Advice on CF layers for longboards

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#1

I’ve built now 14 longboard skateboards as a hobbyist. Gifts for my kids and their friends. The boards generally have 550 corecell foam core of approximately 5/8 inch to 3/4 inch with a wet layup of 3 layers 3k twill (from CJ composites) top and bottom, quality epoxy (west systems or equivalent), vacuum bagged and low heat cured (80 deg for 12 hours). The boards are approx 44-48" in length and 10 inches width with a 3/4 inch concavity (from a curved mold they are vacuumed over).

I release after 24 hours and then trim, sand and lay a final coat of epoxy.

Most the boards have been very successful – strong and flexible.

One epoxy failure due to incorrect pump metering – I now weigh and measure everything.

My question: two boards cracked in the center (compression side only) with light usage. One with 200 lb person and the other with 170 pound. The only difference in the first was a substitute of one layer CF with Kevlar on the top (I understand this was a mistake and that if I am to gain any benefit from the Kevlar it would have been better on the tension side).

The second failed with 5/8 airex core but I mistakenly put only two layers CF on top(compression)

What would be the best combination, in anyone’s experience, to achieve a board that retains some springiness but doesn’t fail on the compression side?
Should I go with a stiffer combination 4 layers 3 k CF top and bottom?
Use a thicker foam core 3/4 " and and stay with three layers?
Should I build the compression side stronger? (4 layers top and 3 bottom)?
Make it bomb proof and put heavy duty 12 k layer top and bottom on my usual 3 layer top and bottom?

Any advice would be appreciated that doesn’t involve adding Baltic birch or other wood cores.

#2

I understand in my post above that both my failures were in boards with 2 layers CF top and 3 bottom-- my concern is that if there is such a fine line to failure that even my “non failed” boards may be too weak. Thanks

#3

Coincidentally, I touched on this very subject (compression vs tension failure) in the last post of this current thread.

http://www.compositescentral.net/showthread.php?t=8424

CF is stronger in tension, and thus it stands to reason that your application needs more strength or plies on the compression face. At least that is the working hypothesis :slight_smile:

Disclaimer: I have yet to fabricate anything from CF, but yours is an engineering problem and not a carbon fiber problem per say.

#4

Yes, your post makes good sense, so it seems if I build it up at all it must be in compression. Any suggestions, given the dimensions of my board, the type and thickness of my core and the expected weight applied to the construct, what would be the reasonable number of layers and the ratio? I.e 5 compression and 3 tension?

#5

Well, now I feel kind of silly giving actual construction advice (see disclaimer above :wink: but one more layer of unidirectional running longitudinally and one more layer of twill on top or even just one extra layer of uni somewhere in the top 4 plies would probably do the trick. The unidirectional will net more strength per unit of weight than the twill.

#6

As others pretty much said, more longitudinal fibre on the top. I reckon you could drop 1 cloth out too and replace that with uni. Unis are typically cheaper than cloth too so this saves money.

Top- cloth, uni, uni, uni , cloth

Bottom- cloth, uni, uni

If this is done with all 200g (5 oz?) you will have 800g of uni on top and that’s almost 3x as much long fibre your original layup (3 x 200g cloth = 300g long fibre).

Show us a picture! if you wish?

#7

I’m having trouble downsizing my images sufficiently to post using my ipad. I don’t have an image of the failed board yet as the young man who has it just notified me. If interested, I will post later.

After doing more research on unidirectional compression of CF and after reading your excellent posts it now seems simple. I need more uni. Since the board generally sees unidirectional compression and tension along its long axis only I need primarily uni throughout the board, but more so on the top. Thanks fasta and vln

#8

Your skins are fairly thin. this means that compression is an issue. The compression strength is about half the strength in tension.
I can add to that that a laminate in compression is held in column by (in this case) the foam core. A higher density foam core is more effective in keeping column.

The thought to just multiply the top layer by 2 is understandable, but keep in mind that the neutral axis shifts towards the top layer as well then. Which causes more tension on the bottom laminate than expected.

Also, there is 2 things: stiffness (springiness) and strength. The stiffness is directly related to thickness, and by careful designing one can vary between several options in terms of stiffness and strength.

Basicly UD is what you want to withstand bending in 1 direction. The 3K fabric makes a nice design, but has only little amount of fibers in 0 direction.
However, adding UDs is not the complete solution. First of all, any amount of UD should be encapsulated in other fiber, preferably +/-45 if the product is dynamically loaded to extremes, and this all will make the board too stiff. But there is a solution for that: Remember the stiffness issue? Making the board thinner (make the core thinner) will create a board which is stronger, but with equal stiffness than the old boards.
The thicker laminate will be more succesful in keeping column on the compression side. The bottom laminate should be altered accordingly to keep the neutral axis approx in the middle, and the core could be shaved a bit to loose some stiffness.

Design boards for the weight of the user. I appreciate it when a board is dead flat with the person on. Definately not sagged in the middle.

#9

Thanks, guys, great advice and heading me in the right direction.

My Next project is to pick up my Probotix CAD CNC 50" router in Peoria, Il, next week to make some truly accurate custom foam cores.

#10

Herman, it is true that adding layers just to the top will cause the neutral axis to move toward the top, but I am not so sure that it adds more tension to the bottom. There is a thought experiment that might demonstrate my thinking.

Imagine his longboard constructed such that failure at breaking load will occur on the tension side and then loading the longboard to within a hair’s breadth of that breaking load. Now imagine removing the load, adding one CF layer to the top, and reloading with exactly the same weight.

If moving the neutral axis upward from adding another top layer increases tension on the tension side, then the board in the above thought experiment should certainly fail after adding the additional layer. But something is wrong with this picture. My intuition cannot provide any possible mechanism for failure merely from adding strength, regardless of where. I’m not even saying your statement is wrong, only that it might be incomplete. I believe the slight shifting of the neutral axis from adding an additional layer to the compression face (and theoretical increase in tension to the tension face from the shifting neutral axis) is offset by the increase in stiffness of the compression face, and by an amount that is greater than the increase in tension on the tension face.

#11

There is a classical calculation of a wooden board, which can hold a certain weight. After reinforcing it, the board breaks at the same weight.

This happened in the past quite some times, when skippers of canal barges had things to say abou the construction of their new barge: as the old one bent quite a lot under load, they ordered their new ones with thicker bottom plating, to reduce the bending. It was not uncommon for a new barge to snap in 2 while loading. Luckily now the classification societies are much stricter.

#12

Also - consider durability…

Weight and stiffness are all well and good, but if you ding your board you’ll dramatically reduce the resulting performance. Personally I’d suggest increasing your skin thickness (although this doesn’t necessarily need to be with carbon).