the neutral axis

General Discussion
121

Oops

Obviously the rules of physics don’t change

I meant the way we interpret the laws

Twang a ruler off the edge of a table or desk. It doesn’t go on forever. In fact it stops pretty quickly. Why? Flex back in the opposite direction is only a fraction of the original flex because in the “flex/counter flex/counter flex/counter flex/counter flex/counter flex” pattern each flex is smaller than its’ parent, until there’s no visible movement. I believe this is what you are talking about in a board. But because counter-flex is so small the bottom only ever receives a fraction of the compression of the deck and the deck a fraction of the tension of the bottom.

I understand this point… but your saying that the deck naturally wants to flex in compression .you would need to frame by frame picture a board being surfed radically to truly analyze this. I believe that its a lot more than a fraction

Do you remember riding flexible morey boogie boards

They naturally want to fold with the bottom being the compression side equally as the deck

Same as a surf mat

That’s a real world observation

Are you saying a surfboard is different because of it curved foil?

even pressure underneath from the water

Um. I don’t think that varies in and of itself (water) enough to matter - water compresses very poorly. Pressure from under the board is more closely tied to low/high pressure planing speed and weight on the deck. As I understand it.

What I meant was the that pressure (from water)under the board is EVENLY distributed

So the axis between the feet has the same resistance as under the feet.

But not as much applied force

Also are we assuming that the skins are indentical in density and strength (this is a huge factor)

The ends of the board would have to flex down, if that were true. Since centrifical force holds the rider onto the board during the turn

Exactly what I think they do… I think its that centrifical force that puts the deck into tension

okay so then the rider hits the lip and the lips force is stronger than your foot pressure(unless your Kong) so then the deck becomes the compression side

Yep. And when you go aerial it twangs back with the reverse force, becoming momentarily under tension

agreed

my flexi fish failed on the deck with a lengthwiswe split from landing an aerial

to me that indicates tension

and bert said Beaus board split as well on the deck

Why does it indicate tension to you? To me it suggests too much sudden donward force rupturing the supporting skin structure.

Mainly because guys don’t land aerial with their feet together or on one leg

So the ends of the board bend down over the centre axis

Causing tension on the deck

And making a long tear or split

rather then an outward crease

Which would indicate the board folding deckwards

Im prolly mostly wrong

But Im sweet with that

And our swell window is so small. if I want to be fit and surfing well when its good

I go for a paddle in 35 knot onshores

It paid off on Monday

Got some sick little sand bar barrels

122

Oops

Obviously the rules of physics don’t change

I meant the way we interpret the laws

LOL! Yeah. I think mostly we just don’t get them. That’s our history and heritage. But in most of macro physics we do get it now.

I understand this point… but your saying that the deck naturally wants to flex in compression .you would need to frame by frame picture a board being surfed radically to truly analyze this. I believe that its a lot more than a fraction.

What I was stating (almost quoting) is that flex, from a force and stress perspective, always consists of compression on the inside of the curve and tension on the outside of the curve (static bend). That curve isn’t the curve of the board, it’s the curve made by applying force to a line. What I was trying to say is that I believe most compression happens on the deck and most tension on the bottom. Because mostly the weight is in the centre(ish) of the board, not completely at one end or the other (otherwise the board flips). When the board snaps back there’ll be some momentary reversal in a flexible board. But that won’t be anywhere near as much as the opposite. Unless you turn the board upside down.

Do you remember riding flexible morey boogie boards

Do I! I still have one. Love it.

They naturally want to fold with the bottom being the compression side equally as the deck

Same as a surf mat

That’s a real world observation

Are you saying a surfboard is different because of it curved foil?

Urm. Not sure I get you here. if force is applied to something and that thing bends then it has an arc. The concave surface is experiencing compression. The convex tension. The difference between mat, boogie and flexing board is in bend resistance and, thus, recovery. We are heading back into flex territory again :slight_smile:

What I meant was the that pressure (from water)under the board is EVENLY distributed

So the axis between the feet has the same resistance as under the feet.

But not as much applied force

Also are we assuming that the skins are indentical in density and strength (this is a huge factor)

OK, gotcha. The axis is just the focal point. The force is spread between the feet from that focal point. How evenly is impacted by flex properties, board angle on the wave, etc. May as well assume that about the skins for now, to simplify things. We can vary it conceptually later.

Exactly what I think they do… I think its that centrifical force that puts the deck into tension

Do you feel as though you are being pushed into you board when taking a big bottom turn? I do. So I am being pushed into the board, the nose and tail will move deckwards. Don’t believe me? Find a waterbed or similar. Put anything flexy with some bend resistance in the middle. Push down on the centre of it down into the bed with your palm. What happens? The ende flex towards your hand. The surface under you hand experiences compression. The surace against the bed experiences tension.

Mainly because guys don’t land aerial with their feet together or on one leg

So the ends of the board bend down over the centre axis

I think I see the problem. To me “down” is towards the water and “up” is towards the deck. The ends bend deckwards. Ref my little waterbed experiment above.

Causing tension on the deck

And making a long tear or split

rather then an outward crease

Which would indicate the board folding deckwards

The ends folding deckwards indicates comrpession. We are talking about the forces the material experiences, not the forces being applied to it.

Compression is typically experienced on the side of the arc to which the force is applied (unless it’s reverse force on the tips, which amounts to the same thing). Tension is experienced on the surface opposite it.

Physics mumbo-jumbo, but it’s about a common language so that ideas can thoughts can be clearly conveyed.

And our swell window is so small. if I want to be fit and surfing well when its good

I go for a paddle in 35 knot onshores

It paid off on Monday

Got some sick little sand bar barrels

On ya! Fingers crossed that swell expected to hit the KI buoy is SW. Then all my paddling might pay off!

All the best,

-doug
123

Hey I was just reading that post… where’d it go?

Whew, I thought that guy had published some some bad e-info(Beware…it could be anywhere!!!). Thanks for clearing things up crafty, I thought I was loosing it. So, any links for me and test_article to check out on load sharing between the skins and core and rails. It seems to me that all this talk about the NS and compression and tension and shear is worthless unless we consider how they are tied in together at the rail. Thank you.

Sincerely,

Tranny

124

Closest thing I have seen so far is the Rails on balsa sandwich board thread. Which is more technique oriented. give it a read and if it’s not what you are after you might have to start another thread.

-doug

125
Quote:
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 …

I’ve been following this thread since the beginning and just didn’t get what this neutral axis was all about. And more importantly, why the need for the floating horizontal stringer. But I was talking to DanB at Cerritos about this and he insisted that he could significantly feel the difference with the floating horizontal stringer. So I decided to reread this discussion, and now I finally get it!

I’ve been studying composites for over 6 years now, and we are trained to make things strong and stiff, and not flexible. That’s what got me in trouble. I kept thinking why not put more structure in the skins if the board isn’t stiff enough. I’ve always been a big fan of flex, so it was unfortunate that I didn’t look outside the text book earlier on this one.

Like Bert says, too much flex isn’t a problem. It’s the type of flex. So adding the floating horizontal stringer isn’t to stiffen up the board, but to stiffen up the feel of the flex. You don’t want to reduce the amount of flex, just stiffen it up. And stiffen it up in the part of the flex where you want it.

Now I get all the rest of the neutral axis stuff and how that effects the floating horizontal stringer. This opens up possibilities that I haven’t even dreamed of.

…getting stoked…lots to think about.

126

If you want both flex and structure in the skin, go thin.

test_article

“No one appreciates the value of constructive criticism more thoroughly than the one who’s giving it.” –Hal Chadwick

127

thats so vague test article

what is “thin”

its relative to the length of the board for a start

is 1 mm thin or is 3mm thin

i know for a fact that you cant get airex any thinner than 3mm

so that must be "thin "on Berts surfburgers

128

Kenz,

Glad to hear you are getting stoked and that the possibilities are unfolding for you!

I like this environment because there’s so many ideas going around and people working things out togethor.

But it has it’s downside. Concepts I understand very well from years of working with them (outside the board building paradigm) are really easy to communicate in person. A bit of talk, some scribbles… But they can be reall hard to explain in this medium.

Your take on the springer jells pretty well with how I see it, so I have to agree. Besides, there are other ways to stiffen up a board without adding internal structure if this becomes necessary.

Maybe one day we’ll get a nice thread rolling on what the different dimensions (and I mean shape in all three dimensions) means for the springer… And skin cross-sections too, for that matter.

-doug

129

hey doug

yeah yeah cool

i understand what your saying as well

theres just so many factors involved in the equation

everyone ignores the tension on the deck

its just something that kinda dawned on me

and id like to try and explain more with diagrams why its important

there maybe not as much tension as the bottom

but the way its affected by it, i feel is an an important factor

especially when we are building boards that flex alot more

ie. being able to be able to flatten a board upside down

if it happens in my living room

surely its gunna happen on the wave as well

130

theres just so many factors involved in the equation

Oh yeah! I think the thing as a whole is at least as important as it’s components. It’s in mixing it all togethor.

On that note - why do we discuss these component factors in iosolation? Just to hear ourselves sound smart? I think it’s so we can understand how all the different parts hang togethor and interact… At least, that’s what I am trying to do :smiley:

everyone ignores the tension on the deck

its just something that kinda dawned on me

and id like to try and explain more with diagrams why its important

there maybe not as much tension as the bottom

I look forward to that! Like you say, if you have a flexing boards that recovers it’s shape the compression/tension loads have to reverse at some time. That’s a mechanical fact. Yes, the reversed forces are less. And yeah, I reckon there are other times that this is more pronounced. One obvious example is when you wipe out, the board ends up upside down and that big old closeout comes along. I think there’s other more subtle examples.

but the way its affected by it, i feel is an an important factor

especially when we are building boards that flex alot more

Yeah, probably.

ie. being able to be able to flatten a board upside down

if it happens in my living room

surely its gunna happen on the wave as well

Uurmm… Just thinking. Noseriding a compsand longboard. One end is held down by the wave. The other has you perched on it. Reverse flex - that reverses the compression/tension forces.

In one way these things are bloody big springs. If you go loading up the centre and then loading it and loading it it will HAVE to spring back sometime. Giving that momentary reversal of flex.

Anyway, enough idle thoughts :smiley:

-doug
131

some one in a factory that gets a lot of broken boards in

would have a better analysis

i guess for me , the theory should be applied to a board in failure .

rather than in performance

if you were to get an aerial surfers broken boards over the years,

say about 4 a year maybe.

you would find that their breaks go both ways.

someone here may have the ratios.

but in terms of failure, I think deck tension really does come into play.

as you say,

a wave hitting an upside down board.

a lip hitting the nose of the board and snapping it off.

a landing with the feet way apart(a aerial surfing friend said this happens alot).

how this relates in regard to construction,

i cant say

anyone whos built a compsand ,knows how amazinlgy light and strong they can be

but NEvs abused board looks to be in a whole different league

132

Well, duh!! :smiley: Something just clicked.

Shortboard, feet way wide apart, aerial. Yeah, that could reverse flex quite well :slight_smile: Especially landing with the lip between the feet. Ala Taj.

Forgive my addled brain :stuck_out_tongue_winking_eye: I suddenly get what you are saying :slight_smile: Still less frequent than than “normal” riding… But sudden and strong. Could result to a snap, hey?

-doug

133

this is getting complicated

my brain hurts

see ya tomoorow

134

okay

so the concept of “performance” is variable to consumer needs.

my ideas of performance would be a board i can

ride good

chuck around, land floaters on the sand.not worry about a girly board bag.be relatively ding proof.

last twenty years .have no yuppie,showoff surf label printed on it

if your talking springers and NA for added “performance”

how does factor into my needs as an average surfer.

surley improving my fitness is far more relevant than improving my stick.

even top level surfers can be absolute piss heads

surely reducing alcahol intake would have a more relevant impact on “performance” than 2% faster stick.

okay say i begin to understand the concept of neutral axis.

is it relevant in application to theory

Re.

speed

manuvres

durability

flex.

it has been dicussed, relative to the tension side and compression side .

And agreed, for planning and turning the deck is in compression.

how about regarding to board failure.tension in the deck obviously comes in to play .

how does this relate to the neutral axis and does the placement of a springer counter the forces applied by extreme surfing.?

135

To be clear I see the NP as primarily related to durability. I see it as having a relationship with flex, so that flex doesn’t compromise durability. I may well be wrong!

Durability - Pretty sure I said earlier that NP is an important factor in durability - which rates high on my personal list. I think that’s relevant to all surfers. If the NP is centred evenly between the deck and the bottom and doesn’t change suddenly you have a far more durable board.

When the NP/NS “steps” suddenly you have a hinging section which is asking for trouble. This area of weakness is the most likely place where a board will bend too much and is where it will probably snap, if it ever comes to that. The bigger the “stepping” of the NP the greater the weakness.

If the NP is closer to one surface than the other then the forces in action on the board are out of balance. Closer to the deck and the board is weak in compression (the deck experiences more compression than the board does tension). Closer to the bottom and the compression force is greater. And there’s a corresponding weakness when the board counter-flexes.

Flex - NP/NS becomes important only in the presence of flex - no flex, no point thinking about the NP. If you know where the NP is going to be and you are ready to manipulate flex with reinforcing (springers, etc) you should try to ensure that the NP is evenly centred between the deck and the bottom. Which comes back to durabiliy.

Speed - More related to flex, I think.

Manouvres - Also more related to flex, I reckon.

how about regarding to board failure.tension in the deck obviously comes in to play.

how does this relate to the neutral axis and does the placement of a springer counter the forces applied by extreme surfing.?

I think it can. Not necessarily always. Probably very likely with styles like Taj’s (rather than the more average surfer). Esp given they’ll almost always sacrifice durability for weight. In that case locating the NP in the central plane is even more important. Not saying it’s the only thing, tho. The way I see the springer is to improve return (aka snap) and control flex. Locating it in the NP (and having the NP central) optimises it for flex and counter-flex and helps retain durability.

-doug
136

Not sure that it applies… such a good thread, it’s worth bumping.

There’s been some talk about hourglass horizontal stringers, etc. to control flex.

Here’s a horizontal springer concept… with credit to Bert, the archery people and the ski people.

Combine arched horizontal stringer (internal/pressed flat) to add spring with positioned sidecuts as shown in ski pic cutaways - (nose:top, tail:bottom) to isolate flex pattern… (note different length of sidecuts on skis one foot up and one foot back.)


137

Those look a lot like the horizonal stringer I’m putting in one of my boards. I researched buiding a long bow for ideas on shape and wood selection of the stringer.

138

Hi Dan,

I bet researching that was interesting :smiley: I think you’ll find that the “traditional longbow” isn’t going to be optimal here (I give you my word as a long-time bowyer). The main problem is that the ELB (English LongBow) cross-section isn’t flat, it’s rounded or “D” section. That’s going to make use of it in a compsand board a little tricky. Another reason has to do with materials. Fact is, the ELB is not the most efficient bow design. The general consensus at the moment is that the ELB was made the way it was, and out from those materials (Yew), for mass-production purposes - the people making them were trying to equip a large army and needed to make these quickly and get as many as possible from a given log. As a result the ELB design only works well with a handful of timbers. The “flatbow” design on the other hand is far more efficient and safe design, and allows the use of a huge variety of timbers. With modern power tools and thicknessed timbers it’s not too difficult to make something basic.

For use in a surfboard you don’t need to worry about tension failure (the EPS reinforces it in tension) so this simplifies things even more. A little searching online will show up a number of flatbow designs that could be used as a springer.

I would particularly recommend the use of a bend-through-the-handle pyramid flatbow design. I’ll even give you a quick rundown on laying it out. Get a 2" wide, around 1/2" thick board of your choice, the length of your springer. Mark the centre and then one inch towards each end. Find the centre of each end and mark out 1/4" from there (giving you a 1/2" section). From the corners of your off-centre marks to your 1/4" marks draw a line (that’s four lines folks). Now cut off the corner sections (blue in image below).

A few important notes if you decide to use this:

  • This is a good way to get a basic springer, but NOT a "real" bow. There will undoubtedly be violated wood fibers on the back, stringing and drawing this will inevitably result in a shower of splinters. To make a bow from here you would "back" it and then tiller it (that's real art of bowmaking).
  • This springer will probably need some tuning - it's a bit stiffer in the centre "handle" area and "whippy" at the ends. To make an area less stiff carefully remove shavings of wood evenly from each side.
  • Using different initial thicknesses of board will result in different overall resistance of the springer. 1/16" thicker will be twice as resistant. 1/16" thinner will have half the resistance. So be prepared to do some experimentation first and don't try it off the cuff with really expensive timber.
  • To drive the point home BE CAREFUL HOW MUCH YOU BEND THIS! Without some kind of reinforcing or protection (like being embedded in EPS) it WILL fail in tension.
Other types of "flatbow" springer are possible. Looking into different bow designs could be very rewarding. And designs NOT based on bow design may be too!

Also, please keep in mind that I haven’t actually used any of this in a board yet. I’ve just built bows this way.

887×75 7.1 KB

139

Interesting that both of you are into bows…

I was able to “arch” a strip of plywood by laying it across a form and glassing one layer of cloth on it. I glassed the inside of the arch but perhaps it might make a difference if it was on the outside? Anyway, it seems pretty springy. In the area of theoretical cutaways (shown in hastily drawn diagram), I suspect it loses some strength so it will flex more in those spots and retain stiffness in others.

Like Doug, I haven’t tried this in a board yet nor have I worked out the details… simply brainstorming here.

140

Yep. That sucker is really gonna bend at the cutaway. From the look of it, twice as much as the rest will. in effect that’s going to promote hinging. The more gentle and linier the changes are the gentler the change in bend. Which might not always be what you want (i.e., what John has posted may be perfect for what he wants). That said I suspect that we will want the COG area on the board to have a bit more resistant to bend than, say the nose or tail. Not sure about that, someone should check using search/archives.

In any case, too sudden a change of flex could introduce a “step” in the NP that will likely provide a focus for force to concentrate at, which would induce weakness.

Glassing the back (side away from which the ends are bent) or similar will be critical if you want to give it a significant bend outside the board. but it will change it’s flex resistance/profile somewhat. You can mitigate that by gluing something quite a bit lighter than glass onto the back.

Interesting.