Flex interests me also.
George Gall (PlusOne) offered his insights about flex a while back:
http://www.swaylocks.com/comment/497273#comment-497273
PlusOneShaper wrote:
Great question. I think a LOT about Flex in Surfboards… …I’ve had arguments with reputable shapers, saying “boards don’t flex” but my training tells me otherwise, in fact EVERYTHING is a spring, even a rock, even water, especially MOVING water (or You, moving across water (a relative intertial frame shift for discussion’s sake.)) First, and I don’t mean to be a smart ass, but nomenclature is Longitudinal for lengthwise flex, also called Straight Bending. Bending from rail-to-rail is Lateral, or I’ve heard some call it “curling” on surfboards is less understood (until the introduction of “parabolic,” or “perimetric” stringers.) The twist, as mentioned is Torsion. All 3 bending modes are important, and I have tested and validated what I thought was desireable (THANK GOD FOR 1" WIDE CARBON GRAPHITE TAPE.) I’ve placed tape in various directions on boards, to completely/partially lock out discrete bending modes. eg. glass on 1" carbon tape at 45 deg X’s on a board to lock out Torsion. (90 for Lateral, etc.) This is where I got in a LOT of HOT water with some kneeboard builders, firmly convinced the twist is what made their boards go, but in general I’ve found Torsion to be the enemy. Surfboards (thin ones especially,) will bog down with they twist too much. Surfboards are glass in such a way to leave them wide open for too much torsion, the cloth comes off the rolls at 0/90 since the beginning. The 0 deg, or “warp direction” is good for Longitudinal control (of bending.) (90 or “fill direction” is good for Lateral control.) Most boards do not have cloth at 45 degrees, thus are wide open to twist and non-immediate rebound. You will benefit with “off axis” glassing, that’s been done for decades, I’ve met success with it, but be careful, if you do -20 deg with one pull, it is suggested to do a compliment of some sort (I don’t mean, “hey bro, good job,”) to counter the single offset behavior. That said, I’ve done a lot of building on Asymmetrics with asymetric layups to promote certain behaviors and to block out others.
http://www.swaylocks.com/comment/497387#comment-497387
PlusOneShaper wrote:
I relate. To create flex (in a surfboard) a load must be applied to it. In most cases we create flex from the rider inputs. Changes in direction create acceleration (sometimes referred to as “G-Forces”) in order to go in the new direction. Our bodies want to keep going the old direction, so we feel the force of change through our legs. The applied load is through our feet to the board (at 2 points.) Of course, these 2 point loads VARY fore/aft (longitudinally) and left/right (laterally.)
To promote flex in the board there are a ton of ways to do it. One of the less obvious but most common is to make a thinner board (thinking HP shortboards that Pros ride.) Board flex with respect to its’ thickness is a cubic factor (assuming Simple Beam Theory.) For example if you double the thickness of a board, it’s NOT twice as stiff, it’s actually 2-cubed, or EIGHT times stiffer. There are other things to consider like Slenderness Ratio and scaling factors, so as you go 1/4" thinner the board is even more flexible in the next 1/4" thinner, but in general you get the idea, “If you want it to bend, thin it out.”
To promote flex you can also use different materials, under this category I’ll also put “material placement.” I alluded to what I’ve been doing (using off-axis glassing) and locking up flex in other degrees of freedom, like torsion. You can also use lower modulus fabrics, like Innegra, or higher performance bi-directional fabrics, like Carbon Graphite depending upon the goal one is seeking. By the way Carbon is an Engineer’s favorite composite because it is bi-directional, its’ tensile strength is just about equal to its’ compressive strength (in a viable matrix,) UNLIKE fiberglass which has tensile strength close to Carbon, but almost no compressive strength. Kevlar is like this too, but has MUCH higher tensile strength than either Glass or Carbon. In general, you can stack fabric patches where you want stiffness, and minimal, off-axis fabric where you want more flex. Likewise you can use different foam (or other cores) to promote flex. I mentioned using iFoam has been very interesting. There are other foams which exhibit good elastic properties as well.
Here is a very interesting thing that happened a few years ago, I built a board with variable flex. I was doing a bit of SUP with a couple of guys when it was small. One of the guys was Jim Weir who owns a company called ULI boards (inflatable SUPs.) He calls me over to his little Jeep and as he’s inflating it and he says “Watch this” and he stands on it between two milk crates and the 14 ft air-filled race board is holding his weight. Got me thinking. Roll ahead a few months and I meet Rouven Brauer (Bufo) who owns HydroFlex. So we end up building the first SUP from iFoam with NO stringer (the shape was like a wet noodle on the shaping racks, ha) and a hard external (minimal 4s glass job. Long story short, Bufo’s boards gain strength by applying internal air pressure. He has a Schrader Valve (tire valve) potted into the board and a small bike pump is used to pressurize the board. His focus at the time was to prevent dents and buckles. What I was after was to make a “floppy” large board and find out the behavior as I added pressure to the board. (NOTE: EPS breathes air (and water,) and iFoam breathes air but NOT water (like Gortex) and PU sorta breathes air, and water, slowly.) I was very surprised to find how much influence internal pressure has on surfing characteristics (and for SUP, paddling characteristics too.) I found that no pressure SUCKED, the board plowed in all phases. On the other end of the scale, too much pressure SUCKED as well, the board would lock in a turn and I’d end up face down splatting in a bottom turn. But the best part was I found there was an ideal pressure for that one floppy board, it really became lively, and sprang out of turns. Higher pressure paddled better. (This is good to know for Racer boards.)
Finally, I came up with a stringer set up which mimiced the optimal flex of the inflatable board. Enter Robb Green (but that’s another story I’ll save,) and the boards work great without having to do the pressure-proof skin and valve stuff. I liken it to the Hotrodders who bag their cars, then leave it at one setting, eventually going over to coil springs and removing all the other supporting gear.
