the neutral axis

General Discussion
41
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… just to clarify…i think it has been said, but the NA is usually midway between the top and bottom assuming the shear stress distribution is symmetrical…to understand the NA you need to know that 1) it is simply a neutral plane where there is NO shear stress and 2) …

i appologise for nit-picking. but bert said something similar is the recent thread about balsa stringers. everyone is entitled to their own theories. but, when applied to engineering principles, some facts are just facts. I still believe the above statement is incorrect. i realise it’s been a long time for this stuff, but the couple betweeen the upper compressive layers and the lower tensile layers, still needs to be resolved (everywhere). I’m quite certain the “classical” defination of NA means that the local shear stress will be at a maximum, despite the fact that the principal tensile/compressive stress’ converge to zero. yes/no? -bill

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but, when applied to engineering principles, some facts are just facts.

so true…

i defer you back to Losos original post, look closely at the second diagram…this is Mech of Mat’ls 101 stuff…dont think it can get more factual…

btw, this is an instructural diagram…the diagram is grossly simplified and totally inaccurate wrt to a compsand board behavior…the shear force dist of a compsand board would be radically different…very little in the core and excessively large in the cored skins…therein lies the core of the problem, no pun intended

btw, personally i dont care much for the first diagram

additionally, as i said b4 the NA is somewhat overrated…its just one part of a stress distribution response for a beam/panel under a certain load…again if the load is zero, the stress is zero and by definition the NA cannot exist…the NA is not an independent subject of engineering study, it is one part of predicting the stress distribution…yikes! talk about nick picking…i’ll have to leave it there

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i think the NA is way more important …

in surfing its all about performance …

forget the engineering terms and put it back to performance terms …

on a trampoline mat where is the neutral axis???

what would happen if for some reason the mat was engineered to not bend in a certain area and had a stiff spot in it???

you wouldnt have as much control and would probably get thrown off in a different direction …

after a while you would develop a style to compensate , but it would always take extra effort to deal with the rigid area , that effort would mean putting extra energy in and getting less out of it …

considering the NA into design means your board does more work for free …

i had one customer once say ( ive never had so much control when ive been out of control )

a mobile NA , means when your on the verge of falling off , the board is still flexing around your centre of gravity and want to help you back to the centre or quickly turn under your center of gravity, like jumping near the edge on a trampoline …

snap out of it and forget classical engineering …

that is so much like what i was talking about before …

it actually hinders learning , coz we lock into a line of reasoning and miss things …

i dont mean to make it sound like a dressing down , but i can see theres something thats just not adding up for you yet …

it was the same when the sandwich thing started , you were the one that took the most convincing , but then you pulled it appart and examinined it that closely , that you ended up being one of the most convinced …

keep digging …

regards

BERT

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while my posts seem contradictory to yours they are for a reason…i have not figured out how to capitalize on getting the most of the NA in a compsand so therefore i was strictly speaking from a pure mechanics of materials point of view, not from a surfboard point of view. my first post here clearly stated that i dont want to get into the pros and cons of the NA as it pertains to compsand surfboards. Maybe i havent been a neutral advisor so i’ll stand to correct that if needed

post edit: went back and read my post #21…yup i was definitely not neutral there…i stand corrected

now Bert, i have no doubt that you have figured out the best way to get the most out of this type of construction but are there other construction methods still available to develop? keep digging …is it fruitfull to keep diggin into something that’s already been invented? there’s so much left to create…whole new trails never even concieved…more features…more choices…

as a side note, you might think engineers in general are these ridgid stuffy types with no creativity…well some buck the trend…

and besides there’s an easy fix to the lack of creativity…weed…the creative ‘genius’ comes out (very loose term)…oops did i say something non PC? and its also an anti-depressent…most of my best ideas/solutions were under its effects…it can turn that ridgid stuffy analytical nerd into a creative machine…another tool in the chest if you will…

back on subject…

for you the NA is a key feature/benefit component…for me the discussion is just titillating intellectual playtime…i could make one very simple statement wrt NA that would turn the conversation on its head

if i spend too much thinking about what your doing, it limits progress on things that are newer and potentially better…

i’ve been posting along here cuz my lower back is f_k’d and im limited on what i can do physically…so heavy thinking and intellectual conversation is on the front burner…

wasnt trying to diminish the validity of your past and current developments…like i said b4, i would rather just buy a sunova…i like having lots of choices…i think most people do too…

cheers

PS -

a mobile NA , means when your on the verge of falling off , the board is still flexing around your centre of gravity and want to help you back to the centre or quickly turn under your center of gravity, like jumping near the edge on a trampoline …

i could see some guys getting really excited about this statement although falling off is just part of surfing (can you imagine those classic surf flicks w/o the wipeout segment…BOOOORIIIING)…could be beneficial in a contest arena tho…but falling off is not a big problem for me so i’ll let the other dogs hunt…

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Meecrafty once said push the design limits till it breaks then slowly back up from there..

When I was little I asked my dad how they figure the weight limits on bridges. He told me they drive bigger & bigger trucks across the bridge 'till it breaks & then rebuild it exactly the same and don’t allow trucks as big as the last one. :slight_smile:

I’m prefering, with surfboards, to work in the other direction. I can’t leap to thin & light without some intermediate steps. Mainly because I’m still not 100% convinced that thin & light is the best thing for the 10’ single fins I surf. So each one is just getting a little lighter & a little flexier and I’ll see how it goes.

I’m also lucky enough to have 2 or 3 guys I surf with who are happy to slide a few on my boards & give me feedback. As cool as it is to surf my own gear, that “holy cow, it really does work!” factor goes WAY up when I see someone with nothing invested riding one of my boards. I know I can make them work through force of will…but someone else has no reason to do that so I get honest feedback.

Anyone else leant out their compsandos?

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but i can see theres something thats just not adding up for you yet …

true.

but here’s a little stab for titillating conversation’s sake…

the NA can move up and down if you limit one of the two skin shear movements available on both the upper and lower panels

severely limit compressive movement while simultaneously maximizing tensile movement on both skin panels would make the NA move up and down under bending loads…the same would be true for the reverse scenerio…

btw, the last few minutes i did try to apply myself to understanding how to make the NA move up and down, so you got me again…dam you bert!!! :wink:

ps - in addition, we wouldnt be having this NA discussion if the parts slid past each other like in the book bending example…something to chew on…

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Arthur C. Clarke wrote that any sufficiently advanced technology is indistinguishable from magic. Many results of science would seem like miracles to people of just 200 years ago.

I agree with what you’ve said about magic boards. The term “magic board” has always bugged me. It implies that for some reason or another its impossible to recreate the board. Berts right that I’ve spent a lot of time reading past posts and the archives trying to make sense of statements that were made in the past. I’ve been very impressed with many peoples understanding of the laws of physics, but I think they are best used as "more what you’d call “guidelines” " (to steal a quote from pirates of caribbean) than as absolute, plug in the numbers facts. Its very possible that the great board builders have an inate understanding of how a board relates to the physical world, but for me the phyics offer a shortcut to building a good boards. You don’t want to put “features” into your boards that goes against the laws of physics.

Things that I’m currently trying to figure out are:

flex & flex bias - both the board and FINS

Relationship between Center Of Gravity and Center Of Pressure - Just starting to look into this but I think its big

Neutral Axis and how it relates to the horizontal stringer - This tread

48
Quote:

i defer you back to Losos original post, look closely at the second diagram…this is Mech of Mat’ls 101 stuff…dont think it can get more factual…

btw, this is an instructural diagram…the diagram is grossly simplified and totally inaccurate wrt to a compsand board behavior…the shear force dist of a compsand board would be radically different…very little in the core and excessively large in the cored skins…therein lies the core of the problem, no pun intended

btw, personally i dont care much for the first diagram

that sounds a little like ‘smoke and mirrors’

It is Materials 101 stuff, but like you said; “losos diagram is a little inaccurate”.

Rules are rules and they apply to rectangular beams, I-beams and sandwich beams.

If you flip to chapter 6 , one explanation looks like this: http://www.aeromech.usyd.edu.au/structures/mos/Mosch06Con.html

The attached example seems to fit the discussion pretty well.

I appologise for dragging this on.

However this thread was started with reference to a very specific term that is well defined with respect to beam theory. I applaud everyones attempt to apply it to their own benefit, but redefining the terms seems a little counter-productive.

… and i’ll stop now.

-bill

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Its very possible that the great board builders have an inate understanding of how a board relates to the physical world, but for me the phyics offer a shortcut to building a good boards. You don’t want to put “features” into your boards that goes against the laws of physics.

Actually we’d be lucky if it was 50% functional design and 50% just to look purty…

In reality I’d guess more like 30-40% functional and 60-70% to look good and feel good in the shapers hands…

Otherwise can someone explain why all boards are designed symmetrically around a center line when hardly anyone these days out side of the makaha crew surfs facing straight ahead. There is no way in hell that your body operates the same surfing frontside versus backside so why are all boards build to surf the same off both sides?

That’s the big engineering question to ponder…

Which leads me to this point…

It’s all been about art and and the arteest sculpting something out of foam than its has been about purely engineering a truly functional device. You can do anything you want to the inside and the outside but as long as it has like a nice sexy symmetrical outline pleasing to the eye is it really any different? Look at stealth or anything pushing the envelope in their arena of technology… Pretty damn ugly in my opinion… An engineer will see beauty in anything truly functional, a mathmetician or physicist will see beauty in a bunch of numbers and a poet will see beauty in a bunch of words… For us to require that what we ride must also be visually pleasing means that we are more focused on the art than we are on the technical aspect of the design i,e, fat penguin, gemini, USO, or roy stewart’s famous turbo finned 14 footers… The desire to always want to bring what ever we build back into the “realm of comfort” design fold must tell us that some of this pursuit of pure engineering is forever hopeless in nature…

I think we have to admit that until we can get over being so vain about what we ride looks like we’re really not going to make any real headway pushing an engineering envelope…

Getting laughed at isn’t alot of fun… but I bet those guys are the ones who come out ahead in the long run… We used to make fun of Bill Gates as a rich kid wise-*ssed punk geek trouble maker when he was in high school. He kicked our butts big times 30 years later… We used to think what the hell does he know when we all were in punch card heaven…

Quote:

I’m prefering, with surfboards, to work in the other direction. I can’t leap to thin & light without some intermediate steps. Mainly because I’m still not 100% convinced that thin & light is the best thing for the 10’ single fins I surf. So each one is just getting a little lighter & a little flexier and I’ll see how it goes

My brother and I are I guess are pushing the envelope building 8’0" performance mini-longboards out of a single sheet of 2" 1lb EPS. We’re on our 4th and learning more with every variation. It’s in that realm between a funboard or fat shortboard and a longboard. I’ve also built two 7’x22"x1.5" thick vector funboards with not alot of success. The stubby ultra light short boards or funboards absolutely suck especially using EPS where as our stubby ultra short fish has seen some success. I don’t know the answer just that building them to existing boards specs as I’ve been doing so far is only making longer lasting versions of things I already have…

Quote:

Anyone else leant out their compsandos?

Been setting mine up at least the ones that aren’t too ugly as christmas presents for those relatives and friends can’t afford to buy a board. We need to do as ambrose and donate more “travel boards” to the cause… As for the ugly ones and and ultrathin borderline designs, been thinking of taking all of them down to the beach on a long weekend if I get the courage and letting the small kine bruddas (no one over 150) play around with them to get some feed back and then maybe leave them with the lifeguards down at makaha or somewhere where there are alot of little kids who don’t have anthing to ride.

Like that IBM commercial… my sando’s like the servers are taking over all the free space in my world…

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Otherwise can someone explain why all boards are designed symmetrically around a center line when hardly anyone these days out side of the makaha crew surfs facing straight ahead. There is no way in hell that your body operates the same surfing frontside versus backside so why are all boards build to surf the same off both sides?

I finally get what you’re saying about asymmetrical boards! I had been thinking - Why would anybody want a board that is made to surf well in only one direction. Now that you’ve gotten me interested, how would you design a board differently for frontside and backside?

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you are correct sir…my overuse of the word ‘shear’ has created some confusion so here’s a clarification, correct me if im wrong:

the NA or NP is a demarkation line where there are no compressive and tensile stresses present

all this stress talk is starting to make me feel stressed…need some surf!!! Ben (chipfish) save me!!!

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Brewer built 'em

Merrick sells them only as longboards

MrJ and CarveNalu have built their own with some success and positive feedback.

Most of the ASSYM designs has been in extending the rail line on the forehand side and shortening it on the backhand side but I think alot of thought still needs to be placed on rocker and flex variations as well since the weighting is so different.

Most thruster kids surf heavy off the back foot due to the nature of having to power and keep moving the thruster fins to maintain the squirt so the differential is a little less than with old schoolers.

Typically you drive off the front foot weight forward on the forehand while on the backhand you drive off the tail with your butt either on or behind your power foot just like you do cutting back off the forehand. Trying cutting back at speed off the forehand with all your weight on your front foot… way different than what happens when you drive off the bottom with the same weighting you’ll probably catch an edge too…

If you drive off your back foot on the forehand like most people today you’ll turn more vertically up the face losing speed as you go against the grain of the moving face which is why you have to initiate your cutback either 1/2 or 3/4 up the face before you lose all momentum. Otherwise it’s smack the lip and free fall to the bottom or do a floater in the lip free falling to the bottom.

I would think you would want a stiffer reaction on the forehand maybe even less tail rocker to increase your drive affect but have the exact opposite more flex and more rocker on the backhand side. How you would do this is well beyond me…

For snaps it seems like on the backhand side you can maintain all the momentum through the arc as you power off the tail back into the pit and gravity is assisting you to drive the board rail engaged back into the pit where as on the forehand side you’re losing momentum the whole time and actually are falling back into the pit due to the effects of gravity. That’s why I think backhand snaps are so much more radical and powerful than a forehand snap which actually turns into a spin out when too much pressure is applied to the tail trying to bring her around. The only time I’ve seen someone come around full speed in the same arc as they can get on a backhand is when the board is micro sized enough to make that arc with the rail fully engaged. Bert claims different but I see like 70% of his board out of the water in the photos he’s published on his massive forehand snaps. There’s so much more power and speed released back you when you come out of a fully rail engaged carve than you’d ever get off a tail swivel. Just look at Occy or Taylor Knox. Again how you’s accomplish the ability to have the same forhand and backhand snap via design is beyond me.

Alot of modern design discussions has been about what goes into the last 18 inches cause that how most thruster surfing is done now a days. Kind of makes you think the first 54" of their 6’2" was worthless. Also makes you think that all you need then is something 18" attached to your rear foot… Looks like foil boarding to me…

I don’t know alot about design and probably never will.

I just know how they ride which is really all that matters to me. Don’t care if they’re butt ugly or don’t make any sense to anyone else. If it improves my ride and what I interpret as the “performance” of the board under my feet in waves that I’m surfing, then that’s all that counts.

No one’s gonna be able to tell you different, cause no matter what anyone says or claims you’re gonna eventually find out for your self whether you like it or not…

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You guys are reading my mind.

2 things: Dan, in another thread you said 4b2-eps-4b2. But I think this is a place where engineering analysis is getting in the way. Yeah, technically, there are 3 different structures, each one of which has a compression/neutral/tension thing going on. But when you bond glass to balsa - especially if you use enough resin to penetrate/saturate the wood - its just a skin. I don’t think the balsa is thick enough to have its own neutral axis in the middle…

I’m going more like 4b4-eps-2b2 or even 4b4CF-eps-4b4. The skins are each a solid unit, espeically if they’re pre-made.

As for asym, I’ve been surfing one of Bob Miller’s new compsandos for a couple of weeks. Its made like a Surftech, but thin & wide to let it flex (24" x 2.75") and its got some nice heft for glide - around 18 lb. 9’10" round pin 2+1.

So…I will make, possibly next, a board with a squaretail on my backhand & a big hip; and a round pin, parallel (to the stringer) rail complete with a sidebite, on my forehand. The nose will be pulled in on the backhand side & wide on the forehand side. The forehand rail will be more pinched than the backhand so I can bury it on a big bottom turn, but the tail & nose will be more foiled on the backhand side to provide some responsiveness for my heels…belly bottom, domed deck. Side bite on the pin side only will help lift the tail evenly as narrow tails lift nicely with side fins but wide tails lift just fine all on their own. It will also let me use the sidebite with my toes & upper body weight but won’t skate out or get squirrely when I’m on my heels.

All this is provided that my current project proves to me that I’m on the right track WRT materials.

Fun fun fun. :slight_smile:

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Your future board sounds very interesting! What are the downsides of going asym (besides lots of funny looks)?

I like the to see the 2b2 on the bottom of your next board. I’m going that way also (except that I’m going 2b4 on the deck). I’ve always though of the the skins as a solid unit, but something Bert said when I was visiting archive land has got me questioning that. Its very hard for me to tell what is “magic” from all the ideas that have been put out so I’m looking at all possibilities.

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Hi Dan

I think if you keep your resin ratio down then you the gbg is a sandwich- a light weight strong composite skin,way stronger and lighter than a single glass skin

If you saturate the balsa then – well i just cant see the point

There are easier ways improve impact strength

Mike

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Whoa! lotsa posts overnight. Slightly rushing this, so I hope it doesn’t come out wrong.

sabs,

Glad to hear you are interested in seeing #001… When it’s done. Hope it doesn’t disappoint :slight_smile:

pinhead,

It gets even more complex than that! Take a log 7’ long and 10" in diameter. Slice it lengthwise into 1" slices. Take the two slices immediately next to each other in the centre. Square these to the exact same dimensions. Now perform flex and break testing on these. You don’t even have to use lab quality equipment to find that there are easily measurable differences between the two. Interesting, huh?

zfennell,

There really is “no stress” at the neutral axis, and at the neutral plane. It is a place where all forces balance out and neutralise one another. if you want to know why, think of this. Take a piece of mythical 0.25# foam 2" thick. Put a 3mm skin (with glass, etc) on both sides but no rails. Now bend it slowly until it breaks, recording the structure from all angles. Now play back the failure in slow motion – the catastrophic failure (snap) will be on the top or bottom. Before this you will see foam failure JUST BELOW THE SURFACES. The last place the foam fails will be the NA of the bend. This test gets done more regularly than you might think and you can do it in your own garage with a few levers and supports.

meecrafty,

I agree 100% on keep digging! Good mantra! Yes, there are certainly various construction methods. I don’t think you HAVE to understand and utilise the NA and NP, but I can see it broadening our understanding of how these structures work.

bert,

I was sure I remembered that there is both an NA and an NP. So I blew the dust off some books AND did some searches on the web. Guess what? BOTH are valid and important. I’d suggest that understanding both will be useful to everyone who wants to understand compsand structures. A nice summary is at http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture38/461_lecture38.html.

Have we been discussing the NP (which runs from nose to tail, parallel to the board surfaces) or the NAs (which run from deck to bottom)? I suspect we have been discussing the NP and calling it the NA. Considering how we have been talking about it and that we have referred to it as “the” NA. Obviously this does NOT invalidate what we are discussing. But an agree vocabulary facilitates discussion and comprehension no end.

I disagree with what you said, tho I’d add that we can and should take pure physics and engineering concepts that do apply and use them to deepen our understanding.

oneula,

I like the art point. But I still want to understand how these things work internally :slight_smile:

All,

Anyone else realise that the NAs and NP change when the positioning of forces (i.e., where you are standing) changes on a structure with non-parallel surfaces?

I am keeping out of the asym thing since I aspire to ride switch :smiley:

-doug

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To sum up then, we see neutral axis not a something you can control to obtain certain flex properties, but as a conceptual aid that helps you determine how well the things you can control are handling the stresses and strains imposed by external forces?

You can go thin if you’re ready, willing and able to deal with the workability issues surrounding high-strength materials.

Real soul surfers build their own equipment. Anyone have an autoclave I can borrow? :smiley:

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Another random thought:

Buckling is a failure due to compression. Fracture, cracking is a failure due to tension. When a typical fiberglass-skin-over-single-foam-core surfboard breaks in service, the failure begins on the side of the neutral axis that is in compression. Yes, the glass bears tensile loads well, but unless the individual filaments in the weave can be rigidly constrained (such constraint being the job of the resin matrix) the skin will not bear compressive loads well. I’m saying that typical boards buckle then break because the resin matrix fails to hold the fiberglass filaments in place.

A good way to get a stronger matrix is to go to a resin that is heat cureable. Buy a cloth that is pre-impregnated with resin (expensive, watch out for shelf life and storage conditions), wrap it around your shaped core (btw, the core and all tooling and materials in the layup must be able to withstand the heat of cure), vac bag it with peel ply, breather, etc., then wrap your heat blanket around the vac bag, plug the temperature controller into the heat blanket and program it to follow the pre-preg manufacturer’s time-temperature cycle. Let 'er rip. Or, if you have the access, a more elegant solution is to use an autoclave instead of a heat blanket and, perhaps, vac bag.

What might you achieve for your trouble? You could shoot for thinness, or lighter weight, or longevity, all without sacrificing other qualities, or you could shoot for failure onset to occur in some other mode.

It only costs a couple bucks more to go first class.

Does any of the above ring true?

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Quote:

A good way to get a stronger matrix is to go to a resin that is heat cureable. Buy a cloth that is pre-impregnated with resin (expensive, watch out for shelf life and storage conditions), wrap it around your shaped core (btw, the core and all tooling and materials in the layup must be able to withstand the heat of cure)

I’m not sure about the chemistry of the resin but I know that different materials have different characteristics. By changing what is going on in the core you are changing what is going on with the board. Bert has often talked about how the benifit of the light eps is how it transfers energy. By going with a more heavy duty core you effect energy transference (I’m not sure how much). I think that where Bert has succeeded is by optimizing his boards shape and construction techniques to work with the materials that he used. Using different materials would likely require different construction techniques to work their best.

60

Well, yes - though failure of the glass/resin-foam bond is what usually makes it all happen in buckling failure. The foam is both light and structurally weak. That’s the fun part with fiberglass, like any other filament it’s great in tension but keeping it in column (as it were) ain’t gonna happen, in compression it’s pretty much the resin and the foam.And when the foam fails in shear, well, it’s all over. In a lot of ways it’s like reinforced concrete structures: the concrete alone is great in compression but in tension it’s the rebar and mesh that does the work.

Would, then, a very tough, dense, relatively thin foam layer sandwiched between two layers of glass/resin have both better bond with the laminate and better compression strength, if said panel was in the shape of a curved plate/plane such that it tends to resist compression quite well? The light, soft foam core is, more or less, there as a form and beyond that it’s just along for the ride, it’s more a stressed-skin structure than anything else.

food for thought, anyhow…

doc…