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Originally Posted by ShannonM
(Post 22197549)
Basically, the bicycle returns almost all of the energy stored in its flex by the first pedal stroke on all subsequent pedal strokes, minus some very-small-but-not-zero amount that gets converted to heat ...
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Originally Posted by tomato coupe
(Post 22197701)
Since you know so much about this, maybe you can tell us the time required for the bottom bracket to naturally return to it's unstressed position and the damping time for side-to-side motion of a bottom bracket. In other words, what are the resonant frequency and Q factors for side-to-side motion of a typical bottom bracket?
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I know someone who had his students do a survey of the resonant frequency of bike frames. They never found one that had a resonance less than 10Hz. They don't resonate much at all when fully assembled. Putting the wheels in eliminates the lower resonant frequencies. The motion is dominated by the static loading, not any kind of vibration phenomenon. I have never quite understood how the deflection of the bb would store energy that would be dissipated by moving the chain in a productive way. But then again, it probably can't take that much energy to get it into the deflected position in the first place. So you can't expect to get anything out of it.
I have seen a slow motion video of fork deflection when someone did a bunny hop. It was scary as all get out. I think the fork did a couple of heavily damped cycles and then stopped. I think the main thing that changed as frames got stiffer was less problems with chains rubbing on front derailleurs. |
Originally Posted by tomato coupe
(Post 22197701)
Since you know so much about this, maybe you can tell us the time required for the bottom bracket to naturally return to it's unstressed position and the damping time for side-to-side motion of a bottom bracket. In other words, what are the resonant frequency and Q factors for side-to-side motion of a typical bottom bracket?
Intuitively, I'd think that the ~3 Hz input from the rider's legs would dominate, but I don't know if that's true. I can't imagine how it would matter... how is the frequency domain behavior of the frame supposed to generate significant losses that would vary directly with frame stiffness? With apologies to the guys at Singer Porsche, everything does not matter. --Shannon |
Originally Posted by ShannonM
(Post 22197808)
I don't know, and I'm not aware of any measurements that have been done.
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Originally Posted by tomato coupe
(Post 22197821)
Give us an estimate then.
As this is almost 200 times the pedaling frequency, it would seem to be irrelevant to the discussion. Why do you think it matters? --Shannon |
Originally Posted by ShannonM
(Post 22197871)
Around 500 Hz for a 1" diameter thin-walled tube around 22-24" long, according to the people who make wind chimes. Q would be quite narrow, or tubular bells couldn't work.
As this is almost 200 times the pedaling frequency, it would seem to be irrelevant to the discussion. Why do you think it matters? |
Originally Posted by tomato coupe
(Post 22197882)
An empty tube is a horrible model for the side-to-side motion of a bottom bracket. Not even close.
And you still haven't answered the question of why this number that you're demanding is relevant to the question at hand. Do you think there is some frequency domain mechanism at work that would make "flex wastes energy" a true statement? Like, what... phase cancellation? Resonant heating, like a 3 Hz microwave? Harmonic distortion in the rider's 5th chakra? In other words, do you actually have a point to make at all? --Shannon |
Originally Posted by ShannonM
(Post 22197888)
Sorry, I thought you were asking about the steel itself. Mea Culpa, my bad, whatever.
And you still haven't answered the question of why this number that you're demanding is relevant to the question at hand. Do you think there is some frequency domain mechanism at work that would make "flex wastes energy" a true statement? Like, what... phase cancellation? Resonant heating, like a 3 Hz microwave? Harmonic distortion in the rider's 5th chakra? |
Originally Posted by tomato coupe
(Post 22197901)
You made a broad statement about frame losses that directly involve the side-to-side motion of the bottom bracket. If you can't even provide rough estimates for the parameters that determine the motion of the bottom bracket, you have no hope of describing the dynamics of the bike+rider system.
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Originally Posted by Moke
(Post 22197618)
My only thought to add to the pile is Sean Kelly didn't seem to be hampered on his old twist -o -flex Vitus. I thought mine was down right scary in a sprint. My second worst bike I had the builder "Make it as stiff as possible". I was a big guy (165 lbs) at the time. It was fun to ride for about an hour on a smooth crit course. An hour on a bumpy road was more than plenty and I just wanted off. So my final answer is yes and no. Horses for courses.
https://www.cyclingweekly.com/news/l...tus-979-194179 |
If I need a bit more suspension, I can just put 5 psi less in the tires, it seems like the most logical place to have, well, flex.
The power losses may be negligible, but you just couldn't argue in favour of "more frame flex".In the modern days of CF goodness you can have a nice, stiff bike which is also light and aero (and good looking to boot), and which feels really solid when you're descending at speed. |
Originally Posted by ShannonM
(Post 22197549)
OK, so Robin Williams' First Law of Comedy has been fully honored in the thread, but I was completely serious and not trying to generate jokes.
When we pedal, there is some displacement of the centerline of the bottom bracket relative to the centerline of the bicycle. Obviously, making that displacement happen requires an input of force. The real question is, is that force lost, to any degree that matters? The answer to that question is, so far as anyone who's ever tried to measure it has been able to determine, no. It turns out that things like frames, crankarms, stems, seatposts, handlebars, etc are very,very efficient springs. Which makes perfect sense... they're mostly made of metal, they have few if any moving parts, and there's not a damper anywhere to be found. Metals have vanishingly low internal losses, things that don't move don't generate friction, and there's not a part whose job it is to convert force into heat, so how can the pedalling force be lost? Where's it gonna go? It's going to be returned into the system, because we're talking about undamped springs, and that's what springs do. Where and when is it going top be returned to the system? This is a more interesting question. Springs release their stored energy when the force compressing them (energy being put into the spring) is less than the force the spring is exerting on whatever is loading it. Now think about pedal loads as a function of crankarm angle. They're increasing between roughly 1:30 and 4:30, peaking at the end of the downstroke. The rest of the circle, they're not very large at all, because almost nobody pulls up in the back half of the stroke, and even those who do don't do it very hard, so not much force is being applied. But, wait... bicycles have two crankarms and pedals, because most bicyclists have two legs. That probably matters a lot, no? So, our right pedal is at 5:00 or so, the force on that pedal is falling very quickly, (it'll be almost zero by 7:00,) the bottom bracket area and crankarm are at their maximum leftward deflection, and there's some energy stored in that deflection. What happens next? The left crankarm is at 11:30, and the force on it is just starting to increase from its minimum. So we have a double null in the force input by the rider... the legs aren't adding much energy into the system. So, the energy stored in the leftward deflection of the crank area becomes, for a brief, glorious moment, the largest force in the drivetrain. And what it wants to do is to drive the deflection to zero. And, since it's displaced to the left, that means moving things rightward. Which is exactly what the rider's left leg is going to start doing when the left foot gets to 1:30 and the left leg becomes the dominant factor in the system. Basically, the bicycle returns almost all of the energy stored in its flex by the first pedal stroke on all subsequent pedal strokes, minus some very-small-but-not-zero amount that gets converted to heat because TANSTAAFL. In other words, your legs only pay the full cost of distorting the bicycle once. After the first stroke, the bicycle itself helps you to deflect it, so you don't have to. And that's why stiffness is not a factor in bicycle performance. --Shannon PS: I'm pretty sure this is most of what Jan Heine and the BQ crew are talking about when they talk about "planing." They go farther than I'm comfortable with in saying that bicycle flex is a net positive for performance... although they do have some data to back that claim up, and they show their work. All I'm confident in saying is that it isn't a net negative. https://www.bikeforums.net/22196241-post13.html |
I love it when people use principles of physics and engineering .... without any experimental data.
Great that you have studied science .... might I suggest that we review Bacon's scientific method? Until you have done experiments and observed the outcomes, and checked whether your results support your hypothesis .... this is all just, to use the precise scientific term, "used food." |
Some interesting comments so far. Some not so. One thing I know for certain from personal experience, frame flex is overplayed as a way to sell. Two guys in the local race circuit that were amazingly fast and won crits all the time, along with mountain bike races, were on Sling Shots. If any of you have ridden one of these creatures it is at first scary, but when your body and mind acclimate to it, all is well. These flexible fliers performed quite well under the legs of two seriously powerful riders and never seemed to hinder their performance.
Eventually they took their Sling Shots to national level races and found another level of talent out there, but they still faired well considering they had zero altitude living in their lives. Flat landers that rode frames designed to flex an inch or more laterally as well as vertically. It worked for them! |
Jan Heine claims a 12% increase in power for a more flexible steel bike compared to a less flexible one. He does not show his work or the data. He shows a chart and says his legs feel warm on one bike and they hurt on the other. He claims the power meters were calibrated, by whom? Where is the cert. IIRC, there was a BQ article or two on the Specialized Diverge and it climbed better than his flexy steel bike but I could be wrong, I threw out all my BQ magazines and ended the subscription at that point. He measures speed with a GPS device to make conclusions on comparative tests. Real scientifically.
I also do not recall any explanation why the Diverge climbed better than his steel bike. Also, Lael Wilcox rides a Diverge and I tend to look at what top riders like she use more than marketing pieces. I'd like to see one other "study" making the claim that a 0.4 mm tubing steel bike flexy as a noodle makes 12% more power. That is really the gist of this thread. If lateral stiffness at the BB and chainstays did not matter, all those Specialized, Trek, Cervelo engineers need to be fired. |
Originally Posted by Darth Lefty
(Post 22197603)
Springy stuff does not dissipate energy. Squishy stuff does. Your bike is made of springy stuff. Your jiggling corpus, the air you are swimming in, and any oil you brought along to squeeze through some valves in a shock absorber are squishy. Stiffness changes the distance the springy parts flex and how fast. The squishy parts make the springy parts stop flexing.
Science! So I guess the OP is correct, stiffness of my frame doesn't matter because I never think about it. I just got sucked into this thread by the double entendre. |
The question of rigidity or flexibility is complicated. Some bikes have brutally stiff seat stays. This was the reason I bought the cheaper 2016 Cervelo S3 instead of the S5 at the time. The S3 had the stays from the R5 essentially but the downtube of the S5. Best of both worlds. Stiff downtube and chainstays = good. Stiff seat stays = bad. Top tube = ?
I find it hard to enjoy the bunny rabbits when my butt is being pounded. The best modernish bike that I rode was a Felt AR1, the BB was stiff enough and it was comfortable for a racing bike. Maybe a lighter rider would have had a different impression. When buying a bike, this is kind of an important consideration. My noodle Vitus in the 80's was so flexible that I could shift gears stomping on the pedals. But many people thought those bikes climbed well, I might have been too heavy (168 lbs on a 60 cm frame). It also shimmied violently at speed. Fun. |
Whether or not stiffness matters for efficiency, it can make a difference in handling and tracking. This is a big factor in mountain bikes, but I think it is also true to some extent for road bikes.
Regarding the effects on pedaling, in particular out-of-saddle efforts.... I don’t think anyone can deny that out of the saddle hammering on a stiff vs flexible frame feels different. So clearly something is different. Until you can fully explain WHY it feels different and give an explanation as to why that does not in fact matter, then you cannot categorically assert that stiffness does not matter. |
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Interesting comments about frame and wheel stiffness by Damon Rinard in the attached.
I can only imagine the vertical compliance comfort of a newer bike with seat stays attached halfway down the seat tube and a flexible seat post. Tour Magazin is the only one I know who systematically measures vertical and horizontal deflections. Minimizing vertical deflection is important because human tissue does not plane, it absorbs the energy and this also contributes to fatigue. https://cyclingtips.com/2018/04/jra-...-still-matter/ |
is it winter already?
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Originally Posted by Kapusta
(Post 22198168)
Whether or not stiffness matters for efficiency, it can make a difference in handling and tracking. This is a big factor in mountain bikes, but I think it is also true to some extent for road bikes.
Regarding the effects on pedaling, in particular out-of-saddle efforts.... I don’t think anyone can deny that out of the saddle hammering on a stiff vs flexible frame feels different. So clearly something is different. Until you can fully explain WHY it feels different and give an explanation as to why that does not in fact matter, then you cannot categorically assert that stiffness does not matter. Your last paragraph is a clever burden shift--nobody can prove that a non-defined parameter doesn't matter. That has absolutely no implication that the "feel" matters in any way related to performance. And feels different = is different is actually a non sequitur. People "feel" like they're going faster when they can perceive bumps, and interpret smoothness in ride as going slower even if there is no difference in speed from the bumpier ride. |
Many riders feel that tires are faster went pumped to the maximum. I don't feel the onus to provide data describing their feelings or other emotions.
All of my fastest climbing bikes have always been stiff at the BB. You know like objectively. Always the fastest up the same climb. Is that because I am a big rider and like to climb in the big ring? Please show your data and calculations. |
In original post there is an assertion that stiffness does not matter for the racks mounted on a bike. Because if they weren’t stiff enough they would break. Umm, racks do break, they break all the time. Current custom builders are using bigger diameter and stiffer tubes than ever previously in the history of bikes. In hopes they won’t break. This would include custom builders who are still using 1” and 1-1/8” frame tubes.
Second post brings up stiff versus sagging car doors. ???? Car doors sag because of wear and free play in the hinges. A stiff car door would be one that required some force to make the hinge move. Discussion of bicycle stiffness is normally continuous non sequiturs. The Eddy Merckx hour record, still unbeaten after 49 years, was done on a Colnago built with Reynolds 22/28 butted tubes. In old style skinny diameter. Converting British wire gauge to metric gives wall thickness of 0.711/0.376mm. Of course Reynolds never produced anything accurate to 0.001mm, that is just how the nominal converts. But the skinny belly of the tube was less than 0.4mm. Every Category 6 rider knows that such a frame is impossibly flexible and noodly. What was good enough for Eddy would be laughed out of current market. When you can put out 750-800 watts continuously for an hour get back to me. |
Originally Posted by 63rickert
(Post 22198311)
The Eddy Merckx hour record, still unbeaten after 49 years, was done on a Colnago built with Reynolds 22/28 butted tubes. In old style skinny diameter. Converting British wire gauge to metric gives wall thickness of 0.711/0.376mm. Of course Reynolds never produced anything accurate to 0.001mm, that is just how the nominal converts. But the skinny belly of the tube was less than 0.4mm. Every Category 6 rider knows that such a frame is impossibly flexible and noodly. What was good enough for Eddy would be laughed out of current market. When you can put out 750-800 watts continuously for an hour get back to me.
So far this whole thread is basically, "Yes, because I said so .... No, because I said so" repeated continually. By the way ... maybe if he had been riding a modern Merckx bike that hour record would be significantly faster. :D |
Originally Posted by GhostRider62
(Post 22198091)
If lateral stiffness at the BB and chainstays did not matter, all those Specialized, Trek, Cervelo engineers need to be fired.
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Originally Posted by livedarklions
(Post 22198216)
Your last paragraph is a clever burden shift--nobody can prove that a non-defined parameter doesn't matter. That has absolutely no implication that the "feel" matters in any way related to performance. And feels different = is different is actually a non sequitur. People "feel" like they're going faster when they can perceive bumps, and interpret smoothness in ride as going slower even if there is no difference in speed from the bumpier ride.
The OP has stated categorically that stiffness does not matter. Period. And this assertion is based solely on the idea that there is no difference in efficiency. What I am challenging is this idea that same efficiency = no difference. I did not say that since pedaling a flexible frame feels different that it is in fact less efficient. (Defined strictly by power into the pedals vs power out in the wheels). What I said was that it is in some way different. Just because two systems ultimately have the same efficiency (power in vs power out) does not mean that we as biological creatures are always able to deal equally well with them. Take the example of gearing. Strictly speaking there is essentially no difference in the efficiency of a higher vs lower gear. Does that mean it does not matter what gear you are in? Introducing a spring into a system absolutely changes the way we interact with it. And that is immediately evident in how it feels. In fact, how something feels is often one of the best indicators that there is a spring somewhere in the system, be it in pedaling, braking, or pulling on the handlebars. I think your tire pressure analogy is off base, here. Are you suggesting that tire pressure categorically makes no difference? Tire pressure is probably a perfect example of when one should NOT be making categorical statements. Where I think you and I may be in agreement is that one should not assume that they know exactly what their subjective experience is caused by or indicates. People should not assume that the harsher ride of an over inflated tire is faster. And people should also not assume that the flex they are experiencing in a bike means that the bike itself is less efficient. |
Originally Posted by Kapusta
(Post 22198368)
People should not assume that the harsher ride of an over inflated tire is faster. And people should also not assume that the flex they are experiencing in a bike means that the bike itself is less efficient.
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Originally Posted by 63rickert
(Post 22198311)
The Eddy Merckx hour record, still unbeaten after 49 years, was done on a Colnago built with Reynolds 22/28 butted tubes.
Chris Boardman, Manchester, 27.10.2000, 49.441 km Ondrej Sosenka, Moscow, 19.7.2005, 49.700 km Sosenka and his bike: https://i.pinimg.com/originals/28/63...e3c5af4234.jpg |
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