This explains a lot. Some of us have to ski on that stuff or we don't ski.Well, I was trained to run away and search for better snow when I can ear my skis scrapping the ice.....
This explains a lot. Some of us have to ski on that stuff or we don't ski.Well, I was trained to run away and search for better snow when I can ear my skis scrapping the ice.....
I remember reading that Kästle also uses the rubber layer for damping. No one in this thread is talking about Kästle. Any thoughts or comparisons with Stöckli?
What you seem interested in is how an impact on the underside of the ski, kind of directly under your foot (otherwise it would create oscillations/vibrations in that part of the ski) is transmitted to your foot. That is not a vibration. This happens when the underside of your ski drifts and catch-and-release with the snow. The frequency of that is driven by the weight of your foot/boot/ski, the stiffness of your leg, the mass of your upper body, the edge angle, the snow, etc. I would generally think that the ski construction changes nothing to that frequency ( around 5ish Hz).
Here is the thing you need to know: the natural frequency of the system has to be at least three times lower than the vibration frequency in order to not transmit the forcing vibration.
(For the geeks, the forcing frequency has to be higher than the third harmonic of natural).
So a 5Hz forcing frequency needs the ski and skier system to have a natural frequency of 1.6 Hz or lower. So, yes, ski construction has *everything* to do with not transmitting your scenario.
The simplest way to figure out approximate natural frequency is by static deflection. Using a static deflection vs natural frequency graph, we can see that we will probably need a ski that bends at least 5inches under the skier in order to not pass that vibration back. How is that not related to ski construction?
Fortunately, most of the harsh objectionable vibrations from snow have forcing frequencies that are a lot higher than 5Hz - or we'd all be skiing noodles.
It seems that you are thinking that the ski as a spring that acts to modulate that catch-and-release behavior with the snow. I don't think that is what happens, although I agree that the tip and tails are vibrating and that depends on the mass/stiffness of the ski.
Yes, that is the model I am using, except I am asserting the opposite: most skis are not soft enough to isolate that catch and release behavior with the snow at 5Hz.
I am also saying that a 5Hz forcing frequency will excite vibration and harmonics in the ski above 5Hz - very much including the 10 Hz -20 Hz range your references are most concerned with.
It is interesting to think about it this way. Many papers cite the spring stiffness of a skis as around 7000 N/m.
The ski is not really flexing when your bases are flat on the snow
Human leg stiffness is on the order of 70 kN/m for both legs (that is, for a 4Hz hopping motion... about the fastest you can go: https://pubmed.ncbi.nlm.nih.gov/15088239/). If you consider a body mass of 70 ish kg on top of that (and the ski/boot as relatively light compared to that), then you get a frequency of 5 Hz. So this would be the resonant frequency of your legs/body...
Both of these models are also failing to explain why you would measure a 5 Hz resonant frequency while going in straight line (see graph above).
. Impacts excite all the modes of your system at all frequencies. ..
Ok, that is about double the stiffness of a beginner/intermediate ski, but let's assume that's correct. I think that's actually closer to expert/race ski flexes.
There is a really easy way to explain 5Hz in a straight line. Using that picture of a ski that is constrained in a center section with the tips relatively free to vibrate, we can ask, how long does it take a flaw in the snow to traverse that section? Let's say boots are between 245 and 355 mm long, let's say the center section of the ski is, therefore, 40 or 50cm. 5Hz is 0.2 seconds. If a skier with a 50cm locked center section sees an incompressible bump in the snow, 0.2 seconds is 6mph.
Neither the mass of the skier nor the size of the incompressible flaw in the snow matters to this calculation - we should start seeing 5Hz vibration at anything over 9km/h aka 6mph. What we're really seeing is traversal of relatively small imperfections - repeated impacts. This is why I say there is, effectively, a 5Hz forcing frequency.
Notice that by the model I presented we should see a shift in peak power density to higher frequencies as skier speed goes over 20mph - at which the tip and tail very much become engaged.
Your model also seems to assume that the bumps in the snow are spaced by 50 cm. I agree that 50 cm is a reasonable length for the center ski section, but bumps could come with any spacing between them and in any size.
You can train to get stiffer legs and loose weight (or gain muscle mass) to modify the 5Hz resonant frequency
I am saying that because we are not seeing the 5Hz drop off with increased skier speed (and because we are using relatively stiff skis which do not have the amount of deflection that would isolate 5Hz at any normal skier mass), it is not reasonable to use the center-section-only model.
Remind me again of why we would even try to change that body resonance point? Changing the frequency of body resonance doesn't seem useful.
Is the goal not to reduce objectionable vibration at the feet?
You can read this: https://onlinelibrary.wiley.com/doi/pdf/10.1002/pamm.200810713
In this case, for both asphalt and sett, and at different vehicle speed, we can notice the car suspension mode at 10 Hz in the PSD. It is pretty much fixed at 10 Hz regardless of speed or surface. They don't give much detail in this paper about how they performed the measurement, but typically you would measure at each suspension corner of a car, with each corner acting like a single point of contact.
I was joking. Yes, the goal is to reduce the vibration of the feet. My argument is that you can't do much in a ski to change the vibration mode at 5 Hz of the ski-human system. I believe that vibration mode is mostly driven by the human leg stiffness and mass...
Since you mentioned cars again, the characteristic of ski vibration that I've been discussing here is similar to automobile NVH (Noise, Vibration, Harshness). Certainly the car suspension plays into this, but there's a lot more to it. Auto manufacturers use multiple methods and materials to suppress NVH to give a car a "quiet ride". Similarly, there are ski manufacturers who incorporate materials, design, and build methods to suppress NVH in skis. One example is the Stöckli epoxy resins and their process for gluing up a ski that is a trade secret because it results in a more smooth feeling ski on slope.
Read this:
https://en.wikipedia.org/wiki/Noise,_vibration,_and_harshness
NVH is about sound. It makes sense in a car because you want to talk and listen to music. NVH technologies are tuned specifically to reduce sounds (in the kHz range) and they might not work well for other vibrations. NVH technologies purpose is not to reduce what you feel through your hands and butt when you are sitting/driving a car. It is to reduce/absorb sound.
My question is: would you be able to differentiate Stöckli's skis if I ask you to wear earmuffs? If your answer is yes, then it is about something that you feel through your feet. It is about vibrations and not sound. It is highly unlikely that you can feel something with your feet above 50-100 Hz. It is highly unlikely that NVH as applied to car will be effective to improve a ski...
If your answer is no, then it means that you care about the sound of a ski that you ear through your ears. How does a kHz vibration of the ski can affects its performance? Can the snow feel the minuscules vibrations at these kHz frequencies? Does it change how the ski interact with the snow? I believe that the effect is more likely that it effect how you ski through psychoacoustic, but it doesn't have too. If you don't care about the sound or don't notice it, it is unlikely to have an effect. But I don't doubt it can have an effect. Many other things affect my skiing, and it is not always about the ski.
Did you ever heard about the stories of car designers spending endless amount of time tuning the sound of a door closing? Or the stiffness of a seat cushion? It doesn't make for a better performing car, but one of the thing that you do at the dealer is to close the door and push on the gas to accelerate during the drive test. If the sound of the door closing is nice then it leaves you a nice impression. If the seat is stiff, you will feel the acceleration more. None of that is about making a better car. It is about making you feel good about a car and closing a sale.
We can talk about it endlessly, but I think we need to blind test skis with earmuffs to be sure!
** There is another thread on this forum detailing Stöckli resin trade secret (https://www.SkiTalk.com/threads/the-never-ending-stöckli-discussion.8747/page-17)... their resin has been used to make skis (and other things) since 1940...
Your own link you posted states that NVH is about sounds and VIBRATION. Why won't you just accept that this is about vibration that is felt through the feet of the skier. Honestly, I'm done going in circles on this. Sometimes I wonder if you've ever actually skied. I'm out on this thread...
--> above 100 Hz : I doubt you can feel anything through your feet above these frequencies. At 300-400 Hz, it starts being sound.
Sound is not defined by a frequency range, it is pressure waves transmitted through a medium as sensed by a being.
Generally humans can hear from 50-20,000 Hz if they are young but bats can hear well into the ultrasonic (>20kHz) and dolphins can hear way down low.
It is being sensed by an organism that makes it sound otherwise it is vibration.
Virtually everybody can hear 60 Hz hum.
A ski scraping across a surface makes a very broad spectrum "white" noise.
The ski may vibrate at numerous resonances to amplify some frequencies.
If these resonant vibrations are transmitted through the ski, the binding system and boot to the body they can cause fatigue quite quickly.
As a Geezer (and proud of it) I get fatigued quickly by high frequency vibration that is most intense with flat mounted bindings.
That's why I'm such a fan of the marker Piston Plate system which I believe (no data) does the best job of damping high frequency vibration of any binding system.
Snowboards on hard surfaces become a huge "sounding board" which is one more reason they suck except in deep snow.
The damping occurs in the soft boots?Snowboards on hard surfaces become a huge "sounding board" which is one more reason they suck except in deep snow.