wwnick wrote:"A material does not have a resonant frequency. A structure made of that material will have resonant frequencies, and they will depend on the shape and size of the structure as well as the mechanical properties of the materials used to build it."
Kim Aaron, Has PhD in fluid dynamics from Caltech
Aaron has a Ph.D. In aeronautics, not "fluid dynamics." I put the latter in quotes because American universities don't typically award degrees in fluid dynamics. Fluid dynamics, which Aaron's dissertation incorporated, is a subfield of mechanical/aerospace engineering. I'm nitpicking here, but the quote is accurate.
wwnick wrote:so you would need to test for "resonance frequency" to understand a little more about comfort properties of a frame.
Not really. Resonant modes are a fairly simple function of stiffness and mass. For metal frames, you could get the mode shapes/frequencies pretty accurately with a hand calculation. You'd need an accurate FEA model to do the same with a carbon frame, and even then frame-to-frame variations in construction will mean the model only gets you in the ballpark.
You can always pluck the frame and and measure the frequencies that way. You can even use an audio FFT app on your phone to measure the ringing frequency of a plucked frame. The ring-down time would be a pretty interesting...the shorter it is, the more damping is built into the frame.
wwnick wrote:as you say... measurebating...
I'm not picking on you, wwnick, but I most often hear this expression as shorthand for "This discussion has become too technical for me and I have lost interest." I get that I care about this stuff way more than most people, and I don't think everyone needs to care as much as I do. But it's funny how no one uses this expression when their pharmacist is titrating chemo drugs or when aerospace engineers are discussing how to make sure the 787 doesn't fall out of the sky due to wing delamination...
wwnick wrote:would you agree, that, in the end all frames are more or less the same, and the only appreciable difference in a well designed frame is weight. carbon cerca 750g, alu, 1100g, steel 1500g.
No, but the right answer is widely misunderstood. Carbon frames have more inherent damping than metallic frames because of interlaminar shear strain, the same characteristic that causes much of the rolling resistance in bicycle tires. Magnesium is the sole exception...it's really dampy (so to speak) for a metal.
People talk about how aluminum frames beat them up because they're so stiff. That's not quite right. First of all, if you made equally stiff frames from carbon, steel, aluminum and titanium (totally doable, by the way) the initial jolt from an impact would be the same in all of them. The difference comes in how quickly the stop vibrating, and carbon has an advantage here.
I recall that Merlin/Spectrum built a series of titanium match sprint bikes for Ken Carpenter, but abandoned the project because either they couldn't build one stiff enough, or they could but it was heavier than his steel bike. This is silly. Because titanium is less dense than steel with similar strength, you can absolutely build a Ti bike that's stiffer than a given steel bike. And it'll be lighter, too. Any competent sophomore mechanical engineering student could prove it, but there's so much voodoo and hand-waving in the bike industry that silly myths just don't die.