Hard Numbers on Flex demystify "flex patterns"

General Discussion
1

Hi Everyone,

I am working toward a PhD in Structural Engineering at UCSD.  My whole career is based on understanding how things flex. 

I talk to a lot of shapers(and manufacturers) and the opinions on flex vary from “Flex is the enemy” to “Flex is the holy grail of surfboard design”.  The bottom line is that all boards flex.  Even if you construct a board out of steel, it will flex.  The only question is how much it will flex and how quickly it will recoil.  So whether you like your board ultra-stiff or ultra-flexible, being able to measure this characteristic is important.

I’d like to introduce a tool that ANYONE can use in their home/garage/backyard to tune the flex to your style of riding.  You don’t have to buy or build anything to run this test. (If you really want to spend some money, we can do the same test in my lab on some really expensive fancy-looking equipment for about 10k).  The link is a video that shows you how to run the test at home.

http://www.youtube.com/watch?v=Z2BdB4NXrMI

And here’s the audio file needed to run the test (at the bottom of page):

http://www.boardformula.com/referencelinks/

NOTES ON TEST:
1)you’ll probably need to standardize your test procedure at 5ft (not 4ft as stated in the video.  4ft works if you make really flexible boards).
2)to avoid math, you can only compare results with someone who weighs the same as you and used the same span between supports
3)run this test on every board that you ride.  Take note of the natural frequency, and you will start to notice a pattern between the boards that flex to your liking and ones that don’t

 

NATURAL FREQUENCY (recoil rate):
The natural frequency of a surfboard is a function of the weight of the rider, the stiffness(or flex) of the surfboard and damping.  Damping factors in minimally, because the frequency of your surfboard is extremely low ( 4Hz…oscillations per second).  Damping might start to really matter at 100Hz.  Heavier rider = lower natural frequency.  Stiffer board = higher natural frequency

 

MY HYPOTHESIS (to be proved with actual numbers in a years time):
For perfomance boards, intermediate surfers should be riding boards with a natural frequency of 3Hz (at 4ft span) and advanced surfers should be riding boards with a natural frequency of 4Hz (at 4ft span).  This is the recoil rate that you can achieve resonance at, depending on your ability.  More skilled surfers have faster muscle reaction rates while maintaining stability, therefore they need a board that recoils faster.  This is a ghastly generalization, but I’ll publish the caveats and details in the future.

 

I’d love to hear about people’s experience with the test!

 

health and joy,

Benjamin

 

2

I’m lovin’ that man… really cool.

I’m doing a HWS for my final year engineering project, and have been thinking about how to compare flex. There’s a flex thread on here with some deflection measurement based ideas.

But I like the idea behind yours, i’ll definitely test my prototypes against my other boards.

I’m hoping its ok to use your work in my report in support of mine? Fully referenced/cited of course!!

I’ll be following your progress on this, awesome :slight_smile:

 

Kit

3

We've have a fair bit of experience quantifying flex. Resonance is one tool, but I'm not sure it's going to ''demystify flex patterns''.  Tools = good.

But that is an easy to perform test, which is good for home analysis. We have access to some sophisticated test equipment also.

I posted a very simple deflection per load test on here a couple of years ago, but I'm hesitant to talk about the #s because of lurking pirates.

4

Hi Mike,

From your posts and website, I see that you are definitely ahead of the pack.  You should publish your test results.  People will steal, but the outcome is good–more people having fun in the water.

The load vs. deflection test is complimentary to a natural frequency test.  However, if only one test can be done, the natural frequency test is more important because it takes damping into account. For instance, if you gave me one of your boards, after bouncing on it, I could reproduce your load/deflection curve w/out ever having done the test.  This is a standard calculation for engineers that work with dynamic systems (like surfboards).  However, with just the load/deflection test you can’t calculate the natural frequency of the surfboard accurately.  You can assume that damping is zero and calculate the natural frequency, of which your error will probably be about 10-20% (which I think is acceptable).  Of course, no one has even proved that natural frequency is of any importance in surfing.  I think it is, but I’d like someone challenge the idea.

Out of curiousity Mike, do you customize flex for different levels of riders?

5

Very interesting. Very clever and intuitive test method. But not sure why youre using the riders full weight in that position tho. Shouldnt it be half weight?

The ultimate challenge with this type of quatitative analysis, is taking into account the natural frequency of loading and unloading while surfing. I've built really flexible advanced composite boards with lots of spring. First time I rode one board the results were amazing, I mean REALLY amazing. Then another day in different conditions the board's flex response was unremarkable.

To say that experience was mind-blowing would be an under statement. But it might explain this statement:

I talk to a lot of shapers(and manufacturers) and the opinions on flex vary from "Flex is the enemy" to "Flex is the holy grail of surfboard design".

Not really tho. Generally, speed freaks, drag racers want little to no flex. Formula 1 guys negociating lots of turns want flex.

There is also a very important feel characteristic that cannot be quantified. 

I have to say that the spring "constant" K is perhaps as important.  Although it really isnt constant. Thats good too. Under load, I want the spring compressed almost fully. Its a really short spring.

Im really intrigued by what your doing, but just dont know how it can be used in a practical repeatable manner. There are so many variables to contend with already. But interesting new trail to be blazed, charge on!

6

First of all, I forgot to welcome you to Swaylock's. Where were my manners?

The fact that all boards flex is a good observation. Anytime a builder talks about flex it's often assumed they make something that wiggles around all over the place. That's not what we do.

I'd agree that natural frequency has an importance, but we're coming at it from a slightly different approach. I'm sure you're familiar with how there's usually more than one approach to an engineering problem. Your test and kcasey's post today point toward a biomechanical ''feel'' that we're very much trying to achieve. You and kc have to get these things merged.

To answer your question, yes, we use all the variables to get the flex and feel as close to individual needs as possible (for customs). But if we offered a ''pro-level flex'' by name, everyone would want it :) 

Couple of questions for you, as well. What support material did you use for the bridge? Doesn't it need to be very hard to avoid damping/interference? (we actually have trouble testing light PU boards because they deform or fail at support points on our surfaces)

Part of my job is having conversations with engineers like you and crafty and kc. I really enjoy it even though you guys are above my level. I always learn.

7

Addressing some questions/comments:

  1. "not sure why youre using the riders full weight in that position tho. Shouldnt it be half weight?"   It may be better to do half your weight, but it's difficult to cut your body in half :) What matters is that the test procedure is proportional to reality (doesn't have to BE reality).  So, assuming your loading the board with half your weight while surfing, the test procedure will always be twice what it should be...consistency is what counts.
  2. "Im really intrigued by what your doing, but just dont know how it can be used in a practical repeatable manner. "  It is repeatable as long as the distance between supports is maintained the same for everyone.  You'll find that if you gain weight, you'll need a heavier glass job, BUT! the desirable nat freq will still be the same.  You would basically stiffen the board to compensate for the extra weight.  This is why you hear big guys say that they like stiff boards 6/6 glass, for instance.  The truth is that those boards are really only stiff for the average 160lb guy.  If you put a 220lb guy on the 6/6 board, it will flex the same as the 160 lb guy w/ a 4/4 glass. 
  3. "There are so many variables to contend with already."  Very very rarely does a test account for all the variables.  This test attempts to account for three variables: rider weight, board stiffness, and board damping.  This is just a small piece of the puzzle.  It can't account for a)damping that comes from the board/water interaction b)where the water is pushing on the board c)where the surfer places his feet...list goes on.  BUT! I promise you that if you start running this test on every board you surf, you'll start to see a pattern emerge.  And you'll start to get a feel for what nat freq works for you.
  4. "I've built really flexible advanced composite boards with lots of spring. First time I rode one board the results were amazing, I mean REALLY amazing. Then another day in different conditions the board's flex response was unremarkable."  That's absolutely right.  Flex is just like shape, it depends on the waves.  So your head high mushy wave board should not have the same flex (glass job) as your head-high barrelling wave board.  In my hypothesis, I was thinking of performance boards in head-high steep or barrelling waves (the natfreq should be between 3-4Hz at 4ft).
thanks for the question!
8

Hi Ben, Cheers for your post. i have been looking for a way to gauge things. thankyou. this is extremly helpful.

9

Hi Mike,

I equally enjoy talking/working with master craftsmen.  Some people see art and science as opposites in conflict, but to me science is only a tool to enhance art.

To answer your question:

“What support material did you use for the bridge? Doesn’t it need to be very hard to avoid damping/interference? (we actually have trouble testing light PU boards because they deform or fail at support points on our surfaces)”  I used 10lb PU foam.  Yes, it adds damping, but it’s a small amount.  As long as you keep your experiment setup consistent, you’ll be fine.  Commit to one material.  To avoid damaging your board, make half round supports, I attached an image.  The round part should be on the ground and the flat part supporting your board, like a rocking chair.  I don’t use half rounds becuase I don’t care if my board gets damaged (I also do impact tests on my boards).

cheers!

 

10

In the “qualitative” world of surfing this is a great way to “quantify” the board in relation to the surfer, imo.  I feel the true value in this is the fact that it is analysis of the board with the rider, after all the board doesnt surf by itself does it?

 

Tfor the non-engineers out there: The engineering algorithm often goes something like this:  start out with an extremely simplified problem and analysis, with a million assumptions.  As a simple model is verified as viable you work on eliminating assumptions to obtain a more precise and accurate solution as you introduce more variables until you are satisfied.  Engineering in practice often utilizes a fair amount of assumptions and estimates… nothing is ever “exact”, but things still work. 

 

Keep up the good work and good luck with the PhD!  When can we expect to see FEM results…?

11

what abourt the natural harmonic?

if you kept going in speed wouldn’t you reach a harmonic to induce a similar reaction or am I wrong

and what does the reaction at other harmonics of the same natural frequency mean to the overall flex pattern?

I’m not so much an engineer in thinking as I am a physicist

maybe that’s my problem…

looking for that equation of everything

good solid case though well presented and thought out 

and it’ll help alot of surfers understand the benefits of rythmn and flow in the weighting and unweighting aspects of surfing.

there’s a move we old timers learned to do on tankers called the hula shuffle which is bsically a quick jive wiggle of the board side to side with your feet close togethor to break the water flow and create acceleration during the trim. You only see us really old farts doing it anymore cause it looks silly but it really works, kind of like a kick stall. Maybe its related

12

Heck, back in the old days engineers drove trains. Now days everyone wants to be an engineer. The sanitation engineers come around in a truck and pick up my garbage! The hotel engineers, well they are just kinda like a handyman. I worked with a bunch of NASA engineers some years ago. Heck , they wore mis matched socks and had pocket liners in their shirt pockets full of pens. They wouldn't even talk to me! But I had to make all the strange things they would think up cause they couldnt make anything. Heck, I allready know everything about Flex. Flex is my friend Bills black lab dog! Or maybe his name was Rex, I forget now! I really would like to see an engineer do a how to thread on building a Hallow Wood surf board though. (they don't flex much, unless they were built by Roy).When you get to be an old fart and lose you flex you shure don't need it in a surfboard anymore. But as an old fart you had better eat you flax or you  aint gona poop! What the heck was this thread about anyway ?  Heck, maybe I will just go out and build me one O those hallow wood boards and I can call myself a  HW engineer. Maybe get me some cards printed too!! Maybe if I do good enough I can even get a job as an engineer !!! Ahui Hou, Wood_Ogre

13

“what about the natural harmonic?”

Awesome question and you’re right it all boils down to physics!  Natural harmonics and Natural frequency are synonymous.  So in the experiment that I demonstrated, you will only find the first harmonic, or natural frequency of the first mode.  If you continue to go faster (5.2 times as fast as the first natural frequency…sqrt(3^3) ) you will start to oscillate again at the the third harmonic, or natural frequency of the third mode.  Of course this is nearly humanly impossible, because if you are hitting the drums at 3 beats/second on the first harmonic, you would have to hit it at 16 beats/second on the third.  If you want to test the physics for fun, support a flexible long board at 10 ft apart.  Time the first harmonic, then go 5 times faster and see if it starts to oscillate again.

Here’s some more info on the mathematics/physics:

http://en.wikipedia.org/wiki/Fundamental_frequency

14

Hi Benjamin,

Boy, this is great stuff, and the posts are also revealing.  I've worked a lot on energy retention and rebound energy in my boards.  Though I am a Mechanical Engineer, I have done most of my designing, building and testing by seat of the pants empirical methods and subjective interpretations.  It is good to see quantification.  Personally I have found Torsion to be of importance (not to clutter your research!) in addition to Simple Beam Theory.  Torsion seems to absorb deflection energy quite well and seems to release it (perhaps) more gradually.  The response of the two deflections will compound that "thrust" or "projection" feel a board has when the demand for drive is made.  About 20 years ago I came up with a way to find the point loads for the rider's two feet using calibrated clay rods called "Plastigage." The loads consistently were well above 2g (front foot) for a decent bottom turn in Head High surf.  Anomalies as high as 4g! but what do I know...

My assumption of an evenly distributed hydraulic support load was incorrect as I found later that the bottom contour and fin(s) of a board have definite "hot spots" of support.

Relating to your work, I am wondering specifically how does Natural Frequency Response relate to a specific performance action? (ie. Will a board of 3.7Hz do well in only small surf with an "active" rider pumping a board for speed, or doing a bottom turn...etc.)

 

Perhaps boards will one day have a Spring Constant as part of their dimensions! ha!

Keep it up and good luck!

George

15

Wondering about stance - since we don’t ride boards with both feet in the middle and all our weight there,  how about running the test with our feet where we surf ?

16

im gonna keep building my boards like i always have. just throw an outline, bottoms curves and surf it. then i make changes. i failed my physics class but i am passing metillurgy class. wait metal has nothing to do with this! unless a metal board would kill at tow-in…

 

how did your impact tests go? did you try different types of glass and composites such as carbon,kevlar and wood veneer? i have a small list of different configurations that i eventually want to test. 2 inch cubes of pu and eps. wrapped with different materials and tested. would be fun to know how to make the toughest lightest board i can.

17

Great work.  But I have a couple of questions.  MInd you I’m not an engineer.  I hated math in school and became an art teacher, if that says anything about my lack of qualifications to discuss this argument…

 

Question number 1- does your drumming technique affect your results?  You’re drumming pretty conservatively there. If you were punk rock drumming , really vigorously, changing amplitude not frequency, would it change the results?  I’m thinking the board would start bouncing a lot sooner if you went all Keith Moon on it.

 

Question number two-  if you go all crazy while drumming, at what point does your test start a process of structural failure?  I’m thinking about how boards lose flex over time.  Might be miniscule but the test would hasten the breakdown. And would my 270 pound neighbor get on your board for the test and buckle it right off?

 

Question 3- Your test measures frequency but not amplitude?  Could you have a board that will resonate at the same frequency but be bouncing up and down a lot more?

 

Question 4- when surfing, a board is never suppported a two points like in the test.  It is supported by a liquid medium, along varying areas of the board.  How can you test account for this?  Also, flex is not distributed equally in a board.  Most people want more flex in the tail and less in the midsection.  Does your or could your test account for that?

 

 

One of the greatest things I’ve found was the CAD programs calculations of volume, something that was missing, except at a mostly intuitive level, in board building previously.  Now we get a cold number for comparison.  I think your on to something similar but it seems you have a way to go before it becomes a viable tool for builders and consumers.  Keep up the good work though. And thanks for sharing.

 

18

Question number 1- does your drumming technique affect your results?  **answer: **If there were no damping involved, the amplitude (vigor) of drumming would not affect the nat freq.  Just like a swingset, no matter how high or low you are swinging, the time between swings is the same.  Just like ocean waves, regardless of the height of the wave, a 7ft wave and a 14ft wave, both at 15sec ( .07Hz) will take 8 hours to get from Harvest buoy to Torrey Pines Buoy (now you can score Blacks with me, when everbody is sitting at home duped by surfline, or would I really give up my solosessions…???).  Amplitude doesn’t really effect the speed.  Now considering damping, which in the bouncing board we have coulomb, viscous and material damping, the amplitude will have a slight effect on the nat freq.  But, without having done the calculations, I would say that, if you beat harder, the nat freq would be slower…maybe not noticeably

Question number two-  if you go all crazy while drumming, at what
point does your test start a process of structural failure?  I’m
thinking about how boards lose flex over time.  Might be miniscule but
the test would hasten the breakdown. And would my 270 pound neighbor
get on your board for the test and buckle it right off?  answer:  Don’t put your 270 lb neighbor on a board designed for a 160lb guy.  I got cocky, tried it, and it buckled :)  However, I put my 240lb friend on his favorite board did the test, and voila, both our favorite boards had the same nat freq!  Also, boards do fatigue, all materials do.  So don’t do the test every night, you’ll reduce the lifespan of the board.  Do the test only as many times as you need to calculate the natfreq: 1-5 times.  Each test is probably equivalent to the damage done riding a head high 100 yd wave once.

Question 3- Your test measures frequency but not amplitude?  Could
you have a board that will resonate at the same frequency but be
bouncing up and down a lot more? **answer:  **You absolutely could! but to my knowledge, no one has done it.  This is where the engineering comes in, because you have to know how to adjust damping.  You want the amplitude of flex to create the optimum rocker outline when you are in a turn.  So you could have two boards that both recoil at 4Hz but one only adds 1/2in. in rocker while the other adds 1in. in rocker through a turn.  You can’t say that one is bad or good, but that they both will work well in different conditions.  If you want to this level of control/performance I recommend hiring a top notch engineer.

Question 4- when surfing, a board is never suppported a two points
like in the test.  It is supported by a liquid medium, along varying
areas of the board.  How can you test account for this?  Also, flex is
not distributed equally in a board.  Most people want more flex in the
tail and less in the midsection.  Does your or could your test account
for that?answer:**  **the test doesn’t account for all of the variables.  No test will.  It is not meant to replace other measurements, it will only fill in one or two pieces of the puzzle.  Unless you are glassing the tail completely different from the rest of the board, the results will represent the flex in the tail.  But hey, feel free to move the rear support all the the way to the end of the tail (as opposed to under the back foot).  As long as you standardize your procedure, you’ll be fine.

 

wow, this swaylocks thing is addictive

 

19

Natural frequency as it applies to a bridge:

 

http://www.youtube.com/watch?v=j-zczJXSxnw

20

I don’t want to throw water on this discussion, but it seems that you’re just generating a standing wave in a structure, the frequency of which is a function of the distance between the supports, the mass of the body on top and the flex of the structure.

I fail to see how this relates to anything in surfing a wave? Since in real life the board is not supported at 2 points, we don’t bounce up and down like that, we don’t stand in the middle of the board with feet together (which completely changes the bending moment and dynamics), and I’m pretty certain that I would not want a standing wave rippling thru my board as I attempt to ride a wave.

On the other hand, I do applaud your efforts, perhaps this will lead to something that I am too dense to visualize.