Just joined Swaylocks, been reading archives for about a month. I forget who wrote it but somebody discussed how the glass bonded to the stringer creating an I-beam affect. As a board is snapped the first thing to fail is the bond between glass/stringer, the I-beam is lost, then snapola. This got me thinking about how to improve the bond. Scoring the foam on both sides of the stringer before glassing is common practice, which makes great sense, and which I always do. The other obvious thing that came to mind was prepping the surface of the stringer. heavy grit sand paper might work but would be a bitch to do a good job of. A tool at my work that is used for testing nerve function caught my interest the other day. It is basically a pizza cutter with a wheel about 2" in diameter. Instead of a cutting edge on the wheel there is twenty or so sharp spikes. I was thinking that if you rolled this up and down the stringer, creating hundreds of little holes, you would dramatically increase the surface area of the stringer being exposed to the resin as well as the bite it would have into the stringer. If you scored the foam deeply on the sides of the stringer you could roll/spike the sides of the stringer as well. Anybody agree/disagree, have other ideas?
Anything that increases surface area of the bonding agent will have positive effects against breakage…at the expense of complicity, expense, and added weight.
Also, sometimes the foam next to the resin bonding agent fails, and the foam is the wink link to the equation.
HOW a board breaks is fascinating reading, but bottom line is more glass = less breakage.
even if the bond over the stringer is weakened, i imagine that the flex threshold of the stringer would still exceed that of the glass still properly bonded to foam. assuming that is true, it would follow that when a board breaks, its integrity is first compromised at the RAIL. or maybe i’m wrong…my brain works funky…this is just how i think of things. clarification, anyone??
most of the boards I have broken, broke where rail had previous damage. just my 2 cents
More glass also equals more weight. My thinking was along the lines of increasing strength at a minimum of increased weight, pretty much regardless of more work. I’ll spend rediculous amounts of time on my personal board and enjoy a lot of it.
I don’t think the rails break first. Either you get #1 a clean snap where the bottom glass and stringer break at once or #2 a buckle where the top glass delams from the stringer first. As far as rail breakage goes, I agree that this is the other main structural point of integrity, where you have a tight radius like that there is a lot of strength. I’ve thought about laying on a 3" strip of glass around the whole perimeter of the board but again we’re getting into a fair amount of glass/weight.
Man, what do you call that metal spike pizza cutter that you torture people with? I’m a dentist and I figure I got some pretty scary things at the office, but never thought about using them on a blank.
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My thinking was along the lines of increasing strength at a minimum of increased weight, pretty much regardless of more work.
well that’s just it…if the breaking point of a board is determined by the amount of force that can be sustained by the rails, then increasing strength (and weight) along the stringer won’t have any effect on what point the board will break. something that might be an interesting (albeit very DIFFICULT) project might be to do a wood inlay at the apex of the rail around the entire perimeter of the board…this could REALLY be the new frontier and board strength!!! granted, more wood = more weight…but with THAT MUCH extra flexibility before reaching the breaking point, an INSANELY LIGHT glass job with epoxy resin could really make for a wild ride.
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I don’t think the rails break first. Either you get #1 a clean snap where the bottom glass and stringer break at once or #2 a buckle where the top glass delams from the stringer first.
visualize the compression of water over a board (or the compression of any powerful force on any relatively stiff object)…even when it breaks clean across, or if it just buckles on one side, it should never happen from the inside out. the force of the water would be distributed evenly over the affected surface area from rail to rail. add in the factor that the center is wood+glass instead of foam+glass (higher breakage threshold due to materials) and is thicker than the foam+glass rails (higher breakage threshold due to mass), and it should be impossible to break a board from the inside out.
again, this is just me theorizing…analyzing images in my head…so please, no one take anything i say as fact.
In addition to the pizza cutter idea, you could use a small laminate router with a 1/8" diameter bit and run it down the length of the stringer at whatever depth of cut you wanted. This would allow a nice clean run of resin down the stringer for strength. Also use a V-parting carving tool and run it up the stringer to achieve the same result.
I’ve seen plenty of bends and breaks and it seems to me the first thing to show stress is the stringer, that little bit of delamination, small cracks ninety degrees to the stringer. The more stress the wider the damage, till a board breaks in two.
There are variations to the theme of course.
People used carbon strips and routed channels glassed with rovings. They still bent or broke, mainly because they were shaped a bit thin and or glassed too light. And because of the extra crap in there they were a pain in the arse to fix properly, much to the disappointment of the owners high expectation. It didn’t look brand-spanking-new any more, oh no.
There is a good discussion about this in the archives www.swaylocks.com/forum/gforum.cgi?post=182389;#182389.
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In addition to the pizza cutter idea, you could use a small laminate router with a 1/8" diameter bit and run it down the length of the stringer. That would be sweet for a 3/8" stringer. If you really went big you could lay some fin rope in there. That might work pretty well on the bottom of the board also, to increase the tensile strength of the glass job. Running a perfectly straight router line down the stringer would be pretty tough though. Might run into some issues with losing flex too.
hey soulstice,
If the bond between glass and foam is more important than to stringer, and the rails are the critical point…how about roller spiking the rails to get some serious resin penetration there.
i can’t seem to picture what sort of effect that might have on tensile strength at the rails. it seems that deep penetration of resin “spikes” might actually have an adverse effect by tearing at the foam from the inside out when the board flexes…but a more shallow roughness that would allow a greater resin bond could prove beneficial. anyone wanna build a couple of boards just to break 'em in half and test this out?