the neutral axis

General Discussion
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[replyWould, 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,

Well I just finished chewing, literly on my pizza and the tail of this thread :slight_smile:

If I followed correctly… The balsa skin (glass/balsa/glass) on a concave or convexed (domed) deck would be better in compression.

i.e. balsa instead of the thin foam you mention and then concave/convex deck in reference to your curved plane?

my sandwich from the bottom up will be:

6oz, 1/8" balsa, 2.3oz, 2.75" 2#(?) EPS, 6oz, 1/8" balsa, 6oz

At least this is the plan for my noserider. I think I have 2# eps but not sure about that.

Short board will prob have 2.3oz, balsa, 2.3oz, 2.25" eps, 2.3oz, balsa, 6oz.

both with solid balsa rails.

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I think that you are going too strong. I made my original boards that way but its way overkill and hinders the way they surf. I think that 4 oz-Balsa-2oz on the bottom is plenty strong while 6 oz-balsa-2oz is plenty for the deck (and thats with the 1 lb eps)

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Quote:
I think that you are going too strong. I made my original boards that way but its way overkill and hinders the way they surf. I think that 4 oz-Balsa-2oz on the bottom is plenty strong while 6 oz-balsa-2oz is plenty for the deck (and thats with the 1 lb eps)

for the noserider, actually looking to add a little weight.

I think I’ll order my next batch of balsa as 1/16" instead of 1/8".

Only have 2oz and 6oz glass, and have a ton of both.

with mixed weight glass, is it better to put the heavier glass on the tension side of the composit? i.e. for the deck on the inside and the bottom on the outside?

I was first thinking 6oz on the outside for impact/dent protection but wonding if tension/compression tuning might be more important?

Will a +3" think 10’ noserider flex? (not doing step deck)

i.e. my cruiser does not flex (~4" thick) so strength means weight (generally a good thing in a classic noserider) and is not a flex issue as it does not flex. Is flex an important part of a +3" noserider? if so maybe I need to sacrafise wieght for flex???

64

I think that a lot of the rules change when you go sandwich. I’m not saying that you need to dump all that is known about traditional boards but rather ask yourself why was it important to the poly board and is it still important on the sandwich board. So far the only place that I miss weight is when it is choppy out. My lite board knocks me around and the extra weight would settle it down. This means that if I plan to surf much in the afternoon (like during the summers) I should probably make a heavy board in order to have fun.

As far as the glass weight theres two ways to look at it - you can glass it heavy and lock everything down or glass it lite and free everything up. In the first case the wood is just a pretty decoration (since it can’t move its not doing much). In the second case you are minimizing the influence of the glass and increasing the benifits of the wood. It just depends what your trying to do.

Here’s a different way to view noseriders. I’ve read that noseriding works like a teeter-totter (you put weight on the nose which makes the board want to pearl while the wave raps over the tail trying to pull it down). The large tail rocker magnifies the effect of the wave. Since this board is stiff the teeter-totter analogy works great. HOWEVER, the flexy sandwich board is more like a teeter-totter made out of some bendy material. When you put the weight on the nose the back of the board still tries to pull out of the wave, but not as much because the board bends to some extent. This means that you do not need as much tail rocker (suction) to balance out the board.

Most of the sandwich talk has been geared toward shortboards, but I still think that longboards have the most to gain from this technology. Light might be enough on a shortboard, but longboards really need the structure that sandwiches provide. I know that others might not agree with me (Bert included) but I think that there is a reason for this comment that Bert recently made:

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the bulk of nevs recognition has been through shortboards and his shortboard teamriders , where as mine has been through longboards and my longboard teamriders …

This applies to more than just doing longboard aires

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4est, I’m with Dan. With your glass, I’d do 2 oz on both sides of the bottom skin. On the deck, 2 under, 6 over. If you want to add some weight, use 1.25 or 1.5" (built up) for the balsa rails. That’s the right place for the weight anyway…but with your 1/8" balsa, it’ll be around 20-21# as it is.

You can also add weight without affecting the flex if you paint a warm (thinned by heat) epoxy seal coat on your balsa before laminating on the outside glass. You’ll get a more even color that way when your lam goes on. Also a nice big tailblock made of something heavier like mahogany will put some weight on the back end…

66

Yeah, I think balsa would be, if anything, superior to foam considering shear with respect to the glass/resin layers. Especially vac-bagged, which should promote better resin penetration. Though there is the question of fatiguing the fibers of that balsa under regular shear loads.

Now, a convex deck ( cambered ) is gonna have more stiffness than a concave one. The whole surface will be resisting compression rather than the two ‘humps’ at the rails; think about the forces in action under a bending stress and you’ll see what I mean. If my mental picture is correct, the more curve, the less that’s in compression, ultimately, and the more that’s in easily handled tension. Besides the tensioned parts of the structure tending to pull the compressed centerline into a greater and more compression-resistant curve.

A couple other thoughts:

The thinner your ‘skin layer’ overall, glass/balsa/glass, well, the less rigidity it’s gonna have. Get down to 1/16" and you are pretty close to effectively no intermediate layer at all. And, I suspect, a fair amount of buckling likely. If you think of the outer surface as in compression and the inner surface in tension, the difference between 'em may well be important.

The solid balsa rails… ahmmmmm, thinking about that. Again, they are kinda along for the ride. Tradeoffs are more resistance to compression vs a light foam but would the overall structure be weakened by the transition from a very soft core to a relatively hard one? On the other hand, that’s the locale that would tend to buckle in compression. I hate to say it, but it looks like this is experiment time.

As Dan is saying, lighter glass layers is going to work better, spaced, than heavier layers of glass together. If you think about what the reasons are for heavy deck glass, then you have a big one, resistance to denting the thing in when normal use is figured, heels and so on. The extra cloth doesn’t really do much, but the extra depth of resin does. Now, your glass/balsa/glass gives at least the equivalent, heavier glass outside to protect the balsa.

Some thoughts on core-foam weights… besides adding weight, perhaps the heavier foam might act as a dampener, such that the flex and vibration would tend to be better absorbed by the heavier foam? The harmonics of the board ( think about the sound a board makes when going through chop- the higher pitched they are, the greater the initial stiffness and speed of snap-back, if I may coin a term) would be lowered somewhat with no lowering of ultimate strength. Flex might be slightly diminished but the snap-back would be slower, though it’d be incremental. Ultimate strength… good question. Depending on the strength and stiffness of your outer sandwich, the core may not be contributing much, especially in compessive buckling situations, as that’s gonna be along a rail-rail line rather than over an area, and the compressive strength of heavier foam isn’t really gonna be a factor. I mean, hey, it’s foam, it’s all weak that way.

And on that note… you mentioned pizza, which I can’t stand, but I do believe it’s time to throw a chop on the fire and mebbe some veg. And another coffee…

hope that’s worth thinking about

doc…

67

dan that was inspirational …

you packed so much understanding into that comment it impressed me , obviously your picking up alot of stuff from the boards your riding now and making some real clever deductions …

just to clarify a comment you made above , in reference to eps transfering energy better …

that may be out of context , i would be interested to see where i said that , you may have misinterpreted my saying something like , lighter eps spreads the load better , or tranfers the load better …

its probably a case of interpretation of words , i know ive had conversations with tom about fins and ive interpreted comments a certain way , but when re reading them could see a subtle difference in meaning …

one material may transfer energy real well and another will absorb energy , the advantage eps has , especially as it gets lighter, is the ability to spread load further through movement , it may not make sense , or may sound like a paradox , but the fact the eps moves , means that its transfering energy away from a point of impact and load is spread through the whole board …

so maybe to clarify if what you quoted was how i worded it , eps transfers energy by redirecting it in many directions evenly …

it transfers energy directly , badly , but transfers energy indirectly well …

imagine a baloon filled with air , as you press on one side , the air exerts the energy throughout the baloon evenly so the effort you put into pressing it ,is transfered in all directions …

its this characteristic , that gives the neutral axis way more range to move through the entire structure , then its your shell that starts to have the features you want for what ever performance your chasing …

dan , if you want to feel less bumps in bigger waves or choppy days , make the bottom softer , so it absorbs bumps …

we only feel bumps when they are transfered through our boards to us …

i once had a team guy win a contest in 40 knot cross shores , where the wind was coming at the surfers and the face , with 1 foot chops on 4 foot waves …

the entire field of competitors laughed and said he would be the first to be eliminated because he had the lightest board and would get blown away and feel every chop …

his secret was a board that absorbed chops through structure not shape …

this left him with a shape that could respond quickly to chops and bumps in the face , where he could change his mind at will and do quick direction changes off a random chop …

the wind was there , so he had the wind to contend with , while others had the wind and the weight of there board to fight against …

so stoked this is even a subject under discussion …

regards

BERT

68

Thanks for the compliment! I wish that I was more in tune with what was going on with my board. I’m only able to notice these things because its so obviously different from what I’ve experienced in the past. I think that when the “true” shapers gets their hands on this tech you’re going to see an explosion of innovation.

Thanks for clarifying the transference of load through eps. The way you explained it helped when I reread some of your old comments. I was over simplifying what was going on. I’ve reread the post several times and it got me thinking. I though that I would put out some ideas:

  • I think that is is salomon that uses the strips of foam to support their boards. This would be a bad thing since in forces the load to travel a set path and not distribute throughout the board (forces direct transfer of energy versus indirect)

  • I originally thought that your board’s free floating bottom skin was a result of convenience so on my first boards I had locked together the top and the bottom of the board. On this board I didn’t. I’ve noticed that this board didn’t want to buck me off it but never related it to the free floating bottom skin. After rereading your post I’ve got a new theory - The combination of the free skin, the flexible epoxy (RR2020), and the light eps work together to absorb the bumps that would normally buck me off by allowing the skin to absorb some of the bumps energy and the eps dissipating the remainder throughout the board. This means much of the bump isn’t reaching me.

  • Because of some of the comments earlier in this thread I had re-evaluated the trampoline analogy of the horizontal stringer. I realized that its main function was to add support to the board without hindering flex. I’ve got an idea of why the stringer works better when its located close to the boards original neutral axis. Instead of viewing the neutral axis as a line (even though it probably is) through the board it might be more useful to view it as a range or zone. By having the stringer close to the axis makes it look like you don’t want the stringer to interact with the skins characteristics. Since the neutral axis moves through the board as you surf if you place the stringer close to the original neutral axis it is likely to remain under the axis’ influence (because its a zone) even though the axis has shifted (I know I wrote it poorly, but its hard to articulate).

I probably shoud clarify that the chop I was talking about wasn’t when I was on the wave, but rather when I was just sitting on the board and some side chop hits me. The boards lack of weight makes it too responsive the oceans movement.

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Quote:

the neutral axis moves through the board as you surf if you place the stringer close to the original neutral axis it is likely to remain under the axis’ influence (because its a zone) even though the axis has shifted

I was rethinking this last night and I think I have it backward. When the board is in a non stressed state the stringer does not influence the board at all (it is in a place where nothing is going on. However, when the neutral axis moves because of stress on the board the stringer is no longer in the neutral axis and it starts to strengthen the board. The more the neutral axis moves from its original location the more the stringer comes into play.

70

if youre referring to a h stringer then your theory holds if the stringer is firmly attached to its surroundings…if it floats then the board’s NA ‘movement’ has very little affect…its more of the bending moment acting on the beam/stringer…iow, only the NA within the stringer component itself when loaded plays a role…which is a very small factor in bare balsa wood

wrt floating h stringers…anyone test this single component individually…meaning just taking the component by itself and bending it between two supports…it’d be a good approximation of a floating stringer inside…what happens if you support the ends and stand in the middle…better yet put in some core foam as if youre buildig a board and retest

some final thoughts…

NA doesnt exist in a no load/stress situation…it is a demarkation line/plane between compressive and tensile stresses acting in a panel

NA in a sw panel is really a neutral plane NP - not a line/axis…and it likely has a shape similar to the deck

making the NA move in a way that favors the ride is probably pure luck or coincidence…unless your making sunova’s :wink:

its more about skin control mee thinks…

71

I’m glad you’re weighing in on this. I think you have the best chance of explaining the mechanics of it. Heres some ideas/answers/questions for your thoughts:

Quote:

wrt floating h stringers…anyone test this single component individually…meaning just taking the component by itself and bending it between two supports…it’d be a good approximation of a floating stringer inside…what happens if you support the ends and stand in the middle…better yet put in some core foam as if youre buildig a board and retest

I’ve done both and this is what made me such a fan of the h stringer. It really transforms the foam.

Quote:

NA doesnt exist in a no load/stress situation…it is a demarkation line/plane between compressive and tensile stresses acting in a panel

In practicality, wouldn’t this be the same thing as the whole foam core being in NA when there’s no load. The end effect is that the stringer isn’t influencing the board.

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making the NA move in a way that favors the ride is probably pure luck or coincidence…

I sort of agree. I don’t have the tech/background/skill to guarentee were the NA is but i would guess that it would be near the center of mass. By placing the stringer near the NA wouldn’t it move from compression to neutral to tension as the NA moved through the stringer (I know that the stringer has 2 sides (one in compression and one in tension) so both would be flipping. If its too far away from the boards neutral center the stringer will come into play excessively when the board bends one way and not enough when it bends the other.

i

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ts more about skin control mee thinks…

I don’t want to downplay skin control. There is so much we agree on with the composite boards that I have was puzzled why you and sabs downplayed the stringer while I over played it. I know that I came across as your way was wrong, but that wasn’t my intent. We were both trying to get a strong, light board. What I finally decided is that longboards NEED the extra support while shortboards can probably get away without it. Its all about the forces that they encounter (try and bend a 1 foot piece of rebar with your hands, now do it with a 8 foot piece). On my boards, a single horizontal stringer adds less weight the any other type of support. In order for Bert to be making 8-9 lb longboards we has to be glassing like someone would on a super lite shortboard. The strength has to come from somewhere.

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thanks dan…

i agree with most of what youre saying but we proly are looking at things from a different angle…

for instance, when i say the NA doesnt exist in a no load sitch and you say the entire core/board is neutral we’re angling differently but we are both correct…the board is neutral and there is no axis…weird eh…your point is well taken…

totally agree on the longie point but i will add that the shell is a major/dominant contributor of strength…

all this is interesting discussion but if you really want to know the real mysterious thing about all this…one of my key last unanswered q’s…how to make balsa wood do the things that are necessary to make these things happen?

or maybe my assumptions wrt that are just off base…

…or maybe im just kooky la dat…

but its not really important to me at this point…just curious from an engineering point of view…

yeah whatever happened to good’ole sabs…proly busy eh?

lurker status?

yeah he’s the classic sways hit and run artist :wink:

proly makin 3 pounders by now…AAAHOOOOOHH!!!

73

Just another random thought, but as load increases proportional to the distance from the neutral axis, a rectangular cross-section’s ability to support a bending load (or, cross-sectional moment of inertia) increases with the cube of the distance from the neutral axis. A surfboard’s transverse cross-section is very seldom rectangular since the rails are rounded and there is probably a little curvature in the deck and contour across the bottom, but to be completely accurate would require more complexity than needed here to explain why it is advantageous to concentrate on the load bearing capacity of the skin.

If you want to add weight with least effect on other characteristics, the neutral axis (or as close as you can get) is a good place to do it.

Fun, fun.

74

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 …

now you can still have shear movement but because you have reduced the shear stresses and friction you get the full spring back without wasted energy …

just to throw another concept into the mix …

therefore to get the most out of any internal memory device , whether it be carbon or timber , placing it in the region of the neutral axis will allow it to work with the least amount of shear forces acting on it wasting 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

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Quote:

that gives the neutral axis way more range to move through the entire structure

How does this quote fit into the mix? I see what you’re saying about the stringer having to work against shear forces and that it would work its best (and most predictably) in the absence of shear (the neutral axis). Hewever wouldn’t this make a fixed neutral axis the goal instead of a moving neutral axis?

Quote:

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

This makes me think that you are using a multilayered stringer designed to reduce sliding friction. If this was the case then wouldn’t the neutral axis become unimportant because the stringer able to accomdate the shear stress

I need to reread what you originally said about neutral axis. It could be that you said that things work best in a neutral axis. This would be very different that placing the stringer in the neutral axis (since its moving and hard to find in the first place). If this is the case your stringer would mimic a neutral zone.

76

There’s a lot of esoteric stuff being bandied about here regarding the ‘best’ way to construct a flexible board, and many references (here and on other ‘flex’ threads) to ‘tuning’ the flex correctly, but no one seems to be quantifying anything, and it seems to me that there are several key variables which haven’t even been mentioned yet.

For example:

  1. So far it seems thst no one here has tried to measure the amount of energy it takes to bend a particular board, other than by the very rough method of counting the number of people jumping on the board when it is parked upside down on land (completely different from what happens in the water). Surely it would be useful to measure the forces involved ?

  2. How a flexible board reacts in the water depends upon how long it is, how much surface area it has and how that area is distributed. . . . . . . all else being equal, a longer board takes more energy to ‘load up’, and all else being equal a board which has more area distributed towards the centre will be harder to ‘load up’

Of course we can still build flexible boards using instinct and experience, (I do) but if we are going to get technical then we have to look at the whole picture.

.

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Meecrafty Dan

Im still here just hell busy at the present time

Im off to West OZ for five weeks in three days time

I ve been doing lots of experiments and a whole lot of new things

Loads of stuff ready for disscussion in the new year

Mike

78

Bert,

At least we could start with the surfing equivalent of a ‘draw weight’ and ‘draw length’ (tested on land with the board supported on the ends) and relate that to the length of the board . . . . . . better than having no idea at all !!

And on top of that a discussion of the effect of length, overall surface area and area distribution on the flex characteristics of a board while in the water is vital !!

Apologies for the digression but IMO the ‘neutral axis’ discussion will not lead anywhere unless the basics of surfboard flex are understood. . . . . . and the topics above are compulsory in order to pass flex 101 !!

Roy

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Have a great time! I’ll be looking forward to the info - you always give me lots to think about.

80

All,

Been away down at Middleton Point for a week for family fun and nice swell. Needless to say this is a harsh Monday morning for me… But I only have to brave 5 days of it and work is over for the year, so…

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I disagree. The NP and NAs are always there. If you understand their placing and the factors that affect them you have good control over them and can (should) use that to your advantage when constructing a boards. E.g., Placing the NP equidistant between the bottom and the deck is ideal, we can also place reinforcing in there.

DanB

I agree with your response to T.A. I’d add to it that part of the trick sounds like being selecting the right material to use in the right place. Gotta say a lot of the rest you said also makes a good deal of sense to me.

Bert,

Your sticks and layers thing is interesting :slight_smile: I’ve looked at this “puzzle” from a different perspective before (bows, etc). Do you realise that it takes for more energy to bend your three pieces of timber when they are glued together. So more energy is stored and released buy the same bend. But it requires more energy to be put in :slight_smile: When you are trying to maximise the energy that can be stored things get interesting! :smiley: The vast majority of the energy stored in this kind of structure is in the very surface layers, the layers in between don’t normally store any at all. Do you know that you can make the NP “hold” energy too :slight_smile: Effectively activating it? Not sure whether there’s any application for this in sandwich boards, tho.

TomBloke,

Actually, I have thought about that somewhat (coming from a bow building background) :slight_smile: I think in this paradigm “deflection weight” is probably the most important factor - that is, the amount of weight it takes to cause a standardised length of the board to flex by a standardised amount. Maybe how many pounds to bend the five feet in the centre of the bending section by an additional one inch. The other important factor would probably be board length, but we already have that.

All,

Reading the rest of the thread with interest. I’d like to suggest something that should probably be kept in mind: Yes curved shapes are stiffer, but they also typically break sooner than flat shapes. We probably need to keep this in mind with our designs - there needs to be a balance between a number of factors.

Materials themselves have inherent abilities to store and/or release energy. But these are constrained by the form of the design.

Take two identical volumes of foam, same length but different cross-sections. One thin and wide, the other perfectly circular. Skin both with identical balsa, glass and epoxy. Now bend both until they break, measuring the force required. Which breaks sooner? Anyone want to guess :slight_smile:

Doc knows my answer and I am sure Bert can tell you off the top of his head.

Interesting to read the thoughts from so many different perspectives, as usual.

-doug