Flex vs "Rebound"

[Gloersen]

..."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?

[chrislandy]

kind of yes but also not really.

 

while yes, a stiffer material generally has a quicker springback, it's not just a function of the materials stiffness.

 

If each ski had the same (EI) stiffness (or flexibility, ie how much it deflects for a given load), a timber ski would snap back slower than an aluminium one which would snap back slower than a carbon fibre one.

 

my understanding is that it's down to the micro / macro level of the material and the actual internal bonding of the molecules and how they store strain.

 

 

[Bruce_Butterfield]

Its probably better to think of flex and rebound independently even though they can be related and will be very material dependent. Think of a car with a spring and shock absorber. If you remove the shock absorber and sit on the hood then jump off the rebound will be very quick. Now put the shock absorber on and the rebound will be much slower even though the flex (amount the car went down) is the same.

 

In this example the flex is determined 100% by the spring while the rebound is determined (almost) 100% by the shock absorber.

[Than_Bogan]

It's easy to experience this at the MUCH slower time scale of the pole bending and unbending in the pole vault. Even the highest end poles mix glass and carbon fibers, because with carbon fibers alone, the "rebound" is much too fast when the flex is right. It basically bucks you off instead of working with you to push you higher.

[kurtis500]

@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.

[Horton]

@Gloersen

 

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.

[Horton]

@kurtis500 will you be conducting ASTM testing on your ski? If so why not?

[kurtis500]

@horton "if so why not" - care to expand?

[Horton]

@kurtis500

haaa ok. multi-tasking leads to poor english.

[Gloersen]

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.

 

[chrislandy]

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

[kurtis500]

@gloersen

 

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_Bogan]

@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.

[tap]

@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.

 

 

[Horton]

@tap

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?

[tap]

@Horton did you just ask if you can make a ski softer by making it more rigid?

 

No.

 

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.

[Horton]

@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.

[Adam Caldwell]

Having played with tip flex/rebound/damping a bit, I will say there is one phase of the course comes into play in a MAJOR way that most people probably don't think about - which is the unloading of the ski through the white water during the edge change.

 

We've added/modified/built some things that slowed down the skis ability to return to shape. All I can say is - it can make the edge change feel very strange. Normally you feel the ski 'land' and almost instantly move into a turn. Slowing down the skis rebound characteristics made it not finished the oscillation until you were almost at the ball. Not a good feeling.

 

Originally the intent was to allow the ski to stay flexed and locked in the turn longer on the back of the ball (only for super short-line ski design/setup). Which it did do, and was awesome - but the negative side affect of the edge change situation was so bad we scrapped it.

 

 

[tap]

@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...

 

 

 

[kurtis500]

Similar thread here with identical questions asked. https://www.ballofspray.com/forum#/discussion/22768/xtrcc-and-9970cc-two-legendary-shapes-supercharged-with-revolutionary-carbon-core-technology

 

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?

[chrislandy]

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

 

 

 

[tap]

@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.

[ALPJr]

Must be mid winter.

[kurtis500]

"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....

[ToddF]

@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.

[BG1]

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.

[chrislandy]

@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:

[tap]

@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.

 

 

 

[Adam Caldwell]

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.

[tap]

@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.

[Adam Caldwell]

@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.

 

 

[dchristman]

This has got to be the most stimulating ballofspray thread ever!

[Adam Caldwell]

@Horton might need to add a "geek factor" to some of these threads....

[AdamCord]

Seriously even I think you guys are nerds

[Than_Bogan]

Classic "takes one to know one" scenario!

[tap]

Yeah well... I thought this was a normal conversation

[kurtis500]

Nerds build airplanes and put men on the moon

[Horton]

[chrislandy]

@Horton I probably use at least half of those equations and diagrams on a daily basis

[Than_Bogan]

@chrislandy Good point! Usually in memes, the "math" is either nonsense or just the most obscure thing someone could find on the internet. But that kid is looking up at a variety of useful tools!

[Horton]

I don't need math. I have Excel (Google sheets most of the time)

[VONMAN]

Ok going out on a limb here. My take is the ribs do give additional torsional strength along it's length and give strength to the top and bottom of the ski. Think of the cross section of the ski, flat on top and tunnel on the bottom. Short side walls and core in the middle. You have a skier on the top exerting down force and water impacting up force to the bottom of the ski. Over and over hundreds times a set. Imagine what the core is doing in hollow rectangular cross section with the forces being applied. With a least one rib at center you can minimize the flexing and crushing of the core and the breaking down of the core material. I know that designers will build shape on the top of a ski to add strength. How about ribs or a rib only under the skier's boot mounting area?

[tap]

@VONMAN you are assuming core failure is what kills a ski. I agree that is a reasonable guess, but I'm not fully convinced. Or at least I'm not convinced that core failure is always the cause, not anymore. Back when skis were made from earlier generations of polyurethane core I would probably agree with you. However, with the better core materials being the standard nowadays for the high end skis (PVC's, SAN's, PMI's, etc.) I think there's room to argue that the carbon/epoxy may be the first to fail depending on the specific ski, materials used, load case, etc... I could even argue that the laminate is the dominate point of failure.

 

Here's my logic... ski designers are always weight conscious, all else being equal. Ski design is by and far an evolutionary design process, meaning that most design changes are field tested by trial and error. The desire to pull out weight affects the laminate as well as the core. There is a reason a ski has 5 or 6 lb/ft^3 PVC foam in it, most likely because someone built one with 3 or 4 lb/ft^3 core and it broke. I suspect the very same holds true for the laminates used, meaning at some point in time somebody tried to make a ski with less laminate and it broke.

 

So with that logic in mind, both the laminate and the core are being pushed to an appropriate limit of the materials respective strength. And, here's the kicker... the higher end foams have a much higher strain to failure than carbon/epoxy. At least that's true for PVC and SAN. I'm not sure about PMI, it may as well I just don't know to much about it. I'm generalizing as the fiber orientations and the direction of strain has a big effect on the carbon/epoxy, but we're talking order of magnitude difference here. So, in the right loading condition, the core may just keep "stretching" thereby forcing the laminate to pick up a higher percentage of the load, potentially all the way to failure.

 

My comments here are purposefully general. Your comment seems to be specific to 'core crushing'. Is core crushing really a problem with slalom skis? I have no idea. I know it is with trick skis as they get heel dented quite often from flips, but I've never seen a dented slalom ski, other than a handle pop. If it is a problem then I would think a slightly higher density foam insert under the foot would also be a very appropriate solution.

 

Don't get me wrong here... you can make excellent ribbed skis as well as non-ribbed skis. I guess I'm just suggesting that structural design is very complicated and takes a fair bit of effort and testing to get it right, and some of the generally accepted assumptions may not be correct.

[kurtis500]

@VONMAN are you referring to the ribs in the other thread? Goode ski?

[VONMAN]

@kurtis500 Yes I was. Most of the ski's I've seen when they break/fail happen between the boots. Sometimes because of years of use. Sometimes because they were designed to thin in the quest to have more flex. Why not add ribs as a strength/flex only in high stress area. With a good CAD Stress analysis software it can be checked for proof of concept.