Hard Numbers on Flex demystify "flex patterns"

General Discussion
21

Its an attempt to quantify the “unquantify-able”…

I think its a brilliant base from which to start applying hard numbers that can be used for reference.

As Lilllibel said, volume statistics are really useful - yet the number alone says nothing about where that volume is distributed about the board.

None the less, still a really useful number!!

Same goes for frequency…

Benjamin seems intent on taking things to a higher level, i’m looking forward to it.

PhD studies… i’m sure there’s some extreme complexity to come, they don’t call you a doctor for nothing! :slight_smile:

22

Bartt,

This is exactly the debate:  Does surboard dynamics matter while surfing?  Does resonance actually occur?  No one has proved it, but I believe it does matter, and I think I will prove it.

Assuming resonance does not occur/matter while surfing, the very least this natfreq test tells you is how stiff (flexible) your board is–which relates to how much your rocker will change when standing on it.

Standing on the middle of the board (as opposed to your natural surfing position) does not negate the validity of the test.  That is like saying we should not measure the full length of the board, becuase the full length of the board is not in the water when we surf it.  All measurements have limitations.  The designer needs to understand how each measurement applies and what are the limitations.

RESONANCE DOES NOT IMPLY THAT YOUR BOARD IS BOUNCING UP AND DOWN WHILE YOU ARE SURFING.  THAT WOULD BE HORRIBLE.  You can do a half oscillation in resonance through a turn and theoretically you will conserve the most energy through the turn.  You see advanced surfers doing turns in 1/3 sec.  I think those are resonant turns (1/2 an oscillation).  ALSO! notice that before big power bottom turns, advanced surfers do a quick pump right before the big one.  Just like a trampoline, you prime the spring before the big bounce.  Watch the first bottom turn on this video,  I see this at least a dozen times a session.

http://www.wetsand.com/video/43624/Lowers+Freesurf/

When you achieve resonance, the second bounce will have greater amplitude.  Your body doesn’t think about it, it learns to do it.  Which is why I think every single experienced surfer often does this little bounce before a big bottom turn.  They need the rocker to be enhanced, to pull a tighter radius turn.

Am I making my case?  Please keep the debate going.

23

Hi Ben-

Thanks for clarifying where this is headed. Perhaps you’re on to something. Please carry-on.

I look forward to further enlightenment…

24

Hey neighbor

Very cool idea.

I have a friend who just finished in MAE his thesis might have a bit of relevance. Not about resonance but 2-way forcing between fluids and (flexible) solids. PM me if you like

also… I have to say the apartment in that video looks extremely familiar

D-Rock

25

[quote="$1"]

Please keep the debate going.

[/quote]

OK....

 

Consider a beam, fixed at one end and having a mass attached to the other, this would be a single degree of freedom (SDoF) oscillator. Once set into motion it will oscillate at its natural frequency. For a single degree of freedom oscillator, a system in which the motion can be described by a single coordinate, the natural frequency depends on two system properties; mass and stiffness. The circular natural frequency, ωn, can be found using the following equation:

\omega_n^2 = k/m \,

Where:
k = stiffness of the beam
m = mass of weight
ωn = circular natural frequency (radians per second)

From the circular frequency, the natural frequency, fn, can be found by simply dividing ωn by 2π. Without first finding the circular natural frequency, the natural frequency can be found directly using:

f_n = (1/2\pi) (k/m)^{1/2} \,

Where:
fn = natural frequency in hertz (1/seconds)
k = stiffness of the beam (Newton/Meters or N/m)
m = mass of weight (kg)

 

One equation, two unknows, one really big assumption.

IMO. its k that youre ultimately after. Because k alters the shape of the board.

Here's the neat thing; a lightly glassed pupe, at the right thickness/volume gets you close to ideal k automatically. Its been determined slowly over years/decades, by thousands and thousands of empiracle iterations. The current challenge comes when introducing new materials, ie eps epoxy compsand, cloths, veneers etc etc, which dramatically alters k.

By the way, i have weights i use to get M/2, I suppose you can drum beat the weights. Been searching for ideal k for a while and as it turns out its right there in my quiver. In the end shape/fins/designsynergy is king.

Flex characteristics adds to the experience when youre trying to squeeze out that last 5%. All things being equal, I like some board flex - it feels great, especially when you synergize fin flex with board flex. When you do, pumping action/feel is significantly enhanced. I started a thread years ago titled,
"board flex fin flex interrelationship" and got some good responses.

26

IMO. its k that youre ultimately after. Because k alters the shape of the board.

Very well explained.

Thanks,

Surfding

27

crafty,

The stiffness constant k itself is actually a function of both the geometry of the object (structure) and the elastic moduli of the material. This is not meant to diminish your point. It does however, emphasis an important distinction between material and structure. In a nutshell, using the same materials, that is if nothing else changes other than geometry, you change the k.

Also, the natural frequency response, here, is also affected by not only the mass of the total system, but also, as pointed out, by the damping factors, which includes, in addition to those inherently apart of the board's construction, all the biological squishy and not-so-squishy bits of the rider.

In addition to getting some sense of stiffness, and energy storage/recovery, I'm also inclined to see a lot of the value here in the abosolute strains (deformations) observed during the resonance trials. The observed strain (deformation) also being a function of the geometry (of the structure.)

.........................................................

BenjaminThompson

Which leads me back to a comment made in the initial post, in particular those which suggest the polarized nature of the flex debate(?). 'Flex' encompasses a big spectrum. There is a real distinction between those who see flex more of an issue of pliability, than with those that view it as energy storage and recovery.

The pliability champions, tend to be more concerned with controlling geometry of the surface area presented to the wave, like the kind of pliability seen in surf mats, spoons, boogie boards, etc. This is often presented as conforming to the shape of the wave face. (The pros and cons of such a design feature aside.)

The other end of the spectrum tends to be more concerned with energy storage and recovery.

All that said, this is a neat beginning! I'm really looking forward to reading more about the amateur vs pro hypothesis. In particular, how much of it can be brought back to the bio mechanical response characteristics of the rider -i.e. like the response characteristics of tendon, muscle, skeletal components etc. And also its application as 'user friendless' design element in general. Sort of the way GM tested the response of car seats so that they would resonant or at least convey the right level of sensation which gave men a little tingle in their important bits.

Nice thread, I hope you'll continue to post as the results come in.

kc

28

[quote="$1"]

 In a nutshell, using the same materials, that is if nothing else changes other than geometry, you change the k.

[/quote]

Many of us know that.

But keep in mind that since 'Blank Monday', the trend has been towards alternative materials, both in the industry and the DIY'r. So yeah I have a bias towards materials. There hasnt been much design change in almost 20 years, at least not in what is currently working for most surfers seeking the most fun. 

Additionally, controlling flex thru material considerations would yield more consistent results than trying to controlling it thru design. Fact is, the easiest way to add flex is to thin the board out, which many riders dont appreciate due to its 'fun' limitations, ie thin boards feel better, may ride better, but generally dont catch waves easier (thats an OPINION).

So if I want a flexible board that is thick enough to suit my needs, Im not going to make it out of pu core with bamboo/epoxy skins. 

Given the choice between riding a great board via design, and riding a mediocre design that flexes better, I'd choose the former. This coming from a flex fanatic. But I can possibly have both with good material/construction methods. This is my goal and Im sure its the goal of many others. My personal built boards have many of the feature benefits of flex, plus high strength to weight, tuned board/fin design and mostly water proof. The only weakeness is a crappy test tank. And the test tank is the truth serum - Re: Greg Griffin; takes him one week to gather all the feedback most others need 6 months to gather.

PS - to illustrate a point, I built a couple of WMD bamboo boards way before the WMD thread started. My immediate thought to the thread was 'that's gonna be stiff'. And as the thread and process evolved that was the general conclusion. Some guys like wood...I like foam. Coil boys knew that way before I did. Not all foam is created equal; neither is the laminate, fiberglass, resin etc etc.  

29

“Given the choice between riding a great board via design, and riding a
mediocre design that flexes better, I’d choose the former.”

I totally agree and I am infinitely grateful that we don’t have to choose.  A lot of shapers ask me if I want durability or flex, or volume or flex.  I say “both”.  They say “well, which one is more important.”  I have a difficult time replying because I won’t compromise on anything when I am paying for a board.  I’ve been on swaylocks for 3 days now, I see a commitment to excellence that I was previously unaware of.  excellence is the attitude that life is not a zero-sum game.

 

Has anyone gotten nat freq numbers on their boards yet?  I’d like to start some data analysis, and improvement of experimental technique.

30

[quote="$1"]

Has anyone gotten nat freq numbers on their boards yet?  [/quote]

Okay I'll throw in (sorry got a bit distracted with too much chit chat)

Tested three boards, all compsand, varying in length from 6'2 to 6'5, followed your test protocol.

Results:

A. Super Flex-O-Matic board from "compsand stiffitus?" thread, and referred to earlier in this thread: 3.3Hz

B. Magic board with trippy "ideal" flex and speed/thrust to burn: 4.0Hz

C. WMD xps core bamboo skinner that feels stiff and never considered it a great board (recent rework in progress) : 4.3-4.7Hz

Ben, I think you've hit a homer with this. Cheers to you and you're variation of 'excellence' below.

[quote="$1"]

excellence is the attitude that life is not a zero-sum game. 

[/quote]

NICE!

PS - check your PM

 

31

Btw, I've never liked the term "flex pattern".

I suppose its easier to say than "flex characteristics"

"flex response" would be cool.

32

Ben,

Very cool.  Nice work and a nice presetation and a new way to look at design.  I'm impressed.

33

What if a board has a wider natural frequency range than another? Yes I know there is really only one but, what if I find larger oscillation amplitude over a wider range? Say 3.7, peaking at 4.0, and 4.3? Compared to another with much lower oscillations at the two ends? 

How do you handle the test with longer boards? A four foot span on an 8ft board would be "throwing out" some data, yes?

 

34

 

Totally agree, I throw up a little inside my mouth every time I say it.  Unfortunately, it became the standard lingo, and when I don’t use the term, people think I don’t surf.

35

 

I think I’ve been on a longboard about 5 times in my entire life, and never a funboard.  So I don’t really know how to adapt the nat freq test to those styles of riding.  If anybody figures it out, please let me know.

36

Hey Benjamin,

 

Have you come up with a dimensionless parameter yet to characterise this by?  Its been a while since I learned Bukingham Pi Theorem or other ways to come up with one, but if I were you, or if I remembered how, Id figure one out, then name it after yourself.  Dont currently have my old textbooks at hand :frowning: … 

 

Nice work.

 

Are you able to incorporate surboard mechanics directly into your research or are you applying your research on other things to this on the side?  Curious as to what you are studying.

37

 

**answer:  **If there is a range that the board seems to achieve resonance, it is probably because you never really achieved resonance.  The natural frequency is probably somewhere in between one of those numbers.  I didn’t have time to produce an audio file with smaller jumps in frequency.  If I did the audio file with  3 Hz, 3.1Hz, 3.2Hz… you would most definitely find a peak resonance.  Hopefully, someone has some time on their hands to produce a more thorough audio file.  Until then, you’ll have to make an educated guess which one is the natural frequency.

38

well i did several boards & had 2 of my staff do some bouncing too. we all weigh between 3 kilos of each other (87kg) so it was interesting. my favourites were around 3.3 to 3.7hz, although its ocillations were over a greater distance even my alaia comes in at 3 to 3.3hz! my pro longboarders favourite shortboards were 4.0 to 4.3hz. can you create a table of info that you would like us all to document for each board? then we could benchmark construction methods, thicknesses and many other variables?

39

I like the idea of finding a correlation between the resonant frequency of a surfboard, and how the surfboard works/feels.

 

We know that the flex characteristics of a board change with it’s dimensions (thickness, foil, ect.), and I assume that the resonant frequency changes also.

As someone noted above, when surfing you don’t stand in the middle of the board, and the board isn’t supported at two points 4 or 5 feet apart. With modern surfing (for the most part) the board is surfed off of the tail, with the riders center of gravity centered closer to the tail.

Since surfboards don’t have consistent thickness and foil throughout (they are thinner at the nose and tail), I would assume that the resonant frequency would change as the support points are changed (like putting a capo on a guitar).

With that said, shouldn’t the test be run with the rear support point near the tip of the tail, and the front suport point be near the riders front foot placment?

How does the resonant frequency change (does it significantly?) for the same board when these support points are changed?

40

“How does the resonant frequency change (does it significantly?) for the same board when these support points are changed?”

 

go get your board and try it… its all about the individual experiance for you… the feelings YOU get from a board co relate to the frequency etc.