B is for Bend

When I’m at a show (like Canoecopia for example) and talking with fellow paddlers, one of the most frequent comments I get relates to how surprised the paddler is at how I can bend wood. Their expectation seems to be one of the wood being brittle and stiff. I’m here to tell you (actually write you) that this is not true. A long (say 50 inches), narrow (say one inch) and thin (say one-quarter inch) piece of wood will behave more like a spaghetti noodle than a railroad track.

This depends on scale as it turns out, at least in my experience. In fact, it even applies to the railroad track mentioned above as an icon of inflexibility. Next time you are in the shop, anchor a long thin slender piece of wood JUST at one end. If you go to the free end of that long thin slender piece and move it, you will observe the spaghetti noodle behavior in all directions. It is flexible.

Now go down half way on the wood piece, which still just has one end anchored. You can still pick this piece up at the midway point and move it around considerably in all dimensions, but not nearly as much as when you moved it out at the tip. Move in again, to the quarter length. Again you will note movement, but again considerably lessened relative to the movement at the mid-point.

Finally, try and pick the wood piece up one inch from the anchor point. You won’t be able to budge it in any direction. For me, this lesson in “wood physics” is all about the radius of the bend you are trying to make. A big radius, like your first move is no problem, the wood is very flexible and will accommodate just about anything within reason.

Each move down the piece shortens the radius within which the wood can flex. It has less to work with and therefore has to make more happen over a shorter distance. Taking this to an extreme, at the shortest distance, the position in which the wood did not move, you have now crossed over from a bend to a corner.

Viewed from a mass point of view, the top surface of a bend is experiencing compression and the bottom surface extension. In a bend, the wood is experiencing the necessity of stuffing or stretching some amount of its mass into a smaller or larger volume of space. Just think about how the top of the bend is shrinking, yet the wood mass stays the same. You have to squish the same amount of wood into a smaller amount of space. Over a long distance (a big radius bend) both the squish (top side) and stretch (bottom side) factors are within the tolerance of the wood. As the radius of the bend gets smaller, you approach an elastic limit beyond which the wood will no longer accommodate a squash or a stretch. It now snaps.

Coming back to bending wood strips while making a paddle, the radius of the bend that the shaft strips make while in the form, are simply within the limits of the wood elasticity, so bending is tolerated. If you force it too hard, usually by moving the first compression block too close to the end of the angle block and/or tightening it too much, you cross that elastic threshold and go from a bend to a corner. The wood strip is now beyond its elastic limit and it snaps.

That’s it sports fans. Thanks for tolerating my diatribe on the letter “B”, all wrapped up in a rant about the wood physics involved in a BEND. I hope you will venture back tomorrow for a dose of the letter “C” as it is used in the word “cadence”.

happy paddling!

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