the neutral axis

General Discussion
81

Build it and they will come, but first, two questions:

  1. What aspects of the neutral axis provide beneficial results when controlled?

  2. What is the beneficial result you will achieve by controlling each aspect you identify by answering question 1) above?

Ah, more random thoughts:

In addition to energy stored and released, isn’t it useful to think about the amount of deflection your construction can sustain before it breaks?

You can have a design that has no material at the neutral axis (hollow), but it’s not yet practical to consider a design that doesn’t have an outer skin.

Greenough’s spoon design is novel for it’s flex, which must be actively controlled by the rider. In essence, the rider becomes part of the structure. Could this concept translate well to a larger board?

Fun, fun.

82

Dear test_article,

I guess I’ll be the first to welcome you officially to da’ Locks. I don’t have any clout around here (I registered the same day as you… how cosmic), but usually someone who shows as much intelligence as you gets a ‘welcome in’ and pat on the back, especially if they resurrect a discussion that’s been passed out on the couch since the Halloween candy binge. Good on ya mate!

I think what you and Bert have pointed out about the placement of and internal structural reinforcement layer is inspiring and right on. Limit unnecessary forces = maximize material properties. My question for you test_article (or Bert or anyone) is: what is the effect of extra rail glass, or a wooden perimeter stringer plus glass on this neutral axis. It seems to me that the NA would be all distorted out towards the rails due to the disproportionate stiffness (tensile, compressive and shear) added by these materials on the relatively thin, non rectangle rails.

A more general question: how do materials on the rail effect stiffness and the NA compared to materials on the skin? Taking the answers to that question can lead us to ask how rail composition and shape affects the NA? We’ve looked at the NA along the length of the board, what does it look like along a compsand crossection? A Surfberger crossection (more cats from the bag please:)?

Oh, and Bert, do you mean to say all shear stress is wasted energy or that sheer stress through the wrong material is wasted energy. (Cheers for your response and all the inspiration Bert!!!)

Thanks for having me gents (and ladies???). Long time listener, first time flapper. I’m stoked on the Revolutionary mentality flourishing here at Swaylocks in the past year or so, even though my Parmenter 6’10" worked great in those 47 degree 6-8ft. bombs this weekend.

Peace

83

Build it and they will come, but first, two questions:

Maybe they will :smiley:

1) What aspects of the neutral axis provide beneficial results when controlled?

The NA isn’t where I place my emphasis. Having smooth NAs is important in that they indicate your materials aren’t violated or faulty below the surface. If you want to talk about the NP (which most on here call NA), that’s different. By centering the NP eqidistant between the deck and bottom we maximise energy storage and return, while giving greater durability.

2) What is the beneficial result you will achieve by controlling each aspect you identify above?

Again, I assume you mean the NP. We maximise durability and potentially have more control over flex and return. E.g., placing a springer in the NP is the ideal (according to bert, at least) and natural location. That gives real sturctural encouragement for the board to spring back to it’s neutral shape quickly. It also means we can make a slightly lighter board beacuse our materials tell us that we can use slightly thinner balsa and lighter cloth on the bottom (since we want the NP to be close to centre).

Ah, more random thoughts:

In addition to energy stored and released, isn’t it useful to think about the amount of deflection your construction can sustain before it breaks?

IMVHO, yes. It should be. That’s a part of the way I am looking at this picture. See the section where I responded to TomBloke.

You can have a design that has no material at the neutral axis (hollow), but it’s not yet practical to consider a design that doesn’t have an outer skin.

Greenough’s spoon design is novel for it’s flex, which must be actively controlled by the rider. In essence, the rider becomes part of the structure. Could this concept translate well to a larger board?

I don’t know. I don’t think it can translate to a larger board in the same way. But flex is something many on here consider as important in larger boards.

As for kneeboards. I understand kneeboarders are far past this design now. Doc??

Fun, fun.

Indeed!

Thanks for your thoughful commentary.

As usual keep in mind that a lot of what I say is conjecture, even though it’s based on sound physic. Always happy to be corrected with fact!

-doug
84

Hi trannydogooder,

Welcome aboard, and the same to test_article. Always glad to see thinkers on board.

…what is the effect of extra rail glass, or a wooden perimeter stringer plus glass on this neutral axis. It seems to me that the NA would be all distorted out towards the rails due to the disproportionate stiffness (tensile, compressive and shear) added by these materials on the relatively thin, non rectangle rails.

Yep, the NP isn’t dead flat. Is a plate of glass flat? No. You might not be able to see the curve, but it’s there. Think of it as a layer, and it does contour genrtly. When it changes suddenly you’ll get a problem… If you consider a hinge or snap to be a problem :wink:

A more general question: how do materials on the rail effect stiffness and the NA compared to materials on the skin?

My understanding is that more mateirals on the rail (e.g., en extra layer of glass) stiffens things up. The extent will depend on the rail and deck shape and curves, I believe. What’s the affect on the NP? As long as the stiffening is equal on the top and bottom the change should equal out and cause no change to the NP.

Taking the answers to that question can lead us to ask how rail composition and shape affects the NA? We’ve looked at the NA along the length of the board, what does it look like along a compsand crossection? A Surfberger crossection (more cats from the bag please:)?

I don’t think it impacts the NP. But I may well be wrong and await responses from others eagerly.

Oh, and Bert, do you mean to say all shear stress is wasted energy or that sheer stress through the wrong material is wasted energy. (Cheers for your response and all the inspiration Bert!!!)

What Bert is referring to, I think, is the traditional view that all sheer stress is to be avoided. Regardless of material it travels through. Sheer stress is normally viewed as a bad thing. FYI Sheer stress doesn’t really go through materials. Unless you consider glue a material. It runs between materials.

Here’s something interesting - from a composite wood bow construction perspective sheer stress has been found to be highly useful. But it has to be used purposefully and designed with that use it from the beginning. Not simply left there by accident and ignored, in which case it will negatively affect durability. So I don’t think we have seen the last word on sheer stress.

-doug

85
Quote:

shear stress is wasted energy …

the neutral axis will have the least shear action happening , that means anything placed in this zone, can flex freely with the least amount of wasted energy …

if you took 3 lengths of flexible timber , wrapped a few elastic bands around them to hold them together …

as you bend all three they will rub past one another to accomadate the new curve , when you let go , they will rub past each other again , this rubbing action slows and stiffles the rate of return , so you end up with less springback or memory than you could have …

if it was possible to do the same experiment , but this time instead of wrapping the elastic around and pressing all the layers together , you glued chunky pieces of elastic between them , so it looked like planks stacked with bricks between them …

[snip]

Is the choice of 3 layers just a coincidence or are you refereing to the deck, springer and bottom? I guessing the foam tied to the deck and bottom and springer is causing the dampening, a flexier 1lbs foam would be better. Placing the springer at the natural axis avoid most of the friction here to minimize lost energy.

imagine a catapult or some sort of device that you loaded by flexing or bending something , then imagine the shaft was surrounded by foam that also had to bend and move because it was attached , see how much energy is wasted by internal friction and bending and moving something because youve locked everything together …

man im sure i just heard a few light switches then …

regards

BERT

The springer could easily be installed without being attached to anything, floating in the foam, but there would need to be something that support it better in some areas(‘bricks’). Secondly, do the bottom skin need to be attached except at the rails? If it floats on the bottom foam there should be little stress here, however I’d be really worried about the board breaking if the bottom skin buckled outwards. Spot glue it to force it to flex in and be supported by the foam although not attached (much) to it?

But still, I don’t know how many times you’ve said “don’t lock everything together”, however I don’t see how this is possible as the deck is attached to the rails which is attached to the bottom. The shear force there must be significant. I can think about a few things, like moving most of the internal (unseen) rail material closer to the NA in a staggered outward fasion, attaching the bottom skin with sikaflex glue (flexible) at spots, not attaching the bottom skin to the rails at all, but then you have the outer skin that locks it up anyway. Creating deep concaves in the bottom foam and have a skin floating over it, limiting the flex if the bottom to the foam would be interesting, but again, how about strength?

Some sparks, but no light.

regards,

Håvard

86

1) What aspects of the neutral axis provide beneficial results when controlled?

please allow me to reword the above statement:

What aspects of surfboard riding are benefitted by the control of the NA NP?

Havaard,

have you tested the concept? if you do you’ll realize that stresses have been replaced with freedom of movement. And there’s more than one type of movement. I agree with your other questions regarding attachments and its affects, but i’ll go out on a limb and say it comes down to proper shaping geometry. And btw, there’s more than one way to generate the movement…

but then again im just guessing like the rest of you.

Sabs, good to hear from you…thanks for all your tips this year. See you next year.

Bert, that was an enlightening post…been kind of bored as of late…I love the bulb.

cheers

87

Hey meecrafty!

What aspects of surfboard riding are benefitted by the control of the NA NP?

Urm… Aren’t we getting dangerously close to the “what is flex” and “is flex good” topics? :smiley: hehehe

Understanding and controlling the NP should allow good control of flex. And yes, other things do that too - as usual there’s more than one way to skin a cat. It should also let us make a wavecraft which is lighter for it’s volume, all else being equal. The third big positive I see is increased durability for mass - tho that wouldn’t directly impact the waveriding experience.

I think there are others, but I have nothing concrete to base that on and won’t go into it here.

-doug

88

i really wanted to get back to this one today , but i just ran clean out of time , i was going to get specfic again , as dan seems to be finding the subtle anomalies which now mean i have to start explaining things a little clearer , as previous posts just highlighted concepts , but now a few of you guys have gone past concepts and actually started equating feelings on the wave with concepts …

i cant stay , so i will just introduce the point i wanted to clarify …

in the past ive made broad statements , these statements have come from a range of construction techniques and different formulas …

so what is starting to happen now , is a mixing of comments as if they all apply to one type of construction technique …

so to put things in perspective …

theres 2 basic concepts being thrown around in here with reference to some old quotes of mine …

a straight sandwich with a free core …

a sandwich with a horizontal stringer …

both have different characteristics and complete set of different dynamics going on …

so when i get back to this i will explain the difference and why the neutral axis is benificial in each style of contruction …

one being more suited to modern ariels and the other being more suited to old skool power surfing and hard rail to rail pushing past the limits …

sorry to leave you guys hanging …

its been hectic lately …

regards

BERT

89

Are you sure that there’s only 12 secret herbs and spices? :wink:

90
Quote:
Are you sure that there's only 12 secret herbs and spices? ;)

Dan, you let me down, there is clearly 13. 1-5 are constants, your selection 6-9 deturmine which you should select for 10-12, and 13 well that one depends on how the plannets are alligned, swell direction, tide, N vs S hemi, and most of all how many cats Bert lets out of the bag for the rest of us cat herders to corral.

:slight_smile:

I really enjoy all this tech design. Keeping it coming.

91

Still curious :slight_smile:

92

The Emperor and his clothes. . . ?

Or the colonel and his secret recipe ?

I’ll go for the basic Longboard of Gwent thanks, it flexes and twangs just fine.

:slight_smile:

93

Thanks for the kind words trannydogooder and for leading me to adjust my understanding of the topic. Even a hollow board will contain a neutral axis, or a contained surface of zero thickness that experiences zero tension or compression in bending…and this would be at the rails. I never thought of the rails as a great place to react the bending loads about which we are writing, but that’s my particular bias. I see them as a great place to incorporate impact resistance though.

And how thick is the neutral axis…or neutral plane…or neutral surface, since it isn’t likely to be precisely planar? I understand it to have no thickness – zero – because immediately on either side of it tension or compression begins. A thing with zero thickness is a thing that doesn’t exist in the concrete world, just in the abstract world.

Regarding shear stresses, it might be useful to think about a lamination of glass and resin to foam. The resin will not support much in the way of tensile loading. That is why it’s so easy to peel the skin from a board constructed in the standard way. In consideration of the vast difference in tensile strengths between the skin of a board and its foam core, how is it that something so weak in tension (the resin adhesive) can transfer large loads between the skin and the foam during bending without causing an immediate delamination? I say it is because those loads are transferred as shear throughout the bond line which effectively spreads the load over such a large area as to reduce the stress (measured in units of load divided by the area supporting that load) below the adhesive’s tensile limit. At any rate, shear loading may detract some from a board’s capacity to react bending loads, but it’s a necessary part of what enables the use of such weak materials (foam, resin) in a surfboard. Resin manufacturers usually include tensile and shear loading data on their specification sheets in the form of peel strength (tensile) and lap shear strength (shear). While those numbers may not be something you would plug directly into a formula, they are useful when comparing different resins to one another as long as they were derived from the same standardized tests.

Comments, concerns, action?

Only the Pope among us is infallible in what’s Holy.

94

as performance becomes more dependant on flex , an understanding of shear forces naturally starts to happen …

without straying to far , ill focus on the 2 points made earlier about dans comments where he pulled down old quotes of mine …

one is related to the placing of internal horisontal stringers , the whole purpose is to off set to much flex …

now to much flex is not a problem , but a board that cant spring back with enough power and speed after being flexed to the extreme is a problem …

the placing of an internal springer in relation to the neutral axis will determine how much load you need to place on the board before it starts to function …

placing it further from the neutral axis will create a stiffening at lower loads or not have it kick in early enough depending on which side we move it to , placing it in the zone where there is little shear movement means at low loads , you dont even know its there , it really doesnt come into action until the board is loaded hard and bent to the extreme , then it will help kick it back into shape fast , this whole design concept is associated with mid face ariel manouvers where the centre of the board is loaded and the release creates the pop for the surfer to get air …

the neutral axis or plane will be the point with the least amount of movement (shear forces ) this movement uses energy , so the further from the neutral axis the more energy is wasted , roy alluded to the point about how much force needs to be put in for what we get back …

it does however start to give a focal point back to where the rider needs to be to have ultimate control over the board , the same as a stringer to a degree , it means our weight needs to remain over that central point …

having a completly free core with no stringer or any other internal device except light core material or air , then gives a mobile neutral axis , the axis or plane is free to move with the rider , because the board is free to flex over the whole area , this means where ever the rider is standing and his weight is focused the board will bend around that point (like standing on an air matress or a trampoline )

this gives a greater measure of control , because we can turn our board and control it without needing our feet in the exact spot …

it also allows the rail line to be used more effectivley because we can load it and get spring from our rail line , because of being able to control our board and load weight near the rail while still having control …

the next logical step is to start using contours that turn compression and tension loads and redirect them to morph our boards into functional shapes under certain loads …

that way we have the contours that suit a wider variety of situations …

with static shape your always forced into making comprimises , because one contour may work well in the pocket and another works well on the flats , one works well at speed and another works well while going slow …

building controlled flex into the structure means we design curves that only appear when we need them …

this leaves other shapers trying to copy curves in total lala land , because the performance isnt in the curves that are visable while holding the board on the beach or measuring them in the bay …

variables in construction give many more variables in performance …

if someone is designing in composites and they dont know exactly whats going into there boards and where ,then they really have no control over the ultimate performance characteristics …

controling shape combined with composite combinations and placement give a true level of customisation …

obviously these comments dont cover everything , but they touch on some of the comments made so far …

these are not the only answers , but do cover some construction styles …

regards

BERT

95

Congrats on the second Surfer article in two months! Its almost getting to be a regular feature.

I was interested in the relationship between flex and contours. In the write-up it sounded like the stringer would be either used or not used. Couldn’t they be selectively used throughout the board to maintain shape in areas where you don’t what board morphing to occur. An example that I was thinking about would be using h stringers in the front third of a longboard to stiffen it up (allowing you to go thin for sensitivity during noseriding and lighter swing weight and probably quicker paddling), while going stringerless in the back half (better nose riding and to gain extra control

Quote:

this gives a greater measure of control , because we can turn our board and control it without needing our feet in the exact spot

and the ability to change the shape of the board under foot.

96

Dan, I think you need a tailblock with a gasket & some clips. Its an open and shut case.

You’d have 3 (or more) different horizontal stringers you can slide in & out. One resin-impregnated balsa, one a glass-skinned piece of corecell or something, one heavy-duty like oiled cedar. Maybe a CF/baltek one or something…

:slight_smile: :slight_smile: :slight_smile:

97

Been away for xmas silliness and just logged back in :slight_smile:

trannydogooder,

Yep, any flexing structure will have a NP. Even a HWC. And the NP is, by definition, of zero thickness. And that can make it rather difficult to locate exactly. I think that the understanding of it’s exact location improves over time, with experimentation and experience. Your idea re rails is interesting, look forward to hearing/seeing more on it in the future. The NP may not be exactly zero thickness in the real physical world, but it will be damn close to it. That’s because it’s not physical in the way a mushroom or a slat of balsa is. It’s this layer or plane where forces balance. And that’s why it can also move around (when the forces in play change).

I see Bert is starting on the sheer questions so what I have to say about that is in my response to him…

Bert,

Sheer and flex again :slight_smile: That is a good summary of what you’ve discussed in other threads about flex, etc. Back to trannydogooder statement about a glass/epoxy bond using sheer to spread force so that it doesn’t cause delams… I think that’s somewhat correct - except that it’s not sheer. This galss/epoxy layer is a stiffer layer that spreads the force. Sheer here is between this glass/epoxy layer and the foam. And that would be quite undersirable. From where I sit relying on the glass/epoxy layer is less than optimal :slight_smile:

Dan, Benny,

Interesting ideas!! Anyone gonna give them a shot?

-doug

98
Quote:

the next logical step is to start using contours that turn compression and tension loads and redirect them to morph our boards into functional shapes under certain loads …

that way we have the contours that suit a wider variety of situations …

with static shape your always forced into making comprimises , because one contour may work well in the pocket and another works well on the flats , one works well at speed and another works well while going slow …

OK so on the dynamic structures front…

I’ve been thinking about a flat bottom that morphs into the traditional single/double barrel concave bottom. A flat bottom board for paddeling and wave entry and a traditional bottom when the board is loaded up. Not sure how much loading it would take to get the bottom I’m thinking of to morph…

So this leads me to the quesiton of when are the concaves most benefitial? I presume when your pumping the board, but I know little about short board design. Time to learn though.

Do the concaves provide benefit in a deep bottom turn? (i.e. when the board is mostlikely to be flexing)

What about pumping to drive down the line?

These questions will help refine my dynamic bottom design (in my head).

thanks

99

One thing that I’ve always wondered is how much change in the boards shape actually occurs when you are surfing. Would the change be large enough to see it in a video of somebody surfing or is it a subtle change that effects water flow?

100

I’d like to officially rename this thread “Neutral Surface of Infinitesimal Thickness” and give birth to a new acronym NSIT, as in, " N’[ow] sit down and get your heads back in those mechanics books so that you boyz can revolutionize surfing the way Simmons did." Just don’t forget to go surf too:)

Great points test_article. I think your summary of this, this…thing is right on. And your description of distibuted sheer stresses was good too, although I think that the foam is usually the failure in a delam (or peel of glass+resin from foam) not the resin, but that’s already been discussed in a Swaylocks galaxy far far away.

Bert, you really brought it all together with that last post, thanks. I think I can imagine the pros and cons of the springboard as you have pointed them out, but I still have to try it out for myself…

T.Dogooder