Stiffness Vs Strength

[Bille]

If Ya re-read this post of --plummet--
You will note that :
Land-boards take WAY more punishment than a Kite board; so he probably has
a Good understanding of "Strength"

I'll start off by saying I haven't build a kiteboard yet. But i have build nemerous skateboards and a landbaord that flexes 130mm and bottoms out on the ground without breaking. Infact i have had no failures in all my builds. the techniques are very simalar. infact the landboard is quite abit more difficult than a kiteboard.

I say use Uni-direction mainly with a couple of layers of Double bias on the 45. Matt in either glass or carbon is the weakest form of layup. All the fibres need to go in the direction you want. 90\0 as handy states is pretty pointless. 50% of 90/0 matt fibres are doing nothing. So..l Uni directional will give you the strength lengthways. double bias on the 45 will get your torsional strength.

Get rid of the pvc core and use a more flexible/durable core. Bamboo is the shiz.

Strength is Different than stiffness.
Strength mostly refers to durability while stiffness is more associated with ability to flex.

A Kevlar board would be Strong, but stiff as a noodle.

A Carbon board would be Stiff,(resistant to bending) but carbon is like
an egg-shell, it will crack and and chip when point loaded.

A carbon board that is reinforced with Kevlar, will be Stiff, + it will have ample
ability to resist a point-load, (ie rock ding).

The core material used will determine the balance needed between stiffness and strength
A Thick board will need Less skins because the stiffness increases by the cube of the height ,(thickness).
The further apart your skins are, the stiffer the beam will be.

So a thick core needs less skins to keep it rigid so a lighter --Lb /cubic ft-- core will be needed
BUT
The heal of your foot can now crush the core because Ya need less skins to protect it.
& just How much Flex do Ya want or need to turn the board ?

So here is the start of Over-all Balance in the Matrix !

Things like:
Torsional stiffness must be considered.
Imagine a 2 X 4 stud was attached to each end of your board at 90 degrees & sticking
out 3ft.
Lift the end of one stud & press down on the other, ( This is Torsional stiffness).

Placing your Fiber orientation down the length of the board,on a + - 45 will Increase Torsional stiffness.
Placing your fibers on a + - 90 will decrease the Torsional stiffness.

Nether Fiber orientation will help with the resistance to crushing your core, (point load), unless
there is a shear-webb like ply or carbon keeping the skins from separating or getting closer
together. Then the + - 45 works better because there are More fibers crossing the sheer-web; but
then your Increasing the torsional strength as well IF their placed both Top and bottom of the board.
placing the + - 45 fibers just on the top deck will Not increase torsional stiffness.
So now were back to Just how strong does the core Need to be ?

If Ya use a Bamboo core, (on end-grain) Ya got resistance to crushing over 100% of the surface aria
but it's heavier.
If ya use 3 Lb / cubic ft foam, then it's Light but will crush after a bunch of jumps.

For me-- i prefer using a lighter 3 Lb ; then strategically place the bamboo or carbon or ply
sheer-web only in the aeries that are Most at risk for crushing. ie-- the center where the tie-down bolts are located, and under the heal of your foot, & ((don't forget about the front of your foot if ya ride tow side a lot)), especially if you land jumps on tow-side !!

The Carbon Uni is the BEST bang for the buck when looking at Stiffness. It should be placed directly on top of your shear-web. When purchased on the roll by the Lb, it's kinda cheap.
The moment they add cross fibers to bind the Tows of carbon, the price increases Drastically!

E-glass is a Good material for row-boats in a flat lake, I won't waist my money on this product
when we got :
***S-Glass which is near 30% stronger and 15% stiffer than e-glass ,--(( "for the Same weight" ))--.

Kevlar is a Good product when combined with Carbon. But a lot of people don't realize that
Kevlar fibers Don't wet-out like F-glass or carbon.Resin will ,(Encapsulate) the fibers of Kevlar, but Not saturate them. For this reason i prefer S-Glass.

& -- OH Yea : Don't even Think about adding MATT to your skins ; all it does is add weight and is fairly Useless .

Hopefully some knowledgeable people will be willing to argue with me
so we ALL can find that "Perfect" compromise in the matrix for our
next board ?

Bille

[gbleck]

I think the word mat is being used instead of weaved. Unidirectional or 0deg, 90deg has your fibers all going one direction on each layer and not woven up and down through each other. Weaved glass/carbon/kevlar is what we usually think of when we think glass fiber. It comes in different weave patterns of over under. Matt fiber is just short little glass fibers in a big flat unorganized sheet. It is not for kiteboard use.

[Bille]

...
Matt fiber is just short little glass fibers in a big flat unorganized sheet. It is not for kiteboard use.

YEA -- Matt, were talking about the same thing; & your correct, it's NOT for
Board use.

On the Fiber Glass thing, E-glass is cheaper but the S-Glass is about
30% stronger and 15% stiffer than the generic E-Glass. Some of the S-glass is
stiff as a board & difficult to work with. I found some 4oz that's good but can't find
the link, so i'd advise Ya to just get a yard of it to see for yourself.

Two layers of 4oz S-Glass will be a LOT stronger than 1 layer of 8oz E-glass,so
adjust your lay-up schedule's accordingly. Also since the fibers are smaller the
area between the fibers will be smaller so the resin saturation sill be lighter than the 8oz.

On Fiber to resin ratio :
I found that placing the materials on a Gram scale and Matching the resin weight to the fiber
weight is the easiest way to get the fibers saturated. now Ya got 50/50
THEN
I will take an old cotton t-shirt & put toilet paper inside, then blot out the 100% resin
down to about 85%. so now your fiber/resin ratio is more in the ball park.
50/50 is TOO much resin !
Be Careful though, you can OVER DRY your fibers real easy using this system.
I press down with the edge of a spreader and look to see how much resin is lifted
when i pull up on it. Practice This on a cut-off & toss it back on the scale to see what
ya got.

The Top skin is easier, Just spread your resin to 50/50 and peel-ply; then place some thin
couch stuffing material between the peel-ply and the vacuum bag. The fiber/resin
ratio will be about Perfect with this method !!
Another benefit with this method is the vacuum is Evenly distributed along the entire board.

If i don't do this then I usually place a 1/2" strip of that stuffing around the perimeter of my
part to ensure even distribution of vacuum.

Bille

[gbleck]

I use cheap rope for that. Cotton or other cheap rope.

[Bille]

I use cheap rope for that.
Cotton or other cheap rope.

NICE !!
A hollow braided cotton cord, would save me 5min cutting time
and transfer the vacuum even Faster !!
Not to mention it's narrower so it won't take up as much
of my joggle space.

Thanks : Bille

[Bigdog]

Triaxial E-glass in 20-24 oz is pretty hard to beat as a skin. This is also more or less a standard for snowboards. It is best suited for a high pressure setup though and harder to handle under a vacuum setup due to the raw stiffness of the cloth.
Upgrading to S-glass or carbon is in most cases a waste of money if your design, core material and local reinforcements are right.
Your mileage may vary

Stan

[windrupted]

Whoa....more information here than I can process. Trying to absorb it all. Ok, if you know so much tell us how to make a bottom mold/rocker table! Thanks, sincerely. Great information.

[Bille]

...
Trying to absorb it all. Ok, if you know so much tell us how ...
.

The way you question the problem, immediately puts me
on the defensive. Others too i'de imagine ?

Yea , i Do know how to do that..
but your imagination may be as good as mine; so at least give
it a try !!
You tell US how to do it & i'll tell You, if it's a viable plan ...
Way more Fun that way. Ya see, i sometimes need several
change-ups before stumbling upon a plan that works.

Check it :

I would like to know the pros and cons of this design strategy.

Basically grooves or chines are part of the base.

As in this design:
http://www.bestkiteboarding.com/2010-Spark

I think the difficulty would be in glassing the base.

Look at this post by flyboyy ---
Ya Don't NEED to be looking at Just a bottom rocker table !

Think from the Other side of the problem, instead of a bottom
rocker table, it can also be a convex TOP !

In this case the builder wants grooves on the bottom, so
what dimensions must the table be, if the Top were layed up
first ??

In this layup, i'd be seeing a multiple build-up of lighter glass so-as
to accommodate the concave surfaces of the pre shaped grooves.
The lighter glass will go around
corners easier. Also a crows-foot weave,would be do the same.
So --- What else would work ?

I'm NOT a genius ; just don't mind failure, if it adds to the learning curve.

Again -- look at the question posed by --Kancler--

Hi, i'm new in here
all time i wanted to build my own board, so after tons of reading i decided to start
so right now i'm in the middle of finishing rocker table, but i need advice on studs length, basically i'm wondering, how much western red cedar springs (not sure if its right word) back after vacuum bagging
i know looks ugly and cheap but frame is strong
studs will be every 10 cm (4,08")
Length of working plane will be about 175 cm (71.4")
width will be 70-75 cm (28.5-30.6)
base gone be mdf plate(10mm i hope its not overkill and it will bend and not crack) , on top of it i'm gone put plexiglas (5 mm), i'm doing adjustable rocker thru plexiglas

So what would happen if the builder had boxed in his frame with 3/4" ply both
top and bottom ?

What would happen if he placed 1/4" all-thread on both sides, the length of his
frame just on the bottom ? Yea-- he could now adjust the amount of flex the
rocker table would induce into the cradle.

I'm asking Ya ALL to ponder your problem from Both sides and think 3 dementionally
instead of just 2 !!
This process, thus illimanites a human flaw known as Functional Fixidness.
That's a cognitive bias in problam solving, once a solution has bin given. It
makes further scrutiny for a strategy, near impossible.
http://en.wikipedia.org/wiki/Functional_fixedness
In other words -- Don't make your S**t the same as everyone else !!

Sounds like BS but it's the reason engineers haven't progressed in the
further advancement for prosthetic design; since Jesus Died, near 2,000 years
ago !!

Bille

[plummet]

triaxial!... now that is also the shiz if you want to lay up a uni and bd lay in one hit!!..

yes when i said matt wasn't talking of chop strand matt but weave. Different terminology in my part of the world.

I say you can make carbon as strong/flexible/robust as you want.

[Bille]

Triaxial E-glass in 20-24 oz is pretty hard to beat as a skin. This is also more or less a standard for snowboards. It is best suited for a high pressure setup though and harder to handle under a vacuum setup due to the raw stiffness of the cloth.
Upgrading to S-glass or carbon is in most cases a waste of money if your design, core material and local reinforcements are right.
Your mileage may vary

Stan

Another point to notice :

I keep mentioning that woven S-glass is about 30% stronger than E-glass.

OK -- but ya Loose about 25 - 30 % in strength the Moment you weave Any
fabric; More in compression than tensile.

SO --Bigdog-- is accomplishing nearly the Same thing as me with his Stitched
E-glass, as what I get with the More expensive S-glass.

Also -- Bigdog is suggesting the Balance for the Entire Matrix is important !
Like a chain, it's Only as strong as it's Weakest link !!

triaxial!... now that is also the shiz ...

I say you can make carbon as strong/flexible/robust as you want.

So Three different builders here, with three different strategies.
None of are Right or Wrong !

I use the Carbon uni for my directional strength, then go with
the woven S-glass to bind all my stuff together.
Plummet & Bigdog go a different route.

Seriously -- the Point of this topic is to make builders More informed
so-as to have more strategies to draw from, when deciding how
their board will be made.

Bille