Foam core monster Door

[Johnny Rotten]

Building a foam core monster door 164 x 50

here is my plan .5" divinycell h80 tip to tail, with .75" for a 120cm span in the middle between foot pads.
3 layers of 6 oz glass on the bottom
3-4 layers 6 oz glass on the top with carbon under the foot pads oriented 0,90 30,60 and 0,90 60,40 Vacuum bagged using SB112 resin

I haven't gone through the stiffness calcs on this yet, and haven't worked with glass enough to have real confidence in my "that there should be about right" assessment that I have done so far.

However am no stranger to vacuum bagging and composites work from many carbon projects done on cars.

someone wanna let me know if I'm way off base here and need to make this stiffer?

I know I can always add additional plys if it's not stiff enough but I'd prefer the chemical bond strength obtained by a single cure, rather than the surface bond resulting from multiple cures

Ultimately I wanna make a big LIGHT board for days when I can barely get my 17m contra in the air.

[Spam Dagger]

From my experience you are right on with the foam core philosophy. My 170 x 52 gets going earlier than any glide or my surfboard. I put a big concave in the bottom but most importantly for stiffness I laid up a piece of directional carbon along the rails. The thickness of my core was more like .75" so there was a decent compound curve with carbon on it that keeps the board from having pretty much any flex at all. I don't really have any intention of riding this board with anything less than my 19M Assault so I don't really care about flex. I think you are a bit heavy on the number layers of glass. I had some Kevlar so I have 1 layer of Kevlar underneath a single layer of S-glass. Carbon along the rails and a bit underneath each heel area.

[aeberl]

I build boards exclusively with long grain 10mm balsa cores and achieve a very good flex patterns laminating as follows:

TOP to BOTTOM:
6oz S-Glass over the entire top surface, orientaton
6oz S-Glass covering center section (rail to rail) up to about 8cm pased the outside strap inserts
6oz S-Glass under the foot pads (about 6cm wider than footpads)
3" 6oz S-Glass tapes along rails
10mm balsa core, tapered down to 4mm at the tips
3" 6oz S-Glass tapes along rails
6oz S-Glass over the entire bottom surface
4oz S-Glass over entire bottom surface.
All fibers oriented 90/0.

This results in flexy tips with a stiff center section and stiff rails. There is some twist which comes in handy when riding waves, it does however sacrifice a little speed. If you want to reduce twist, you could consider 45/45 fiber orientation for one bottom layer. Compression sternght of balsa is good enough to skip the under-pad reinforcement, but I put them in amyway (to get rid of cut-away colth). I don´t know about your foam core.
Keep in mind that you have the largest bending moment just on your outside strap insert and on the rails next to your heel. Dont worry about fiber strength on th bottom of the board, they work very well in tension; on the top however, you have to provide sufficient reinforcement to take the compressive loads without buckling your laminate either separating it from the core or compressing it beyond its yield strngth.
This setup has worked well for my freestyle boards up to 134 x 40. You are probably considering the monster door for light wind riding, so a stiffer setup might be appropiate, consider a thicker core for longer boards, thicker cores make lighter and stiffer boards

[stvandev]

aeberl,

Are you using balsa cores that you build from smaller pieces or are you getting full length/width cores? If you're using full size one-piece cores where are you getting them from? I've been trying to find a source for balsa and haven't had any luck. Thanks.

[kitehok]

I'm also interested in balsa cores, who's your source? Thanks

[Johnny Rotten]

"Keep in mind that you have the largest bending moment just on your outside strap insert and on the rails next to your heel. Dont worry about fiber strength on th bottom of the board, they work very well in tension; on the top however, you have to provide sufficient reinforcement to take the compressive loads without buckling your laminate either separating it from the core or compressing it beyond its yield strngth."


Thanks a ton Aeberl this makes a lot of sense. and forces me to think about the top ply a lot more. Originally I was just going to target the same lengthwise bending stiffness as my JL board. Assuming similar flat shape and neglecting concave, (My JL doesn't have a lot) Stiffness is dependent primarily on the thickness of the core and the amount of glass I put on the bottom. I guess if I flip the board upside down and measure stiffness in the other direction it should give me a good idea of what they used for the top ply.


For a kite board when you are carving hard typically the front of the board is out of the water with the rear in deep. This creates a bending moment that is as you say near around the rear foot pad, However this would be off Axis with the 0-90 layup that most people suggested.

On landing from a jump, the rear would hit first, likely on axis with a 0-90 lay-up and would create a bending moment trying to snap the tail off (bending moment at rear foot pad) as the back fo the board stops and my body weight tries to keep going.

Time to pull out the materials text book. Designing an optimal plank isn't as easy as I thought.

[BWD]

Lots of ways to go but I think it boils down to wanting a more isotropic layup for the top deck as well as having a little more cloth.
Thanks from me also for posting those layups.

[zfennell]

......a more isotropic layup for the top deck .........

I'm pretty sure "isotropic" is the one property NOT available for any fiber reinforced plastic.

regardless of the method to determine the necessary strength and stiffness (trial & error, copy, calculation, guess) the layup properties in any particular direction will always be dominated by the fibers aligned in that direction (no fibers = no strength)
..maybe thats what you were suggesting?

regarding Johnney's layup:
if you do any math , i'm sure you'll find that
1/2 " core with 18oz of glass (top and bottom) in high stress areas is a good place to start.
divinycell core is ok but some have found it to be a bit brittle and fail over time,
corecell ( or similar non-cross linked pvc foam) has a reputation for being tougher and more durable with its higher 'allowable strain'.
Its not often that you'll find a legit use for carbon if the bulk of the board uses glass.
generally too stiff, too expensive and the light weight is overshadowed by all the other ingrediants.
Using the top and bottom cloth to overlap the rails will give good reinforcement.
Its probably a bit more difficult that 3" tape, but less sanding and fairing to get a clean finish.

i would have no problem using aeberl's recommended layup.
Just remember the flexural strength of his balsa core will be a significant contributer to the overall strength of the board. Something you will not get from any of the pvc foam cores.

folks on yahoo board building group are an excellent resource for what already works if you dont wish to experiment.
http://groups.yahoo.com/group/boardbuilding/
search there for common layups.

-bill

[aeberl]

I use 5mm and 6mm thick balsa available at any hobby shop. Board are about 90cm long and 10cm wide. The bottom layer has about 80cm of longrain balsa in the center and towards the tip I place the balsa crossgrain such that the board has more flex at the tips.
The top layer of the balsa core is all long grain, but thickness is reduced towards the tips, again, to ensure more flexy tips and stiffer center section.
Initially I used to work with a solid long grain balsa blank I made myself out of logs I get from timber companies here in Peru of out of Ecuador. It's not worth the hastle, They tend to be very expensive because I had about 50 to 60% material loss during the shaping process and the final product is very stiff.
Now I only use them to make tow-in surfboards. They are indestructible!

[BWD]

I'm pretty sure "isotropic" is the one property NOT available for any fiber reinforced plastic.

apparently several years ago someone realized this
and invented something called "fiberglass cloth."

The point of using cloth at all, whether woven, biaxial, even triaxial and quadraxial, is to make laminates with "isotropic" qualities (or "quasi-isotropic" I guess if you are a semantic stickler).

If you just want more strength/stiffness in one direction, put a layer of uni in that direction.

If you want to have a more (quasi-)isotropic laminate,
than woven or biax gives, you can cut one or more of your fabric layers at an angle to the fiber orientation of the cloth. For example a deck patch cut at 45 degrees to the fiber orientation. Or maybe a bottom laminate of 2 layers of 9oz S glass cut at say, 15deg to the fibers orientation, just for example. Either would give more isotropic qualities than the same weight of cloth in 0-90 deg only, but plenty of boards are done with just 0-90.