COP

Sean is right, eveything has harmonic properties and points in which the wave of force traveling through it is narrower or more concetrated. not only swords, but rifles, and ships and bricks all have harmonic nodes, though good luck finding the harmonic node of a brick. my point is, yes katana have sweet spots too, even though I have seen people convinced otherwise due to the stiffness of the blade typically found on katana. on my ship there is a certain RPM of engine speed we will not spend sustained amounts of time on because it vibrates at a rate that is harmonically very bad for our equipment.

ok back to swords, the thing with the pommel tap test that I don't like is that it sets up a wave of force going through the sword 90 degrees off from the direction the wave from cutting would be. this is because you tap the pommel on the flat side not the edge side. if you did tap the edge side it wouldn't dance as well. True, the tap test will give you an idea of where the nodes lie and if you know what to look for there's a lot of info there (but I'll leave that to Tinker and Gus) BUT for most of the subtleties are lost. I like the drop test because it sends the wave through the sword in the same direction that using the sword would.

in this review's cutting video I show the test pretty clearly, mostly because the sweet spot on this sword is amazing.
viewtopic.php?f=6&t=111

I personally use a combination of TomK's method and the "waggle test," detailed from this article


I recommend that everyone take a look at that article, as I believe it contains valuable information on the subject. It also deals with saber-like weapons, a category into which I believe Japanese swords such as katana and tachi fall. If I am interpreting it correctly, it calls into question what a lot of people mean when they start spouting out "COP" when describing a sword. It states that the optimal cutting "sweet spot" and general handling dynamics have more to do with how a sword acts like a lever, especially when its pommel comes into play during rotational behavior, than where the blade vibrates when you slap it.

Basically, there are two areas in the article that I believe do an excellent job of talking about a sword's actual "COP" and optimal striking points. The first is here:

"Interestingly, the sizes of the pommels we find on authentic swords indicate that the result must be a pivot point very near the tip. So swords with real pommels must behave quite differently from cavalry sabers. Using an overly large pommel would move the pivot point past the tip, which would guarantee that no physical part of the blade could ever be used for an impact that doesn't cause hand shock. In addition, a pivot point far past the tip would make the sword rotate very poorly, like an extremely long and unwieldy sling. So the existence of extra large pommels is doubtful. Likewise, if the desired pivot point was anywhere near one third back from the tip, why use a pommel at all? Just leave it off and have the sword pivot around a point about a third back from the tip, just like a saber. So I would say that if a sword has a real pommel, it would only logically be used to create a pivot point very near the tip.

Suppose that the desired pivot point is exactly at the tip, and go back to the drawings of rectangles, where we can quickly see just how sensitive a sword is to the location of the balance point, which is set by the size of the pommel. If you have a 45-inch blade, with a balance point 5 inches forward of the cross, and a natural pivot point, relative to the cross, located at the sword's tip, you'll have a 5x40 inch rectangle, with an area of 200 square inches. Note that the rectangle is already eight times taller than it is wide. If you add a large pommel to pull the balance point back to 4 inches, then you've squeezed the sides of the rectangle, making it much taller, and the pivot point moves 9 inches past the tip. If you pull the balance point back to 3 inches, the pivot point moves more than two feet past the end of the tip. You can buy swords like this from some of our best manufacturers, so don't think it can't happen. This is all the result of the mistaken notion that the pommel is there for balance, which is all that Sir Richard Burton, and many others, could figure out. I would argue that the pommel was used to control the sword's rotational behavior, pure and simple [ed. and also to prevent the hand slipping off during certain tehniques]. Maybe the early sword researchers should've asked why a short sword needs a pommel for balance, while a heavy, forty-five inch cavalry saber does perfectly well without one.

To find out where authentic swords place this natural pivot point, I did some very simple waggle testing with several 16th century swords in a private collection, while attending a recent ARMA retreat. I already had some accurate data from six reproduction swords, from some of the best modern manufacturers, and their pivot points, relative to the cross, vary anywhere from 10 inches behind the tip to 20 inches past the tip. Oddly, all of these authentic swords pivoted within about an inch and a half of the tip, which is also close to the accuracy limit of my simple waggle test. Considering that the odds of any one particular sword doing this are about one in twenty, given the similar sampling of reproduction swords, the simplistic odds of six in a row doing this are one in sixty-four million. So it seems that the people making the authentic swords were probably paying pretty close attention to this rotational behavior, and carefully controlling the location of the sword's pivot point, which apparently is a trick that was lost far in the past."

And the second is here:

"As to why we’re still obsessing over the percussion point, given that we haven’t known what it is in quite some time, I must admit ignorance. Once we’d decided that it was the node of oscillation, about a third back from the tip, a location that can’t vary much, why continue to search for it on each and every sword? I assume that earlier authors had somehow conveyed the importance of finding its location, without clearly explaining what it was. Sir Richard Burton did say that every swordsman must know the location of his percussion point, but was a bit vague about how to go about it. He did say that to find it the cavalrymen go into the woods and hack at limbs, looking for the impact point that gives the least hand shock. But most notably, he never said that you can just slap the slide of your blade to find the percussion point. Since he talked about swords to just about everyone, and apparently no one told him about the blade slap, I assume that no one in that era thought that the percussion point could be found by slapping the blade. But he did tell of an ingenious technique for finding the percussion point, in use at the Tower of London, which involved swinging the sword like a pendulum and timing its oscillations."

However, for those quotes to make sense contextually, I highly recommend reading the whole article - at least the first page. In case the first link was too hidden, here it is in its entirety: www.thearma.org/spotlight/GTA/mo ... mpacts.htm