#61
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Pretty much every component on a bicycle is designed to operate on a very linear plane. The more a frame flexes, every bearing, bushing and connection/contact point is being loaded and operated outside of this ideal plane. That is friction and loss in the same way chainline losses occur. The stiffer the frame the less loss to these tangential loads.
Again, you can argue that this loss is miniscule and insignificant in the real world (I'm guessing you don't invest in oversize pulleys and ceramic bearings) but it is watts. It cannot not be. The only possible argument for frame flex leading to a more efficient bicycle is rider comfort and fatigue factors on the rider. But 2-5mm wider supple tires and 5-10psi less tire pressure will do magnitudes more to increase comfort than a flexy frame. This conversation is like the the ontological argument for the existence of ____... the sasquatch. I get it.... it's not so much loss to frame flex, but it sure as heck is no gains. So hey, if you like the flexy frame.... ride the hell out of it!
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cimacoppi.cc Last edited by rain dogs; 02-07-2018 at 02:35 PM. |
#62
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Just from my humble and non-professional experience:
1) Stiffness is a fine line. 2) I've compared two bikes side by side on a computerized diagnostic set up - one was a fillet brazed number made of Columbus Foco tubing (anyone remember that light weight stuff?), the other was a new 2007 Orbea Orca. The Orca was more efficient compared to the Foco tubed bike, but please note that the Foco tubed frame was shot. No really, I had ridden the snot out of that thing for 8 years, and say what you will, a lugged steel frame will hast longer than a non-lugged one. 3) Carbon can be VERY stiff. So stiff, in fact that over a road course, the accumulated micro vibrations can soak into you and wear you slap out. 4) Carbon is it's own worst enemy - What I mean is that it's a great ride for the first few years, but that stiffness helps to also degrade it over time. In the end, it's way less efficient. 5) What happens to plastic when you leave it in the sun? Think what happens to your carbon bike. 6) Last year I reconditioned a 1971 Raleigh Pro for Eroica California. I road that almost exclusively for 2 months. At the end of that time, I got back on the Orbea (now 10 years old) and felt invigorated...like "yeah, this beast is fast!"....until I checked Strava afterward. I was worn out and exhausted and it wasn't even a fast pace. Two days later, I repeated the same course with the 1971 Raleigh. Result was a faster time and I felt refreshingly great at the end of 30 miles. I repeated this little experiment a few times to see if it wasn't a fluke. It wasn't. 7) Sold the Orbea frame and purchased a new lugged steel frame (Bottecchia Leggendaria), and have had incredible rides ever since. So, for me, yes....a little flex is good. Too much flex is not. FWIW....just my experience. |
#63
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I disagree that bigger tires are a better solution than flexible frames. The frame flex appears to be energy we can recover, but sidewall flex of larger rubber tires is heat loss. A balance between the two would be more efficient than a stiff frame and big tire. |
#64
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2. Bigger tires are faster and more efficient that has been proven time and time again. 3. You cannot recover 100% of anything. It's better to lose as little of it in the first place.
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cimacoppi.cc |
#65
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Energy stored in a spring is inversely proportional to the spring constant (stiffness). Since the linear stiffness between the BB and hub is so very high, there is virtually no energy stored in this deflection. Instead, there is far more energy stored in lateral and torsional deflection at the BB - but since these deflections are orthogonal to the chain force, you'll have to come up with a mechanism to redirect these forces/energies. |
#66
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A simple exercise: Standing my bike up and leaning it like it would be in a spring, I apply pressure to the crank arm (at the bottom of the stroke) and the frame flexes. The quicker and with more pressure I do it and let off, the quicker it snaps back. In a sprint under hard torque where does that rebound go?
Interesting to note how much flex this guy is getting in the handle bars and built up stationary bike frame when he is applying power. He can't rock the bike as much as in the real world but consider if the guy in the GCN video could flex that frame by just standing on the pedal and holding the brake, how much flex will there be in a full on standing sprint when he mashing and torquing the pedals and handle bars back and forth? Quite a bit I would imagine. https://www.youtube.com/watch?v=kiBw8mSgjRU William |
#67
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If you look at that guys related video with the push down/pull up exercise, there is a lot of torque being generated.
William |
#68
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#69
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I don't know why you don't understand what I mean. Quote:
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#70
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I was a pole vaulter in high school. Sprint, plant the pole, pole smoothly flexes, you change direction, pole expands to full length and you let it go way up in the air. But one time the pole broke just at the beginning of bending, and a tremendous amount of energy was released all at once into my arms and I just fell forward into the mat. It was awful, but demonstrated how that controlled release of energy is useful and immediate release is not. |
#71
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https://web.archive.org/web/20060214.../Frameflex.htm As for heat loss, which yes exists, I've been looking over this paper Elastic Hysteresis of Steel, which happens to use drawn tubing almost identical in stiffness to a 25.4 steel tube with .5mm wall thickness (although admittedly it isn't the exact composition or manufacturing process as bicycle tubing). Given the results on page 535 and some quick "broscience" calculations I did, the heat loss under very hard pedaling (1-2cm lateral flex) is something around the magnitude of 10^-4 watts which doesn't even put it on the same magnitude as oversized pulley wheels. Also, the damping ratio of composite and aluminum is in the same order of magnitude of steel. The argument for lateral flex doesn't have much to do with vertical compliance. In fact, I'd almost argue that bikes that are more laterally flexy tend to be less vertically compliant, since flexy bikes tend to correlate with horizontal top tubes, meaning longer seat tubes, meaning less exposed seatpost (which is the only thing that really flexes vertically on the rear end of a bike). |
#72
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If there was any heat generated due to frame flex, it would be so small that you would never notice it by touch. |
#73
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Sounds insignificant to me. Meanwhile, I suspect the role of frame stiffness has mostly to do with ad copy/sales...
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#74
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I relate with this post a lot.
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#75
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2. Bigger tires are faster if we are talking 20s vs 23s or 25s. Above 25s the rolling resistance goes up when you are riding the correct pressure for your weight. The only graphs that show decreasing rolling resistance with size are when all the tires are at the same pressure. No one is riding 28s at 110psi. 3. That's correct, but you can choose where you lose it. Which is faster: A four wheel drive sports car or a mechanically more efficient two wheel drive? If (and this is an if) a flexy bike aligns forces with bearing better and keeps tire contact better, than the flex loss would be worth it for increased bearing and tire efficiency. |
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