brake pads (and then customers would complain the wheel wasn't stiff enough because it was rubbing. Customers would have had it backwards!)
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  21. #46
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    Quote Originally Posted by Pirx View Post


    That's trivial, in principle: Imagine the pedal fixed in space, held in place by a foot that's not moving. Now have the BB rotate (obviously, it's not just a side-to-side motion) as it will when the frame un-flexes. Result: there will be an incremental rotation of the crank axle, which can generate a propulsive force. Of course, in reality such motions will be superimposed on the "normal" crank rotations, and relative phases matter. Here's the thing: For someone who actually understands the physics of this, these details are irrelevant. Moreover, it is understood that the exact details you are asking for are extremely complex, and that common-sense intuition is mostly useless in an attempt to assess the energy gains or losses.
    I believe I was penning my previous contribution when this went up. FWIW, it's the same model and explanation offered by the Calspan boys 41 years ago. That's one of the beauties of physics - valid principles are enduring.
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  22. #47
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    Quote Originally Posted by Jwiffle View Post
    Very crude analogy: (Pirx, correct me if I'm wrong and the analogy makes no sense)
    If you apply a sideways force to a lightweight ball rolling down a hill, it will go sideways, but continue downhill at same rate as it was. Apply same sideways force on a much heavier ball rolling down the hill, it, too, will continue down the hill at its same rate, but won't go sideways as easily. The sideways force will not slow either ball from its downward roll.
    That's a perfectly fine analogy.


    Quote Originally Posted by ibericb View Post
    I believe I was penning my previous contribution when this went up.
    Yep, you did. Read the article, and, yes, the mechanism described there is the same as the one I put forward. It's better explained in your article, though

    Thanks!

  23. #48
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    Quote Originally Posted by ibericb View Post
    The determination that a frame's lateral flex does not lead to power loss is not new. The BQ work relied on a new FEA to ascertain or confirm what was explained well in 1974 by Calspan in a research report done for Schwinn. In that report you will find both the mathematical evolution and proof of the concept , as well as some limited practical test results to determine the extent of lateral and vertical deflection on different Schwinn frames of the era.

    Quoting from that report:

    "For all pedal force deflection curves which go through zero at the deflection extremities, any energy which is lost in frame deformation at a particular instant is subsequently recovered fully in the form of propulsive energy, since the instantaneous spring energy, and therefore the cumulative energy lost in propulsion, returns to zero at every half cycle of the rider's force deflection curve. The only qualifications to this conclusion arise from conditions previously stated, specifically:

    • that the forces between rider and bike are as shown in Figure 2c

    • that energy lost in the form of heat in the frame material is neglected."

    An explanation of the physical mechanisms that follows the preceding proof will be found on page 13 of the report, which you can read for yourself.

    There is a very important caveat to note, that is well explained in that report, and that is the elastic return of propulsive force is dependent upon the force applied to the cranks vs. crank position (the first qualification noted above). That result is predicated upon zero force application with the pedals in their 12 and 6 o'clock positions. That is the fairly normal and typical seated force profile, or very near it. A departure from the efficient return of propulsive force would be expected when the rider applies appreciable force when the pedals are near vertical. That might typically be expected in an out-of-saddle extreme pedal force situation, such as climbing or sprinting. In those situations then some small fraction of the force applied by the rider to the pedals at those positions should be expected to be lost by spring work being done directly on the rider himself in a way that cannot be converted into propulsive force. However, force being applied at those positions in likely from supporting the rider's weight, and would not be constructive in propulsion to any significant degree anyway.
    Another quote from the Conclusions and Recommendations chapter of the same report:

    "These initial results show that the stiffness of a frame can be directly related to its efficiency of energy utilization, but the characteristics of the rider's input forces must be better known before this efficiency can be quantified.......the study of the rider's bicycling characteristics could be done together with energy measurements of the bicycle under actual pedaling conditions......"

    and another:

    "The relationship of frame properties to energy use is only part of the question of bicycling efficiency. Also of importance is the rider's ability to utilize a certain bicycle's design characteristics and the importance of efficiency /weight ratio in various modes of bicycling."

    which brings us back to "Try it to see if you like it".

    Now, this from someone with a bit of hands-on experience on the matter:
    Frame Flex | Kirk Frameworks

  24. #49
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    Quote Originally Posted by dcgriz View Post
    which brings us back to "Try it to see if you like it".
    I think that's a perfectly healthy way to look at it, even apart from any performance benefits or lack thereof. Don't forget that, given the very small amount by which any modern frame flexes, if some frame indeed turns out to be less efficient for a specific rider, then the loss of efficiency will be minute, and well within the noise of other effects. Thus the best advice is to ride the frame you like, even if it's a more flexible one, and ignore the illiterate marketing drivel.

  25. #50
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    Quote Originally Posted by Pirx View Post
    That's a perfectly fine analogy.




    Yep, you did. Read the article, and, yes, the mechanism described there is the same as the one I put forward. It's better explained in your article, though

    Thanks!
    It's true that the frame is storing and returning energy elastically given that the frame is not deforming, but I wonder.

    The frame is storing energy during that part of the pedal stroke where the force on the crank arm is increasing, let's say from about 2 to 5 o'clock for the right side arm. It's returning that energy as the force from the leg to the crank arm is decreasing, say from 5 to 6 o'clock. I think the unloading must be much faster than the loading. Do both flexions balance in terms of torque to the drive train? Neither one is going to be balanced by force from the other pedal.
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