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Flex vs "Rebound"

GloersenGloersen Posts: 1,074 Mega Baller
edited February 2 in Technique & Theory
..."features a softer flex that makes turns quick, symmetrical and automatic, which when combined with a carbon core that rebounds more quickly out of the turn, leads to faster side-to-side speed and more space before the next buoy."

Can flex (measured standards; "softer" = lower flex # measurements) and rebound be inversely related?


  • kurtis500kurtis500 Posts: 91 ★★★Triple Panda Award Recipient ★★★
    edited February 3
    @gloersen "flex" and "rebound" (in this context) are determined by the cured composite structure. In this case, the type and amount of carbon/glass, foam and resin all create a ski with a determined amount of "flex" once cured. Change any of those or the amounts and the product acts a little different. The only way to know is for a composite product to go under ASTM type tests to get baseline readings. Afterwards changes can be made and tested against the baseline. From there a "soft" ski or "rebound" can be measured.
  • HortonHorton Posts: 30,178 Administrator

    You are quoting the press release for the Goode WideRide CC?

    It is not a ridiculous claim. The Carbon Core construction is pretty interesting and theoretically makes the ski more durable and allows it to be a softer flex. Because the stress is potentially carried more by the carbon fibers and less by the core rebound could be faster.

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  • HortonHorton Posts: 30,178 Administrator
    @kurtis500 will you be conducting ASTM testing on your ski? If so why not?

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  • kurtis500kurtis500 Posts: 91 ★★★Triple Panda Award Recipient ★★★
    @horton "if so why not" - care to expand?
  • HortonHorton Posts: 30,178 Administrator
    haaa ok. multi-tasking leads to poor english.

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  • GloersenGloersen Posts: 1,074 Mega Baller
    Interesting shock absorber analogy. Wouldn't the amplitude of rebound in the no shock/spring scenario be greater (rebound above static height)? Does a slalom ski rebound beyond its static state(?). The core construction material/technique being the shock absorber?

    The thread is not posed to challenge the Goode ad, but regarding flex, standardized measurable findings are derived. Whereas with “rebound”, it appears this is a quantity (or just a quality noted) empirically derived?

    For slalom skis of identical shape (thickness, profile, length, etc., etc. ), given conventional core vs a carbon ribbed core, can there really be a demonstrably quicker rebound in a ski with a softer lay-up? Same goes for vaulting poles (same conditions of shape equivalence), is “rebound” quantified with a standard measuring technique?

    If construction techniques (as in the carbon core) permit a ski to have a thinner profile with a lay-up providing favorable flex (keeping it thin and not too soft); compared to a “thicker” ski with identical flex; it’s difficult to perceive the “rebound” of either being appreciably different, if measurable. Though it is conceivable that the “thinner” ski (in this comparison) would have a greater flex “displacement” relative to its thickness; call it a displacement/thickness ratio (the thinner and thicker skis having the same flex displacement). So a “thinner” ski, even if rebound is no quicker than the “thicker” ski, might be felt (skiing) to actually do so given a greater displacement/thickness ratio.

    Interesting stuff, and like most aspects of choosing a ski, favorable qualities all depend upon what seems to function best below the skier’s feet. Curious though as to what has been used to define and determine “rebound” as it has been applied to a slalom ski.
  • chrislandychrislandy Posts: 178 Solid Baller
    Does a slalom ski rebound beyond its static state(?)

    Yes, pretty much everything does when using a load and release pattern, by how much depends on the damping of the system, if it's overdamped then it will return very slowly and not necessarily go past it's static state, but generally it won't recover to it's natural state before the next flex starts, underdamped and it's still oscillating before the next flex starts
  • kurtis500kurtis500 Posts: 91 ★★★Triple Panda Award Recipient ★★★
    edited February 4

    The spring back doesnt have to exceed its natural state. Memory foam comes to mind

    "but regarding flex, standardized measurable findings are derived." - 'Standardized measurable findings' would mean using standardized equipment to take measurements from...... i.e. ASTM. Measurements can be made but kept proprietary also. The data exists and is not shared or compared, or ??. My guess is your asking for 'empirically derived' information to understanding how it works but without certain data its like a math problem without some numbers. The ad tells them what it is and what it will do for them and skips over the 'why'. Most people dont care about the 'why'.

    Sandwich construction in it basic form uses the face sheets to carry the bending loads while the core carries the shear load. Its basic, but I attached a pic that helps visualize it. Since the face sheets carry tensile strenght carbon fiber has been succesful here since its inception. Change any one thing in the strength/thickness of the foam and face sheet strength/thickness and its a different product.

    About vaulting poles, its been a while but I believe all the world records since the 60's has been on good old fiberglass...not carbon. May have changed recently. There are many applications where fiberglass is superior to carbon fiber.

  • Than_BoganThan_Bogan Posts: 6,875 Mega Baller
    @kurtis500 It appears you are correct that (current WR holder) Mondo uses a pole with no carbon fibers. (Certainly Bubka did, but that was a long time ago.)
    I haven't yet verified the pole composition for the women's world record.
    Nathaniel Bogan -- GUT Padawan
  • taptap Posts: 105 Crazy Baller
    @Gloersen i feel like you’re baiting me.

    Have any of you ever taken the bindings off a ski and tapped on it. Modern skis are tuning forks, not shock absorbers. The argument that all of this is simply explained away by damping is just plain wrong.

    If by “rebound” you mean the time it takes for the ski to cycle through a forced deflection and back, then to me you are describing the frequency of vibration in some form or another.

    For any physical system, the frequency response is proportional to the stiffness over the mass. If stiffness goes down, so does frequency, aka rebound. Yes damping plays a role, but we’re not even in the right ballpark here. Stiffness and mass are king. And... by mass I mean mass of the system, i,e, bindings, water, skier all contribute. If all you care about is how fast the tip of your ski vibrates while you wheelie out of one ball then I’ll concede it’s mostly just the ski in play, but that’s still not the answer you’re looking for.

    Think in terms of a mass on a spring. Now pull it down and let it go. If you want it to “rebound” faster you need to take away mass or use a Stiffer spring, not softer. And if added a damper it’s just going to rebound even slower.

    The only positive assumption I can make is that the statement is a description of perception and not anything based in physics.

  • HortonHorton Posts: 30,178 Administrator
    edited February 5
    the Goode carbon core construction uses ribs the length of the ski. effectively the core is much more structurally rigid than it would be if you just wrapped foam in carbon. so the question is could you make a soft ski with that construction that would still have comparatively very fast rebound properties?

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  • taptap Posts: 105 Crazy Baller
    @Horton did you just ask if you can make a ski softer by making it more rigid?


    Stating the obvious here, but you can make one part of a ski softer and another part stiffer (faster rebound). Or make bending softer and torsion stiffer. But you can’t make the same part and the same mode of deflection softer and stiffer (faster rebound) at the same time.

    You’re only other option is to pull out mass, and I’m not sure there’s any mass left to pull out.
  • HortonHorton Posts: 30,178 Administrator
    @tap I guess I assumed it was possible because the core would be so much more efficient with the carbon ribs. my thinking was that no matter how high-tech the foam core is it would not be as efficient as carbon fibers carrying the load vertically.

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  • taptap Posts: 105 Crazy Baller
    @Horton I agree, carbon ribs are interesting for sure, certainly opens up the design space a bit more than foam alone and that’s pretty neat from an engineering perspective, but they’re not magical. Fact is, all skis have carbon (or glass) ribs, they’re called sidewalls.

    We could probably fill a couple hour conversation on the mechanical effects of adding more ribs, good and bad, but I doubt anyone cares that much. I started typing a bunch of stuff, but my thumbs started to hurt. One thing is for certain, adding more ribs is not more economical.

    There’s an infinite way to configure a composite rib, everything depends on the details. Stiffness, strength, failure modes, fatigue life, weight, cost...

  • kurtis500kurtis500 Posts: 91 ★★★Triple Panda Award Recipient ★★★
    edited February 5
    Similar thread here with identical questions asked.

    Looking at the cutaway in the ad its hard to tell what the ribs are designed to do specifically. Are they 6 individual I-beams or box-beams...hard to tell.. Obviously co-cured.

    What are the reviews on riding these? I couldnt find any besides whats on the other thread. There are some in the snow ski realm but a different animal.

    @tap how was he baiting you, is this your ski?
  • chrislandychrislandy Posts: 178 Solid Baller
    as we're putting pictures in ;) this describes the "rebound" quite nicely, their terms are slightly different as it's probably American ;)

    "Hard" damping is Overdamped - like @kurtis500 example of a memory foam mattress
    "Light" damping is Underdamped - like a @Bruce_Butterfield spring without a damper or a bell ringing
    The last is "Critically" damped, this is the ideal damping characteristic

    in a dynamic system, when trying to model it (note trying, as there are some variables you need to use as assumptions) there is the force, the mass in the system, the deflection (or movement from the force), the release (gradual, or instantaneous, or somewhere between), then the spring stiffness (EI) which is a combination of the material stiffness, shape stiffness (second moment of area) and length, then you have the damping, this can be external damping (like a damper on a car) or internal damping (damping characteristics from the layup, material properties / characteristics etc). Then you have to model how many degrees of freedom the system has and then you get bogged down in maths for a few days.

    This diagram is typical in calculating the response in structural dynamics modelling

    Changing to the ski, the internal ribs will be a combination of torsion boxes and shear stringers and I would presume that they've also altered the layup and thickness of the ribs to maintain or increase the stiffness as the ski thins down in the centre for the concave and possibly vary the stiffness along the length of the ski.

    That's how I built my wakeboards anyway

  • taptap Posts: 105 Crazy Baller
    edited February 5
    @chrislandy your graph is misleading. Your ‘hard’ and ‘critical’ damping signals both start at time t=0 with a peak amplitude while your ‘light’ damping signal starts with an amplitude of 0 at t=0. These signals are not in phase, so the visual comparison of “rebound” is wrong.

    the statement that “critically damp is ideal” is a stretch and unfounded. There are many many applications where that would be a very bad thing. In the discussion of ski rebound, a critically damped ski will take longer than a lightly damped ski.

    skis are no where near critically damped, they hum like a tuning fork.
  • ALPJrALPJr Posts: 2,382 Mega Baller
    Must be mid winter.
    Stathis VentourisDaveD
  • kurtis500kurtis500 Posts: 91 ★★★Triple Panda Award Recipient ★★★
    edited February 6
    "skis are no where near critically damped, they hum like a tuning fork."
    "Modern skis are tuning forks, not shock absorbers. "

    I have to admit, I'm not getting the tuning fork comparison. help me out here, tuning forks resonate....
  • ToddFToddF Posts: 596 Solid Baller
    @kurtis500 and @Than_Bogan
    Many of the poles from spirit are wrapped with s glass, which also makes them more expensive, and their high end poles.
    Mostly because the s glass is stronger and can use a smaller mandrel also feel lighter, which helps the mental part
    This also gives them different characteristics.

    I jumped on a few of the original carbon poles from pacer back in the 90's and they sucked. They would recoil way to fast for me, was that real or imagined?. Other vaulters loved them and jumped well on them.

    Poles like skis have a specific way they are made and each individual athlete has a physical as well as technical style that leans them in one direction or another.

    I also knew guys who could jump high on any pole, just like some skiers can ski amazing on any ski.

    Pole vault poles have a sail piece to control bend charasteristics. Maybe someone needs to build a "leafspring" into the middle of the ski, it could control or manipulate lots of variables.
  • BG1BG1 Posts: 205 Baller
    A ski does act like a tuning fork when you bump them while holding them in one hand. In the course, the water and the pressure on the ski would dampen a huge portion of vibration from the waterline to the tail, especially when under load. I’ve ridden skis that would oscillate plus and minis 1” when riding flat in chop before turning in for the gates. I could feel it in my front toes but nothing from there to the back heel. I’m sure Regina’s whole ski got a few good cycles in during some of those 10.75 turns she scrambled through this past summer! She’s so good she just made sure the ski was back in phase when she loaded LOL.
  • chrislandychrislandy Posts: 178 Solid Baller
    @tap you are right on the graph, I missed that when I grabbed it. BUT if it's lightly damped then it will still be moving when when another force hits it.

    It's more an engineering principle rather than a specific example on a ski. Although, the damping in the ski example will include the ski material and the effect of the water rather than thinking as them as separate entities.

    and yes, @ALPJr 6 weeks to go :disappointed:
  • taptap Posts: 105 Crazy Baller
    edited February 6
    @kurtis500 sure. communicating via short blurbs of text is not exactly ideal. The "tuning fork" comparison was just to imply that the inherent structure of a ski is not a highly damped structure. Granted "highly" is a very relative term. I was trying to stay within the context of this discussion which was specific to the structure and materials of a modern ski and how "flex" and "rebound" are or are not tied together. Somehow damping got thrown into the mix and everything went sideways.

    If you are just talking about the ski itself and the materials and design, then you would want to measure it by itself. No skier, no water, no bindings, just the ski floating in air. Best I can do is to hold it in such a way as to try and not interfere with its vibration. I just grabbed a modern high end ski off the rack, no bindings, gave it a whack and held it up to a microphone... here's what we get:

    For perspective, that's a ~47 Hz signal. At 47 Hz it only takes about .02 seconds per cycle, so that entire graph is roughly 0.8 seconds long. Not particularly important, just context. What is important is that the signal does not decay anywhere near something that is 'critically damped'. So... I'm suggesting that the structure of a ski is much closer to a tuning fork than a shock absorber. Tuning forks are better designed for radiating noise, but both tuning forks and skis resonate fairly well. Note, if you hold the ski in the wrong spot it won't do anything... the boundary conditions are hugely important. That's why tuning forks have a nice little handle built in at the perfect location.

    All that said... @BG1 put it all into context in a much more straight forward way. The inherent damping of the ski is almost meaningless by comparison to the effects of the water and the skier bolted on top which do provide an abundance of damping to the "system".

    I'm not a ski designer. But I would suspect that the characteristics of importance are dominated by shape, then stiffness (flex) as a close second, then weight as a distant third, and damping is just a fun thing to talk about but not actually important as the water and the skier will take care of that. I say weight is a distant 3rd in the context that a 3.0 lb ski and 3.5 lb ski are still both extremely light relative to the system of the ski + water + skier. Obviously you don't want a 20 lb. ski. I'm not saying someone couldn't eek out that last 1% of performance by going after internal damping within the ski, but I'd be pretty impressed if it was done well and without taking away from the other more important characteristics.

  • adamhcaldwelladamhcaldwell Posts: 729 Open or Level 9 Skier
    There is a very real aspect to damping a ski and how we feel it on the water.

    6 or 7 years ago we built up a lot of skis that had "ribs" through the core of both 45/45, 0/90 and uni carbon. The most dramatic shift was with the skis torsional stiffness. However, far more noticeably was how it felt in the gate and preturn when skiing in choppy water. Made your eyeballs chatter in your skull.

    Years later I stumbled on the old Rathbun/Mapple video where Andy is in his old shop talking and showing off stacks of old prototypes. There were several with "ribs" in the core. I figured they realized the same thing I did. All it is good for is making your eyeballs chatter in the slightest of choppy water at the expense of more meticulous/difficult layup process.

    I'm curious if anyone on these new Goode carbon core skis have noticed something similar or not. I will say that back when we had ribs in the core, we were using heavier PU and not PVC for the core, which I think is also true for the old Obrien skies in Mapples chop shop.
  • taptap Posts: 105 Crazy Baller
    @adamhcaldwell I'm pretty sure we've had this conversation before, but out of curiosity... completely thinking out loud here and only half baked... I wonder if that feeling of chatter is a function of the wave slap impulse passing through the thickness of the ski more so than the ski having less flexural damping. In other words, with a series of very stiff ribs directly under foot tying the top and bottom skins together you may feel much more of the wave slap hitting the bottom of the ski. A foam core ski will behave a bit more like an isolator for the impulse traveling directly through the thickness. I don't know if you could tell the difference, but do you recall feeling excessive bending vibration or just 'slap' for lack of a better word.

    If I had a ribbed ski I'd be happy to measure it and compare against an all foam core ski.

    And so as not to contradict myself from earlier... yes, all skis have sidewalls which are ribs, but sidewalls are not directly underfoot. So that is a unique distinction.
  • adamhcaldwelladamhcaldwell Posts: 729 Open or Level 9 Skier
    edited February 6
    @tap - thats exactly what I deduced from it, but I think the thing to consider is the potential of that wave slap impulse exciting the ski and influencing the magnitude/frequency of the vibration. Not all that different then a seismic event acting on the foundation of a tall building.

    There was a very clear distinction between the energy transferred into your feet/binding plates with the addition of the rib. Given that my binding is a hardshell with no foam anywhere between the bottom of my foot and the top of the ski - it doesn't help the eyeball chatter situation, but it does give me amazing feedback as to what's going on and what I am feeling with the ski.

    If I remember correctly, watching the tip of the ski (which was hard when your vision is blurry), it appeared like the skis tip was oscillating at a much faster rate, but lower amplitude. You could "see it". Other experiments we played with the last couple years attempt to add a 'strut' effect to the skis flex/rebound. These were dramatically easier on the body with less chatter, as the tip oscillations are much slower, but appear to be higher amplitude. We have taken this concept 'too far' to where it looked and felt like the first cycle of flex & rebound still hadn't finish by the time you landed on the buoy line. Not a good feeling.

    Obviously I am being fairly vague on purpose, but with how our skis are 'built' today, I would say they have the least amount of eyeball chattering vibration of any ski on the market when skiing in chop.

    One thing I look at when I watch ALL skiers on ALL skis is the characteristics of tip vib/oscillation during all phases of the course. Some brands do it WAY more more then others especially in chop.

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