Build theory ?

I'm in the process of building another compsand and starting a similar build. I've not posted any pictures because it's reunadnt to prior threads, but I'm interested in an engineering question. Are you reading Ed?

I was really impressed with Matt S's stringer glue up. Where he cut the board in two and then glued the machined edges together. Nice work Matt!

When I do the low density work I have to reinforce the fin boxes in some manner. In the past I've just installed divinycell, but that has proven to just spread the torque a little further and not really stop the twisting on the boxes in the board.

You can't see it and I didn't save any pictures, but this board has some internal stiffeners that coincide with the red lines in the pcitures. Basically just balsa and basswood. It got rid of the box twisting, but I'm not so sure about preventing rail twist off or unwanted flex in the tail.

I've thought about doing the bracing diagonally, but haven't tried it yet. You see diagonal bracing everywhere, so I'm wondering if THIS would provide a stronger/stiffer tail and still tie the fin boxes together to prevent their twisting in the 1# foam?

Now lastly...as I mentioned I was really impressed with Matt S's stringer glue up. What I was contenplating was sectioning my blank and gluing it back together. So, after hotwiring the rocker, I would cut the blank at the red marks. Then insert stringer material and glue it all back together. The glue would be stronger than the foam or the wood, plus I'll be skinning it so I think that I can achieve a reduced flex in strategic areas and also prevent the fin boxes and rails from twisting off.

Ed, as our resident engineer if I may pose this question. The perpendicular cuts are the easiest, would that give me the desired results (preventing box twist, reducing rail twist and stiffening the tail) or is the diagonal bracing a better option?

If I could have my answer in the next 5 minutes please

Jeff, I might need help with your terms and problems. Also note that if you were having guitar amp problems I might be more helpful.

You say you want to prevent the fin boxes and rails from twisting off. Do you mean the rails, the edges of the board from tip to tail?

Are the fin boxes showing signs of damage? Do the fin boxes become detached? I'm assuming no.

I assume that the fins flex and give and therefore provide poor, uhm… mushy control.

It sounds like you want the tail to be rigid.

Ok so I have to be more articulate?! FINE!

The 1# foam is easily compressed and allows the fin boxes to move or twist. It is shear movement between the skins. When you turn or pump, there is torque that is applied to the fins. The idea should be that the boxes remain stationary enough to allow the fins to flex. What happens, though in practice, with the low density foam is that the foam has less resistance to shear or movement than the fins. A higher desnity foam prevents the boxes from moving about in that high density foam, BUT then the entire insert moves. So all I did was move the movement.

When I placed the perpendicular cross braces in the foam THOSE prevent the fin boxes from moving in the foam. The wood being more resistant to shear than the fins themselves. THIS is what I wanted.

The longitudinal stringer - like a traditional stringer, prevents unwanted flex lengthwise on the board. I want some of that or I do NOT want lengthwise flex, but in the tail, I want to also prevent rail-to-rail or latidunal? flex, as well as, prevent the longitudinal flex.

Yes, you have concluded that what I want to achieve is a STIFF, RIGID tail that has virtually no flex lengthwise or widthwise. While achieving that, I also need to prevent the fin boxes from twisting around in the foam. The high density foam inserts had a minimal impact on reducing fin box movement and did NOTHING to minimize flex in the tail. James has actually pulled a fin box out of a board and also caused a spilt in the tail of a board.

So...in short, I want to prevent the tail of the board from flexing in the nose to tail orientation, I also want to prevent the board from flexing in the rail to rail orientation AND I want to prevent the fin boxes from moving at ALL.

I'm sorry that I was unclear Ed, is that better? Thanks for taking the time to look at this.

Jeff, The lateral stringers should firm up the fins nicely which you indicate works.



An unloaded board


Now load the board with a rider actively carving and pumping - well as active as a 50 year old can be any way


From the side view the unloaded rocker


The loaded rocker dashed line is unloaded


The tail of the board in the picture is fairly square. If you made a triangle out of wood stringers I think that you'd have a really stiff board. I show two lateral strings. One at the fin and one at the tail.

Jeff, have you tried putting a layer of glass under the fin boxes and wet it out lets say rail to rail. That might provide the added strength needed there.



(Message edited by Bigshow on February 24, 2008)

I love cartoons, I love the scale in the width of the above cartoon also - I think that is a 7 foot wide board.

The board that I am building has perimeter stringers - so I do have "somewhat" the triangle. I have some carbon tape leftover and I was thinking about putting that across where the boxes are, but wasn't quite sure that would do anything. So, if I am correct in interpreting your elequoent description, the triangle or diagonal bracing DOES provide the stiffest structure.

Can I achieve this by virtue of a perimeter stringer? That is instead of the stringers above bisecting the fin boxes, the stringers are out towards the rail - say 1/2" inboard? The lateral stringers then connect to these perimeter stringers?

Truly Ed, the visuals with the cartoons are not only helpful but make me chuckle no end. I want to get my icon printed on rice paper for a board.

Jeff, I don't know it hit me on the last thread where I made cartoons - Charlie Brown is a good icon for you

Your stringers are kind of parabolic, right? If I were to make a board and place stringers where I wanted them that's what I would do too. In fact I would try to arrange the stringers so they ran near or through the fins. I showed a triangle set up because I thought that would be easier to construct.

As far as the carbon fiber goes, I’d glass the board, router the fin boxes, lay down the carbon fiber, epoxy the fin boxes on top of the carbon fiber.

Stringers near the fins


Stringers pass through the fins

I was scratching my head, trying to figure out what the similarities are between Charlie and myself.

I like the second configuration, but I don't have that available to me today...I do believe the folks at Segway and Greg Loeher developed a blank just like that with 3 pound density eps on the rails and 1 pound eps in between...I'll have to figure that out for the next build.

Thanks for all your input, as always, invaluable!

This is what I wound up with for today. I sectioned the foam and glue it back up The next build will have the stringers running through the boxes, at least the front ones and then something to stiffen the last bit of tail section.

How are the long-lat stringers joined? Are they simple butted or are they inter-locked

They are interlocked - rudimentary, but I think that I may have screwed it up. I'm going to cut a concave and the deck height has to come down...I'm afraid that when all is said and done, it will just be a butt joint with a slight step

I think that the carbon fiber tape will become imperative to keep the strength in that section.

The blank all glued up and out of the bag. It's as strong as any glued up blank that I have ever seen, so maybe it will be alright with appropriate glass/carbon reinforcements.

I'm thinking 2001.

A space oddesy? You always say that about my foam experiments, what is this fixation you have with apes?

I tried roughing out a parabolic stringer that would hit both boxes for a quad. That looks like it's going to be tough to accomplish. See first picture above. So, I'll have to do a "crossmember" in some sense.

If that's the case, would it make more sense to run a piece of carbon tubing from the tail through the two lat "stringers" so that I have a solid tail? In this way I'd not have to worry about the interlocking location. I'm thinking that if I have the tubing imbedded in the foam and through the center of the lat stringers I'll achieve a more robust joint.

Then I can run my perimeter stringers as normal.

I'm an engineer, I need pictures I don't really know how or where you intend to run the tube.

I think whatever you do it will be better if you can directly connect structural members

I'm not an engineer - I need cartoons

Does this help or just cause you to run screaming from the room?

I wasn't able to download the picture, for some reason, but this is the blank I was talking about - it's from American Blanks:

http://www.segwaycomposites.com/ParabolicSystem.html

They use a small insert at the tail for the trailer fin and that's all in the 1# density foam, the perimiter stringers are designed to have the rail fin boxes cut into it. Principally the design you came up with above.

Interestingly enough...they propose that you can glass that 1# and not skin it. I'll have to try that after figuring out how to keep the boxes from doing the "twist"

this might sound wierd,but from what i know about pro surfers, is they like the flex and the best riders can tell when their board has too many miles.it basically looses all its rebound flex.when surfing mavs and pipe and huge waves,fin and tail flex obviously becomes a factor.imo for wakesurfing all that extra weight is exactly that.

Yeah...the stringers I placed across the back of this blank REALLY increased the weight - the basswood isn't a viable option. I have such an investment in Future's fins I'm reluctant to change, but I think a major part of the fin box ripping out is their design combined with the lightweight foam.

Billy what fin boxes do you install most often in ASC boards? Do you get folks bringing in "other" boards for box repair and if so, what boxes do you find you repair the most?

This may be a stupid question but why don't you just glass your fins in? It would be interesting to see if you had any issues with your son breaking the fins off where they would be glassed on at. I know for the average rider it probably wouldn't even be an issue. Most average riders can't tell the differences between certain fins. I am very average at best. When riding my longboard I can't really tell the difference between just my single fin, my turbo tunnel, or my longboard set up with the sidebites. I can tell a major difference between my thruster and my quad.

I've thought about glass-ons but we are still messing with size, type and location. I doubt that James could break them off riding, but I'd probably snap them in the shop!

The other thing I'm contemplating is just using a higher density foam - that would resolve the box twist issue also.

I think that I am going to glass this as is - 6 oz bottom and 6/6 top. It's pretty stiff as is, due to the stringer(s).

Is EPS like coffee cup foam? Is it more difficult to shape than PU, I assume it is?

Right EPS is what coffee cups are made of - they are typically referred to as styrofoam, but that's not accurate - styrofoam is the trade name for a blue foam made by dow for insulation and dow claims it repels water.

The higher density eps shapes pretty easily - 2+ is fairly straight forward - the 1# stuff is floppy and you can't put a tool to it without it bending - the stringers really corrected that.

The other issue is that EPS tends to "chunk" or get pukas.

PolyU is easy to shape, doesn't chunk and sands like a dream I wouldn't say that EPS is difficult, but it does require more care than PolyU.

i like the standard plug,it fits fcs and couple more brands..if you scrape the shiney off em before ya epoxy them.they will stay in there for ever..you would not believe how many people have thrashed sticks in austin.right now in my shack i have a 9'6 velzy,6'6august,6'8stewart all for glass on fin repair..id have to say i get those the most.

Hey Jeff,

Just curious....
What was the thickness in the tail of the board(s) that James ripped the finboxes out?

Of course you should be able to go as thin as possible with no worries with the perpendicular stringers...

It was on a quad...the tail thickness was 7/8" and I do believe I was using 1/2" thruster boxes for the trailer fins and the standard 3/4" rail boxes. However we prefer the 3/4" rail boxes for the trailer fins on a quad. The lamination was good, but you could feel the fins flexing the foam...it felt almost sloppy. When it let go, it took about 3/4" of foam all around the fin box. It was UGLY.

When I put in the basswood inserts that went perpendicular across the board, the boxes stiffened up and I didn't have any box failure problems.

The perpenducular stringers really stiffened things up - but I'm really concerned about weight. The futures quads are heavy and the use of the basswood seems to add a ton. I don't know, maybe having the balance point 3 inches in front of the tail will be a good thing!

I'd like to find a way to lock the fin boxes down without going to heavier foam and without having to add so much weight with the glue and stringers. It's the second set of boxes that is my problem.

Any ideas Matt? Anyone?

Go thinner. If you have a thin sheet there’s little strength in the thin dimension. The deeper dimension will be very strong.

WHAT?

Cartoons!!!!!!

With the thin structure you retain a lot of strength up and down and loose a lot from side to side. You only need strength in one direction. Make that stringer side-to-side stringer thinner, save weight, get the strength you need.

how about some carbon "football" patches? or a carbon "butterfly" patch from rail to rail?

Matt, the other part I am having problems with is the boxes flexing - that's not accurate. The boxes flexing IN the foam. I need something on the bottom side of the box to prevent that flex in the foam. Ed S did a good job of showing it about with the cool rider.

I think that Ed is making sense...I can probably go exceptionally thin with the stringer and retain the strength I need.
The greatest strength, in the picture - would be in the length of the stringer material. Which is what I've found already.

I wonder if I can use a lightweight material...either balsa or a carbon sheet?

Ed's thin stringer makes a lot of sense.

Or how about a carbon wrap on the tail instead of football patches. Almost like an 8inch or so wide ring of carbon going from the deck to the bottom. That could add significant stiffness without too much weight... just another idea to throw out there.

Matt - I'm sorry, I'm not sure I understand - are you thinking basically covering the entire tail top and bottom with carbon?

We were in the snoe over the weekend and I saw a snowboard that had a clear laminate as the top shet so that you could see the wwod core and also two pieces of carbon tape running the length of the board on either side of center.

I know that Calibratd selectively uses carbon - my guess is in this manner - might be an interesting way to stiffen the tail with no weight gain. Thanks Matt

Yes - just wrapping it. You could wrap it from the bottom and connect the ends on the deck, bag it and then once it is out just put a traction pad over it and you wouldn't be able to see where it connects.

CARTOONS! Now I understand That's an interesting concept - your proposal of the modularity of the build. Rather than do the entire board in CF, just bits and pieces as needed. To stiffen the tail, do a CF section.

I like it, plus that's a cool look.

Don't all of these cartoons show that we're all working at an 8-year old level At least we're a happy lot and easily amused

(Message edited by Bigshow on February 27, 2008)

seriously,the cartoons rule..even i could follow along

I love the cartoons! I aspire to work at an 8 year old level - can you even remember all the energy?!

The sub 4 foot board...I just spackled the deck and this boards currently weighs 10.2 oz.



This board is 6x6 / 6 and I "think" it weighs 52 oz. I think it will be ridable like that. I'm going to shoot for a 64 oz finished weight.

yo jeff, just wondering how some of your other boards came out. too bad i never got to come up to bring you a board. i havnt even seen the lake since the svfara shoot at tulloch. im collecting some rust and i really want to get out. how is the carbon fiber board and the fire wire style one?

Hey Johnny,

Any time you can make it up this way you are always welcome to a pull. We have been out just about every weekend since the weather turned nice. We'd love to have you out on the boat before too long.

The hollow carbon/balsa still needs to be polished...after I melted the eps core it just weighed a ton. It came out about 7+ pounds before traction and fins...so I haven't pursued finishing it. It looks cool, but sheesh it is going to be a slug. No doubt after I get everything else completed I'll go back to that one. In order to make the board hollow it has to have incredible rigidity - but at these short lengths the weight savings is so negligible that the final board is heavy.

The firewire came out sweet, that construction methodology really does work well. I didn't like the rocker on that first build though, so I have a second one in the works. The carbon around the rails is a bugger! BUT like you thought, it stiffens up the rails something crazy. I was talking with Loc on the phone yesterday and was telling him that has some of the stiffest rails I've ever ridden, in combination with ultra light weight. I love the combination of corecell/balsa/carbon/eps.

The D-cell skinned boards have been mixed. Mostly because of the design I chose, as opposed to the materials. In comparison though, I prefer corecell to d-cell as a skin. Corecell is stiffer and doesn't require as much epoxy to maintain that rigidity in the final lamination. I think that D-cell tends to suck more resin than corecell too...so at least in my experience, corecell winds up lighter.

I'm currently working on some prototypes for James - PolyU so that he can have a board for the current year, after we get that hammered out I still have the Rohacell I want to build from.

Keep in touch, miss hearing from you.

yea i know i havent been following the posts to much latly, trying to focus on school right now, but its almost wakesurf season so i need to get back on the water.

Resinheads will find this thread over at swaylocks interesting. If you have the time and bandwidth the video is priceless:

http://www.swaylocks.com/forum/gforum.cgi?post=365609;#365609

There is a link to a video in that thread.

While I was visiting Dave's shop - and for folks that can get a board shaped locally, it's great fun to watch your shaper do it...Dave allowed me to watch him shape a few boards.

Anyway...Vernor is known for his I-beam suspension system. I was sworn to secrecy! but he describes the material in the video. It's a layer of coremat, which is exceptionally light bulking material, (I've used it to thicken up a mold) then carbon and I'm guessing fiberglass on the outside. Torsionally what it does is prevent the thin tails from twisting and it also ties the futures boxes into more of the board, reducing twist of the boxes.

I really thought it was an innovative use of materials.

That's pretty cool. To me it looks like he's building a spring. Jeff you wanted to build a stiffer board. If you wanted to build a spring your V would be better.

You changed your user name? I don't feel comfortable using any part of Dave's design or materials...but I think it's seriously cool the way he builds it.

My name was all over the place. Full with middle initial. Not really all that cyber secure, probably too late to matter. I don't think I'd let my kids out that much out there.

Think about the first guy that built a fiber glass surf board. Your copying that work aren’t you?

Yep - only better. I don't disagree with your logic and I've had my own designs show up for sale by other folks but I guess that Greg coined it most appropriately when he referred to a sense of honor...it would just feel like stealing, I guess. So...I'll just admire his ingenuity and craftsmanship and try and do it better, without imitation