What about hanging a board from a
string and doing a frequency response
test on it. It would be interesting to
see where the resonance (natural)
frequencies lie for different construction
methods.
Just another way to quantify flex/twang.
-Hein
Quote:
What about hanging a board from a
string and doing a frequency response
test on it. It would be interesting to
see where the resonance (natural)
frequencies lie for different construction
methods.
Just another way to quantify flex/twang.
-Hein
I’m highly intrigued. What a good idea. How do we standardize it?
I know our vacuum-laminated stuff has a very unique ‘‘sound’’.
Mike
hi mike, what have you concluded with your flex measurments, my son weighs about 11 stone but is a very powerfull surfer and extremly hard on boards, i don/t know if his 20 lb poly is the ideal for his weight /style but its what i am going to aim for with my woody boards , i have been fishing in the dark before so i am glad you posted up your flex measuring thread so from now on i can at least have data to see if some things are stiff or not ,
what have you found to be a popular range for your riders and their weights,
also i was thinking , if i made him a poly board i could lay the bottom with cloth on the bias so i could achieve a flexy tail but more strands of glass would be across the area where he normally breaks them,
this is what i will try with the woody boards ,lighter glass ,2 oz inside and out on the tail but on the bias to gain back snapage strength and also a more parallel thickness from the tail for the first 2 ft,
As silly (and others) have pointed out, the thin, high performance PU boards have a great flex pattern,
but fatigue rapidly and otherwise fail in some manner all too quickly for many of us. In particular, the
single 4 oz both sides pro boards ride unreal but last anywhere from 3 waves to 3 weeks.
So that’s the target we’re shooting for; the weight and feel of those boards, and their flex. As far as
adjusting to rider weight, the thickness increase/decrease that corresponds to the riders weight seems
to proportion the flex as well, with no change in lam schedule. As you test, you’ll see that thickness
has a profound effect on flex, if you hold everything else constant.
Any surfboard worth riding can break if the ocean wants it to. You can build your son boards that are more
resistant to breakage, but if he’s a big kid and he surfs hard things will go wrong occasionally.
Mike
Hi Pete,
Urm… Wasn’t the consensus on the neutral axis thread that boards, in normal use, predominately increase rocker and then return? I’d be interested to hear any contrary logic you have (here, in PM, email, whatever). Fascinating subject, for me.
In any case I think the difference between deflection up and down will be slight, but present, in most cases. I could be wrong of course - but if I’m not that difference probably doesn’t matter too much at this early stage.
Cheers mate!
Guys,
Sounds like we are reaching some agreement… Talking about parallels to bows I am interested in another posible relationship. And that’s flex in the centre of the board.
With bows we avoid too much flex in the middle of the bow, as it’s associated with a jarring feeling as energy is transferred up the shooters arm, rather than into the arrow. Some flex through the handle area can provide for a pleasant springy feeling in lighter weight bows, but with more powerful bows this is often unpleasant and also associated with a short life for the bow. Heavy weight hunting bows typically have thick, inflexible handles.
That’s why I posted a method that made allowance for measuring flex in the middle of the board. To make it a little less scientific for the moment I’m wondering if anyone is interested in indicating whether they feel any flex in the centre of favourite boards. Perhaps just “lots” or “a little” or “a tiny bit” or “none at all” would suffice for now.
Mike - No thanks needed - the trick with this kind of topic is shutting me up 
actually i see flex working similar to a skateboarders legs to some extent
but primarily to increase rocker through the tail
thats why when you design a flextail board
its tail rocker should be reduced by up to an inch
the board feels fast and stiff in trim but as soon as you weight the tail
and reduce weight on your front
water pressure increases rocker
enabling loose turning then flattens out again back in trim
so when you unweight the front foot and bury the back rail
your rocker increases
then when you weight the front it flattens and you get the down the line drive back
the FASTER it springs back the better the board recovers
that why you want a stiff board that flexes
the overly flexible board i built which i gradually stiffened was important
it amplifies everything so i have to consciously weight the tail and use more front foot pressure in trim to get my drive back
it turns like no other off the tail and i can get it vert at low speeds
when i get on a balsa its a lot easier as the movement and weighting isnt as conscious to get the board to perform
when i jump back on flextail balsa or hybrid i surf a lot better
anyway this is just my personal observations
if anyone has similar belief or experience
maybe we can discuss this concept further
gday Hein
im right with you about resonance
it was my first ever post on swaylocks
only roy stuart had any ideas on that one
my proposal was to use quartersawn spruce to better distribute frequencies
and tunning the frequency of each skin before applying to the blank
both skins must be tuned to resonate in harmony
if they are not in harmony they board would not be perfectly tuned
i still believe that
but it might better with its own thread
Negative Rocker?
I curious Doug, are you about to suggest that comps be built with a negative rocker in order to pre-stress the board?
Bows are beautiful machines. It is my understanding that, though not all are, most are pre-stressed, hence they operate higher up on the stress/strain curve, and as a result store more strain energy per strain. (I referring to the amount you pull back. My apologies, I’m not that familiar with archery terminology.)
You could control the response of the board by pre-stressing. If you know what I’m referring to, and have considered it, it would be nice if you’d write a little about it.
So, to ‘stiffen’ the response of the board you’d want a sigmoid (curve) rocker? Down in the tail moving to a flipped nose? My guess is that you’d probably be the first on your block with one, but maybe someone has played with this before. The negative rocker would be slight to small but likely visually present.
Of course, sticking with the bow analogy, I’m under the impression that the one sure way to ruin a bow is to fire it off without an arrow - not to mention cause some pain to the archer, or surfer? I’d be surprised if this was actually a problem - it’s a lame attempt at an archery related rub. That aside, I do find your approach interesting.
kc
Hi Kevin!
No, I’m not suggesting they be built with negative rocker… Just observing that flexing compsands generally have flatter rocker than an equivalent “normal” board and that, from earlier discussions, flex is used to momentarily increase rocker. Thus flex measuring upwards from the deck is probably most appropriate.
Negative rocker would be interesting, to say the least 
For performance and safety reasons all bows (at least ,close to) should be pre-stressed once they are strung. We call that stress/strain curve the force/draw curve and profile bows by measuring the force to draw increase the bow’s draw each inch. We sometimes then graph it.
Because a surfboard doesn’t get pulled into rocker by a string before riding it I think the mechanism for pre-stressing would need to be different. With compsands using wood skins I think that can is probably easily taken care of by the skins - the issue is often how to get enough flex! If you were using high density foam or making a hybrid HDF/wood skinned compsand I reckon you’ll need to look into other methods… Which is where the springer (discussed in the neutral axis thread) comes into it. I believe that’s the most applicable use for a springer - promoting rapid recovery, as a compensation for the skins.
And you are spot on - “dry firing” a wooden bow is extremely bad for it. I have personally had wooden bows explode into a shower of splinters as a result (arrow falling off the string for example). Most of the splinters scatter away from the archer so it’s not so dangerous, but gives you a hell of a fright! Modern fibreglass laminate bows can, for the most part, survive that without much harm… Except for the wear and tear on the string and the shock that runs up the archer’s arm.
hi doug , my belife that the boards predominatly flex flatter then return are more from observation than anything else, maybe when you drop down a wave and most of your weight is on the front foot there will be some added curve but as soon as you bottom turn and your weight is on the back foot , all your weight on the end of the board with just your front foot controling the turning rate , i can only see a flatter rocker , you turn and weight the front foot , flatter again,
my son comes hard off the bottom and uses his back foot to hit the lip , the boards he has broken at this point have all creased between the rail and the stringer,just on one side , his heel side, the board has flexed flatter and twisted until the bottom is under to much compression and inevitably breaks,
as i say these are just my observations , burt also demonstrated his flex by flattening and then returning , pete
It is hard to make my mind between PETEUK’s and SILLY’s views, because from the same phenomenon, things can be seen one way or another. What you’re suggesting Silly is that it is the water pressure that increases the rocker when you lighten your front foot and push on the back foot, but PETEUK suggests (I think) that when you’re pushing on both feet you’re flattening the rocker, which means that if you lighten your front foot and push on the back foot, the rocker will immediately increase as a result of the board regaining its original rocker (pre-loaded) rather than the water giving energy by pushing under the front and increasing the rocker. For PETEUK energy is put (by the rider’s weight) to flatten the rocker, and then is given back as the result of flex when unloading the rider’s weight, while for Silly, energy is put when the water pushes on the bottom and increases the rocker and is released when the rocker flattens back. The timing when energy is stored is completely different.
I would think that the rider’s weight may be a higher force than the water pushing on the bottom on the board when the rider unweighs his front foot. When you lighten your front foot and push on the back one, there is no real oposite force to resist water pushing so the nose will get higher, but there wouldn’t be a real flexing point. For making a relatively stiff board to flex, we need 3 points where force will be applied, at least 2 on one side and one (theoretical) resisting on the other side (for example 2 feet on the deck and the water on the bottom, or for flexing a board upside down, ground pushing under the nose and tail and some weight in the middle). I would tend to think that the board flattens when the rider is equally on both feet (and water pushes on the bottom as the opposite force), and then the rocker would increase when he unweighs his feet. For silly’s theory, the back foot pushes down, the water on the bottom pushes up, but where is the third pushing point if the weight on the front foot is minimal at that moment. It could only work if the rider would join the front foot to the back foot on the tail and push very hard on this front foot so the rider dosn’t fall behind his board. Opinions?
Ive got a couple of flex majic compsands, and they are insane going frontside where I typically pump/drive/bottom turn off my front leg. On good days with steep fast drops and hard bottom turns theres absolutely no question; rocker increases.
Backside is totally different cuz I surf way more off the back leg. I learned something from Kendall tho…when the wave sets up right, I do alternating quick but smooth big S turns, bottom to top, in the pocket. After one bottom and top turn, the board comes alive with flex majic…and feels like Im accelerating on every turn (5 finner). I do this mostly off the back foot but when I get the board in that freak zone, its more evenly distributed stance, and the board flex tends to flow with the wave curvature and my style. Trippy to say the least. Btw, my boards dont use balsa, I prefer higher density woods in the rail. Low density balsa is too whimpy for good twang…thats why twangy balsa boards (Sunova) req the use of more wood…deck, bottom etc. Im not saying using balsa is wrong, just saying you can have like affects more efficiently with better wood in the right places.
Btw, I tried bending some using a short span and it doesnt flex much at all. The length of the wetted wave side rail is key, which isroughly 3/4 length.
It seems the logical conclusion is that boards flex both ways, that there are loads
that increase rocker and loads that decrease rocker. I know that I can cite examples of both.
The dynamics that produce those loads are not part of our static test, which is admittedly
crude. But the test is easy and repeatable, and those were my primary criteria.
I think we’re off to a good start and hope to see more people posting up #'s as we refine
our technique. One adjustment I’d propose is that we lengthen the spread on the mid-flex
test to 4’ (instead of 3’) simply in the interest of dealing with slightly larger #'s and decreasing
the margin of error. We started out doing it at 3’, but it was more of a check just to make sure
they didn’t flex too much, when we tried new stuff. The tail flex test has long been our favorite.
I should mention here that the tail flex test was the brainchild of composites limit-pusher Eric
Brasington, co-owner of Coil. Apparently Doug’s archery experience led him to think along the
same lines.
Mike
Hi Pete!
Thanks for sharing that! Interesting. I always got the feeling the Bert and a number of others on here thought that flex was most important deckward. Maybe that’s not so… Equally maybe I misinterpretted. Possibly because of the typically flatter rocker on a compsand.
I always thought Bert just jumped on boards upside down because it was such a dramatic way of illustrating flex. I though he had said that flex increasing rocker was most important. I haven’t checked the archives and there’s every chance I’m wrong.
In any case I don’t think it’s that important to measuring flex.
Can I ask you a massive favour? Could you measure the tail flex on that board UPWARD? And then tell us. I’d just like to get a handle on what, if any, difference it makes. I suspect it won’t be a lot.
Cheers!
Hi Mike,
Interesting!
I haven’t heard of Eric before. Can you tell me more about him? Maybe a link?
Cheers!
Warning: This is a little off topic. Also numbers are involved, but in a good way. That said…
Quick Summary
Free falling on your surfboard and landing upright can be brutal, even when falling a mere meter.
How much force?
Case 1. Free Falling (… when the ‘air’ goes bad?)
Lets say you find yourself and board a mere 1 meter above the surface of the water and dropping towards that surface. An ‘air’ gone bad? Who knows.
We know that, neglecting air resistance (you’re only dropping a meter with zero initial velocity, so this isn’t that much of a stretch), the time it will take you to fall will be given by the square root of [ 2 times the distance you’re about to fall divided by the acceleration due to gravity (9.8 meter per second squared)]. Here, falling a mere meter means you’ll fall for a short 0.45 seconds. We also know that in falling through the gravitational field you’ll acquire some velocity, and it can be calculated by the time you just calculated (0.45 s) times the acceleration due to gravity, which gives a speed 4.4 meters per second upon impact with the water.
Now for a collision with a head on flow, you can approximate the mayhem using something engineers call stagnation pressure, its given by one-half times the density of the fluid times the fluid velocity squared. If you want the total force, you just times that by the area of impact. Here, using a density of sea water of 1025 kg per meter cubed, and say your where on your 6’6” or something like that, which has a total bottom area of say 0.8 meter squared, you get a force of 793 N. Big deal? Lets see, we’ll compare it to your weight.
Your mass is a mere 63.5 kg, or in the Land of the Free (and slightly retarded when it comes to things metric), your weight is 145 lb. In metric, you weight is of course, 622 N –i.e. mass times the acceleration due to gravity.
What a difference a little air makes, huh? Here the fall is only a meter. You may wish to do it for 2 meters, or use the bottom area of your big boy thruster(?) But the real killer, in the free fall case, is the surface area - it works against you. Here, when coming down from such a drop, virtually the whole board can impact the water, in fact its a matter of style to make it so. However, most of the time, when surfing, the wetted area is usually significantly less than half the value of the whole board’s bottom.
So, you’ve got the formulae, you’ve got the logic, you’ve got all you need to estimate the 'slap’ force an ‘air gone bad.’
One point is certain, free falling and ‘slapping’ the surface like a belly flop is likely to be brutal on a board. As a design consideration, it is a unique case. Factor it in if you care to, or simply request that customers avoid the full belly flop when ‘airs go bad’, or on air landings of any kind (… yeah, good luck with that.)
As always, somebody check my Math, please.
kc
the trick is not to land flat footed…
the landing is tail first, then the rest of the board, thus no impact spike.
most pros land airs in the foam anyway…
if not, it shouldnt be tried
I guess the last few comments of my post could be interpreted as instructions, my apologies; they were a lame attempt at humor.
The point of the exercise was to gauge the forces involved in this kind of maneuver and the way surface area can work against you under those kinds of conditions. Also, that it is pretty rare to use the whole bottom when surfing (paddling is different) so in a way, this kind of maneuver tends to be at the extreme end of things.
In my opinion, it’s all part of the package which seems to be coming together here.
kc
Hi Kevin,
Actual speed at time of impact will be a little less because of drag on surface area (less momentum overcoming that drag).
Not bad tho.
Hey Doug,
Kirk and Eric Brasington are twin brothers from Satellite Beach, Florida , who’ve spent the last 15
years developing the Coil Construction technology. With their brother Scott, they own a popular
restaurant(Eric is also a trained chef). They have proprietary vacuum techniques that make it possible
to put an incredible amount of fiber in a single skin laminate. Their unique process doesn’t fit any of the
composite industry labels, i.e., it’s not exactly infusion, it’s not just a vacuum lam, etc. It’s Coil.
We use handshaped stringerless EPS(urethane is an option) and Resin Research epoxies, fused with a
mix of exotic and not-so-exotic weaves to make some halfway decent surfboards that people seem to
like. There was an article about us in Eastern Surf Magazine back about March, and an article about
them in one of the kiteboard mags a few years ago when they were doing Hana Crew Kiteboards. Doesn’t
surprise me that you didn’t see those being in Orstralia(you actually have surf to play with). Sorry about
the lack of a website, we spent all the money building surfboards.
Cheers to you too! Now I need a nightcap…
Mike