Let's talk about flex

[Horton]

Let's talk about flex. Flex is just one design component of a slalom ski but it perhaps the simplest to understand because it is most often expressed with just 4 numbers. Perhaps it is also the most misunderstood.

 

How is flex measured? A standard flex machine measures how many pounds of force it takes to bend a 24 inch section of a ski 1/10th of an inch. These measurements are taken at standard distances from the tail of the ski (17", 25", 33" and 41"). Some factories measure at additional locations but these 4 are the standard.

 

 

In the most simple terms a softer ski bends more when the skier pushes on it. A stiffer ski might be harder to turn or it may turn more aggressively and might be faster across the lake. Maybe. A softer ski might turn better or smoother or not. Maybe. I say maybe because you can not think about flex independent of all the other design components of a ski such as rocker, shape, tunnel depth, tunnel radius, bevel shape, ski thickness and the always mysterious torsional flex.

 

The trend in the last few year is for ski designers to design skis with softer flex patterns. Not all factories are going in this direction but maybe ½ of the skis I have reviewed in the last few years are much softer than typical skis from 2007.

 

As an example the 2016 Radar Vapor has a softer flex pattern than the 2015 ski. The problem with this information is that EVERYTHING else about the ski is also different from the previous ski. Ski design is an evolving science. The flex patterns on the 2016 skis would likely not work well on skis that were designed in 2006. All of the components of a single ski design have to work together. Simply changing the shape of the bevels may mean that the flex should be changed.

 

My understanding changed three years ago who I had two Warps that were identical except for the flex. One that was the standard flex and one that was the softer flex. I was very consistent on the standard flex. The soft flex ski was more technically demanding and I was less consistent but when I was skiing my best it was clearly better than the standard flex.

 

If you could take out all other factors I suspect that as a whole the current softer flex trend means higher high scores and slightly lower median scores.

 

One last thought. None of this information should drive an informed buying decision. Trust the design team at whatever ski company you like.

 

This is an incomplete brain dump.... written on an airplane

 

[Horton]

 

[Horton]

The top 3 images are sort of new school softer flexes. The last image is a 2014 Radar Vapor. That is a more old school(ish) flex pattern.

[Than_Bogan]

Good info.

 

4 samples of a possibly-not-very-smooth continuous function is a pretty severe under-representation of the information. But it's better than nothing. And when all four of the values are bigger than all four of them for a different ski, calling one stiffer than the other is probably valid.

 

Other than that, I'd be pretty hesitant to use those 4 numbers for anything except measuring the flex degradation of one particular ski over time or testing the manufacturing consistency of two supposedly identical skis.

[Gloersen]

@TAP needs to chime in on this

 

[Wish]

What @Than_Bogan said.

[Horton]

@Than_Bogan it is a VERY small sample set of flex numbers but I am pretty sure the trend is real. Maybe slightly stiffer tails and softer tips for less is the trend. I am also sure that there is something to the softer skis.

 

Testing for break down is not something I really believe in. A ski can die but flex almost like new.

 

It is just one design parameter so as I said above no one should choose a ski from flex numbers

[tap]

@Horton I'll bite, what's your theory for how a ski can "break down" without any change in the flex numbers? This may be a setup, but I am curious.

[Horton]

@tap Below is my understanding from talking to factory guys. I can not claim to have flexed enough skis to say this is from personal experience.

 

The "standard" water ski industry flex test method does not usually reflect the various things that you would call break down. More accurately, a 1/10th inch deflection over a 24 inch span is not sufficient to show that the structure of the ski has changed very much.

 

I am unaware that anyone has built a better flex tester but it can't be rocket surgery. I have always thought a measure of rebound might show us a lot about break down.

 

[Jordan]

@Horton, do you think that general flex, soft or stiff is the biggest influence on how a ski feels or do you think that the difference between say tip and tail flex has a bigger impact?

[Horton]

@Jordan it is absolutely the balance - the way the entire ski flexes

[chrislandy]

@Horton & @tap stiffness & deflection are completely different from "breakdown", I believe breakdown is more a loss of response or rebound as the internal structure of the ski changes over time.

 

This is more a dampening measurement rather than stiffness - just look at material science & engineering, I can design a structure in timber that will be as stiff as steel but the dampening or resonance / natural frequency of the frame will be completely different.

 

I believe this is what happens when the ski "breaks down", the bond between the fibre and matrix (and sometimes core) may over time and with repeated deflections start to degrade, but as long as the fibres are kept in the same place relative to each other, the deflection will be immeasurably similar

[Horton]

@chrisroddy that matches my understanding.

 

I am for sure NOT an engineer but I understand that a fair part of breakdown is the structure of the core degrading. I have also heard that if you look at a used ski under a microscope (electron?) you will see a far amount of micro-fractures in the resin. As you said the bond between the carbon/epoxy skin layers and the core is the other prime suspect.

[teammalibu]

@horton Thats a nice looking flex tester! TW told me after I sent it to him that he tested his old HO that he won a pro event on and it was still extremly stiff compared to current skis!

[chrislandy]

@Horton that makes sense, cores, no matter what type (although some are worse than others) break down with repeated lateral shearing forces.

If you imagine a cut though section, this can either happen within the core or at the interface between the core and the fibre/matrix layers (top and bottom), it's called interlamina shear, it occurs the most when you have a material interfacing with a material with much lesser stiffness. Each part attracts a different amount of the force and thus at a macro/micro strain level each moves laterally (with the bending moment) by a different amount. Over time this bond breaks down and the top/bottom layer detaches from the core.

 

For reference, I'm a structural engineer and did my masters degree thesis on advanced composites ;) I've got some nice pictures which describe the above quite well but can't find them at them moment!!

[Horton]

@chrislandy What you describe is exactly what I have heard for years - you are just more concise : )

[buoyboy1]

My 2016 67" Vapor flex numbers are even softer than what Horton had

(76 96 117 133). Would be curious to know what others found for this ski size.

[Horton]

@Bruce_Butterfield did you mean to give me a thumbs down or did you fat finger it?

[Bruce_Butterfield]

@horton fat finger. Sorry.

[Horton]

@Bruce_Butterfield meh. It happens.

[tap]

@Horton @chrislandy I don’t know how I manage to waste so much time on this stuff, but here we go. To start with, I’ve never subjectively experienced a ski “breaking down”, so I’m not exactly certain what people mean when they say that. However, I can theorize how it could happen. Like most materials, the stuff skis are made of (fairy dust and unicorn tears) will degrade through fatigue loading given sufficient cycles and sufficient load/cycle. The mechanisms of which are varied, you’ve described a few. All of which would show up in a static test via change in compliance (i.e. flex number). The idea that resin micro-cracking has no effect on static properties is a bit far-fetched, when a ski is flexed one side is put in compression while the other is put in tension. Maybe… maybe… the tension side could be measurably unchanged with a Harbor Freight dial indicator (doubt it), but no way would the compression side not show up.

 

The rebound theory, as I understand it, goes something like this: The time it takes for a ski to return to a non-loaded displacement after an applied load is suddenly removed has gotten longer (i.e. the snappiness has diminished). Is that about right? To keep this simple let us consider a ski in a 3-point bend, i.e. a standard flex testing configuration. The flex of a ski can be modeled as a linear spring, F=-kx (Hooke’s Law) where x is displacement, F is applied force, and k is the spring constant or stiffness. Anyone with a flex tester can validate this, simply record multiple flex numbers on your way to 0.1” of displacement then plot applied force vs. displacement. You will get a straight line, pretty much guaranteed. The slope of that line is k, the stiffness of your ski. If you get something other than a straight line then something is wrong with your flex tester, or you really do have a ski made from fairy dust and unicorn tears, or you just crushed your bevels… that sucks. This is the reason why the amount of preload on a flex tester, whether 20 lbs. or 50 lbs., has no effect on the final flex number (within reason). So now that we can think of a ski as a linear spring the discussion of rebound should be much simpler. If I stretch a spring and let it go it will accelerate back to its starting position and then continue moving in the opposite direction beyond it’s starting position until it reaches the same displacement only in the other direction and so forth, simple harmonic motion. The time it takes for a spring to compete one full cycle (stretched – compressed – stretched) is the period (T). So a “rebound time” could be considered as T/4. The period can also be expressed as a frequency (f) or 1/T. So the question is, what affects f? Fortunately, there is a simple relationship for that as well… f=(1/(2pi))*SQRT(k/m), where k is still the stiffness and m is mass. So, the only way to change f, and therefore rebound, is to change either the stiffness of the ski, or the mass of the ski. This is all for non-damped systems. For a damped system the level of damping does affect the frequency, but it is completely negligible for anything we’re talking about here. What damping does do is decreases the amplitude of how far the spring stretches/compresses each cycle, absolutely nothing to do with rebound time.

 

We’re looking for a change in stiffness (flex) or a change in mass. If your ski happened to have a balsa wood core I would buy the change in mass option… but it’s been awhile since anyone has been ill advised enough to do something like that. It could be interesting if someone wanted to weigh their ski over several years of use, but I doubt you’ll gain much more than a few binding screws worth of mass (assuming it’s well constructed). So we’re down to stiffness. This is where I’ll defer to “Science”, as I don’t want to give up too much. But… the idea that you can degrade your stiffness to a noticeable point without a static compliance change (flex) is hard to believe... or that the core breaks down but somehow still maintains its static modulus.

 

If you really want to get to the bottom of this, I’d start with this… the flex numbers written on your ski may or may not be the real starting flex numbers. I have measured a few where the ski stiffened up since it left the factory. So be careful what you compare your numbers to.

 

And just for fun to put this all into context, an actual “rebound time" is on the order of magnitude of milliseconds, or about the same amount of time for the boat to travel roughly 1-2 inches down the course. Which gets me back to where I always end up with this type of thought exercise... how much change has to occur before you can truly tell a difference. I suspect you'll notice that your ski is flexing like a banana long before you notice a change in rebound time.

 

P.S.

Horton: “I am unaware that anyone has built a better flex tester but it can't be rocket surgery.” Instron. Just need someone to fund the testing.

[Drago]

I've always heard what @chrislandy said. Prolly a core-less Carbon ski will not break down, ever . I had a ski that suddenly broke down after I chucked it about 40 feet. Grew up since then.

[Gloersen]

@tap - super cool, thanks for the elaboration and f -formula.

 

is there a triple Dexter award?

 

 

[chrislandy]

@tap it is a damped system though, the materials used (carbon / glass fibre & others) set in a matrix have high internal losses compared to other materials, providing a noticeable damping factor. If the material is subjected to repeated high deflection cycles then the bond between the fibres and matrix fatigues, thus theoretically the "breakdown" is actually a loss of damping.

 

You could also view initial breakdown as the ski just bedding in as the materials relieve any internal stresses from the manufacturing process.

 

I'll have to put one of the wakeboards I've built through a series of deflection cycles and see how it reacts

[Jordan]

@tap I'm interested in your statement that you have tested skis that have become stiffer since leaving the factory. do you think this is from incomplete curing at the time of testing at the factory, or do you think that the materials in fact harden or change somehow?

 

Also very interested in everyone's take on the effect torsional flex has on performance. I have to admit that when I first read about the torsion slot (?) on the Mapple it sent my head for a spin.

[Horton]

@tap all I can tell you is what I feel.

 

If you get two skis - put one in storage and ski a season on the other (200 rides x 36 balls per ride all @ 32 off or shorter). I would bet a few bottles of good scotch that if you pulled out the one from storage it would feel faster off the ball and just "better".

 

It would be a analogous to a ski with a PVC core vs a ski with a PU core. Clearly PVC vs PU introduces other factors but the PU ski will be just a bit sluggish even if it has the exact same flex test numbers.

 

Note: I hope we are all clear that the water ski industry standard flex method works for us but is not a state of the art scientific method. It may not match engineering theory because it is a crude method of measurement. Behind closed doors someone may have a better method but I have not heard about it.

[DaveD]

So we all need to build a flex tester and a natural frequency tester? Anyone know where to get a cheap data acquisition system and accelerometers?

[tap]

@chrislandy For sure, I just don’t think it’s significant, the numbers just aren’t there. To me the smoking gun is change in stiffness. It would be very interesting to hear your results if you do fatigue one of your wakeboards. Sounds like a great use for a wakeboard :)

 

@Horton I’m not doubting your subjective testing, I think subjective insight can be pretty meaningful. I just think it’s worth keeping change in stiffness as a primary suspect. Just need to find a NIST traceable flex tester. As far as PVC vs. PU, everything else being equal, I suspect what you're feeling is a difference in mass. As you're well aware, you would typically run a little denser core with PU to make up for the lower strength properties, at least I would if I was making skis with PU.

 

@Jordan All I can say for certain is some of my numbers have been higher than what was written down. It wouldn’t be unreasonable (not saying this is common, just plausible) to see an increase of a couple points in flex from taking a ski straight from the factory and letting it sit out in the Florida sun for a few days (black skis get hot!). I like to think of flex as a quality control number, i.e. a comparative number from ski to ski to catch any flaws or drift in the manufacturing process. If the numbers are all taken on the same tester after the same amount of cure time then the system should work reasonable well. Plus, I doubt anyone is paying to have their flex tester calibrated, I’m certainly not, so the numbers from one tester to the next could certainly be off by a few points. My point is, if you’re looking to measure degradation where a couple points in flex may be meaningful then make sure to take your own initial readings to compare to. And if possible, do an at-home calibration of your scale with a known mass.

[Horton]

Rethinking this again....

[SkiJay]

A significant consideration with soft ski flex is how smoothly and consistently you load your ski into turns. A soft ski will rip the most incredibly tight carved turns ... so long as tip pressure is loaded early, smoothly, and consistently into and around the ball. But if you end up scrambling and jump on the ski for an "emergency" turn, it may fold like a cheap suit and deliver overwhelming tip pressure—a.k.a. crushing tip-grab.

 

So if you approach balls with a high tip-attitude, then dunk the tip for an abrupt turn after the ball, you'll do best with a stiffer ski. If you get the water to break ~10" forward of your front toes before you arrive at the ball, a soft ski may blow your mind with how hard it carves.

[BraceMaker]

@SkiJay How about torsion. Back to the mapple box construction, raptor skis stating high stiffness with torsional softness, what are your thoughts on how torsional stiffness correlates to that scenario?

[Horton]

@SkiJay I could not agree more.

[SkiJay]

Well @BraceMaker, I have to confess that I've not spent any time testing that one first hand. But FWIW, my opinion on torsional suppleness is that to a degree, it allows a ski to track and behave more smoothly. I'd guess that a torsionally soft and longitudinally stiff ski might be fast and smooth ... depending on all of the other design variables.

[Adam Caldwell]

I invested a tremendous amount of energy from 2012 to 2015 specifically tuning flex and torsion on skis -literally every set.

 

Torsion is a huge contributor to performance and IMO, what will be a significant contribution to the feel of a ski breaking down.

[Horton]

@Ilivetoski

[wawaskr]

Take a look at the picture of the bottom of this Goode Nano 1 XT - see the dimpling? I bet this is caused by holes in the PVC core used to tweak the flex of the ski. Anyone ever notice these? I have them on my XT. The dimples did not show up until I had skied on it for a couple of years.

 

[Horton]

@wawaskr I am 99.9999% sure Goode does not dimple the core to tweak flex.

[wawaskr]

@horton Im not talking dimples....I'm thinking thru holes in the pvc core itself. The dimples are result of this ("mark-off" in composites speak). For Goode, every oz. matters, right?

[JoelHowley]

Well this thread is legit…

 

From my experience it is common to see skis that use prepreg carbon stop performing despite the longitudinal flex remaining the same.

 

However if you take that same ski and flex it torsionally those numbers will generally be softer.

 

Despite having heard this many times I have only seen it confirmed once so there is still a chance that a ski may flex both torsionally and longitudinally identical to the day it was made despite having lost its performance due entirely to changes in the rebound characteristics.

[JoelHowley]

The ideal window for longitudinal and torsional flex is inversely correlated meaning that a longitudinally soft ski which is stiff torsionally will still ski good as will a longitudinally stiff ski which is soft torsionally.

 

My first ever request to Connelly when we were starting the GT was a ski with carbon strands at 45 degrees crossing through the underside of the ski to stiffen it up torsionally. The ski kinda sucked so I put some unidirectional carbon on it to make it stiffer longitudinally and then it sucked even worse, haha. I couldn’t run 28off on it. I have learned a lot since then.

[vtmecheng]

This is a really cool thread, at least for nerds like me. The only part I'm willing to comment on is the testing aspect. Doing any sort of dynamic test is going to be more expensive, probably prohibitively. You would need a test stand that could strike the ski in such a way that it isn't damaged. Then you would need an accelerometer with data acquisition and an A to D converter. That's a lot of money, not to mention I'm not sure how to strike the ski sufficiently and ensure it doesn't get damaged, haven't thought about it enough yet. Boy would that be fun though. If anyone does it, I'm your guy for examination of the accelerations. Acceleration response analysis from shock inputs is what I specialize in.

[eleeski]

Realistically, a slalom ski doesn't get too many critical shock inputs. Jump skis, yes. But a slalom ski is supposed to be on the water carving smoothly.

 

It also should not get a lot of torsional loading. The water is pressing evenly on the bottom of the ski. The water ski tips appropriately for the turn radius. Crossing the wake also doesn't have too much righting moment (ever been lean locked?).

 

It's nothing like a snow ski on ice.

 

There is a synergy between flex and rocker. A stiff ski will need a different rocker than a soft ski. Either can work but just stiffening (or softening) a ski probably won't feel right.

 

Human testing in a slalom course is probably more useful than analysis. More fun for sure!

 

Eric

[vtmecheng]

I should have said that I'm not commenting on if a dynamic shock test would be useful for a ski, I just want to be part of the test and analysis group if it were done.

[Horton]

Bump because of @adamhcaldwell

[Horton]

Bump

[kurtis500]

Curious...

 

Has anyone made skis without a foam or honeycomb core? Flotation cant be the only reason for the core, im guessing the sandwich construction is for the stiffness.

[Horton]

@kurtis500

I highly recommend that you buy Composite Basics by Burt Rutan and read up on sandwich panels. Questions like why skis he have foam cores and pretty fringe conversation for this forum.

[kurtis500]

Maybe you can look in to how autoclaved cured primary structures are NOT filled with a foam core and how they are stiffened. Its a basic of Composite Aerospace construction , and non-cored construction in general, and likely a part of the route I'll be taking. Maybe get the book yourself.

 

I've sold to Scaled Composites before BTW. You'll never guess who ran it.

 

If I come off a little rude, that makes two of us... was simply trying to ask a simple question for a simple answer.

 

[chrislandy]

@kurtis500 I have run the numbers and prototyped in the past to make ski's and wakeboards with controlled stiffness & reducing weight. It's a fine balance and very small things, even a slight deviation in the manufacture of each item can cause it to behave very differently.

 

As you probably know, you can use cores of different thicknesses to vary the stiffness while keeping the layup similar or the same. BUT in waterskiing/wakeboarding etc there is a very significant factor that doesn't effect any other sport the same. The ski can separate from you - while this sounds obvious, the finblock is denser than water, carbon/glass & matrix are denser than water, bindings are denser than water and you need to displace the equivalent volume of water using the ski body (a very short, narrow and thin "void") to stop your very expensive collection of parts departing to the bottom of the lake forever.

 

If you are taking about honeycomb cores, (ali / nomex or other) aircores, flexible & forming coremats, voided stringers etc, they have all been done, they all have benefits, they all have drawbacks.

 

And BTW, I can understand @Horton response as the questions you've asked in this thread and the other are essentially beginner "know nothing about composites" questions so his advice was certainly given in good faith.

[Jordan]

@kurtis500

The Warp ski was made without a foam core. It was hollow. @Horton liked it.

It was a very expensive ski and they could develop leaks sometimes which is obviously a problem.