Hello,

does anybody knows how to calculate the "point of percussion"?
I have read some articles about it, and I was wondering if it can be calculated with a particular formula by using the measurement and weight, or is there a particular practical method which involve some particular striking/cut testing?
Thanks in advance for your help

Gianluca

My guess is no. You have to know the stiffness of the material, as well as the stiffness of the individual blade, so you have to know the profile in both dimensions. This is probably only one reason out of several. It is most likely easier to do it experimentally.

Any simple equation (as far as I know) to calculate such things would assume a rod.

Not too many swords are shaped like rods.

If anyone says yes, I will be interested in seeing the equations(s).

For swords, the Center of Percussion can be determined easily.

Read this:

[ Linked Image ]
Understanding Blade Properties
An article by Patrick Kelly

The original post asked if it was possible to "calculate" not to "determine" the point of percussion.

I stand by my answer, although the poster seemed satisfied with finding out that it can be determined empirically.

If anyone has any magic equations, I would still like to see them.

OOPS! So he did.

My response was intended to be definitive, not argumentative.

If what you're looking for is the "harmonic" Center of Percussion (the one linked to blade vibrations), Richard is right. It would indeed be a rather heavy calculation, needing a lot of data on the sword. It is however easy to measure.

In fact I'm not even aware of a way to compute how the harmonic CoP varies when you had or remove mass somewhere on the sword. For all I know the only way to get that is to try and see.

Note that there is another, less used definition of the CoP, based on inertia. I call it the dynamic CoP. It is not discussed in the featured article, but that's the one the wikipedia page speaks about (the harmonic CoP is a term rather confined to swords enthusiasts, it seems). It is equally easy to measure, it is in fact the pivot point associated to your hand (see http://www.albion-swords.com/swords/sword-terms.htm for the definition and the way to measure pivot points, two thirds down the page).

Sometimes they are the same, sometimes not. The dynamic CoP exists on anything from stick to swords to axes. The harmonic CoP exists only on flexible blades (for example it is not obvious on katanas as far as I know). Working out where the dynamic CoP moves when you add or remove mass is possible mathematically, though the resolution is a bit long to explain it can be done by hand with some simple geometry.

As I said in my recent "literature review" (http://www.myArmoury.com/talk/viewtopic.php?t=10758) the question of the relevance of both of these CoP for cutting is still open. I personally consider the dynamic CoP as more useful to figure out handling, alas it is almost never given in reviews...

Regards

Hi Richard-

Sorry if I sounded defensive. I took no offense to your post and was happy that you shared with us all. Sometimes when I'm in "work mode" I get far too "matter of fact" and short and to the point in my posts.... I'm trying to take a moment to share some helpful stuff here but likely don't have the time to really do more than little snippets here and there. :\

Good intentions and all that...

I'm not sure if anyone has bothered to come up with a formula for something with so many variables (length, width, amount and rate of profile taper, thickness, amount and rate of distal taper, cross-section, tightness of assembly, weight/shape of all the components, etc., etc.) when it's so much easier to whack the side of the pommel. :)

Since it's a vibrational issue, the easiest way to find it will be to make the sword vibrate by whacking the pommel. Then you can see where the waves cancel out to no (or little) vibration.

Vincent,
I think it's easier to stick to terminology like "pivot point" rather than "dynamic CoP." Not every sword's pivot point is made to be struck with. In some cases the CoP and the blade pivot point coincide. Sometimes, they don't and shouldn't. CoP is a static vibration thing. Pivot points are dynamic rotational things. Terms like "pivot point" and "point of rotation" don't cloud the issue.

As for why pivot points (hilt and blade) are never given in reviews, it's a much newer concept than CoP or PoB. Not everyone knows about it or cares about it or realizes why it may be important. For our reviews, we have a set of standard stats we try to collect for everything. Pivot points aren't among them, as most people won't know how to make sense out of them. Also, our standard set of stats was compiled before people started really talking about rotational points. We're not going to be able to go back and get pivot points for all our previously published reviews. :) As has been discussed, PoB is not the best stat, yet it's so common we always include it for A) consistency, and B) because people still want to see it.

The primer articles on swords linked above don't include pivot points simply because no one was really talking about them when the articles were written. Way back when, weight was the most important stat. For a while after that, PoB and CoP were the hot stats to know. Then, distal taper rates were big. Now, some people want pivot points or other things. In 2 years, there will be other measurements. :)

For me, I have never made a purchase decision based on PoB or CoP or even pivot points. All of these measurements are a by-product of the design of a sword. They all work together to make a great sword, an okay sword, or a bad sword. To take one measurement or other out of context doesn't always help. What's more important to me is to be able to hold it myself or to be able to read the opinions of those I trust when they talk about their handling experiences. The handful of dry stats seen in reviews mean much less to me than the paragraphs relating how the sword performed in hand. :)

When cutting, I will try to determine the CoP just so I can make sure I'm cutting with the part of the blade best suited to deal with the shock of impact. That's the only use I have for knowing the CoP these days. :)

Chad, thanks for your answer... I hope my post did not come out has reproachful or anything, maybe I wrote too fast as well :)

Chad Arnow wrote:

I think it's easier to stick to terminology like "pivot point" rather than "dynamic CoP." Not every sword's pivot point is made to be struck with. In some cases the CoP and the blade pivot point coincide. Sometimes, they don't and shouldn't. CoP is a static vibration thing. Pivot points are dynamic rotational things. Terms like "pivot point" and "point of rotation" don't cloud the issue.

Problem is, there is an infinite number of pivot points, as many as there are points on the sword where a force can be applied (from the user or the target). As a matter of fact, pivot points relative to the back and front of the hand on the hilt are different and sometimes quite far apart. So I came up with "dynamic CoP" when I needed to refer to the specific pivot point associated to the spot in your hand where you don't want to get a shock (for me it's between the index and middle finger, empirically), as a short-cut. I'm quite unsatisfied with the term as well...

Technically, looking at how other disciplines use the terms, I think it's the harmonic CoP that should change name and be called the sweet spot or something like that, but there is no chance to change that, ever ;)

Quote:

As for why pivot points (hilt and blade) are never given in reviews, it's a much newer concept than CoP or PoB. Not everyone knows about it or cares about it or realizes why it may be important. For our reviews, we have a set of standard stats we try to collect for everything. Pivot points aren't among them, as most people won't know how to make sense out of them. Also, our standard set of stats was compiled before people started really talking about rotational points. We're not going to be able to go back and get pivot points for all our previously published reviews. :) As has been discussed, PoB is not the best stat, yet it's so common we always include it for A) consistency, and B) because people still want to see it.

Just to clarify, I was in no way criticizing past reviews or anything. I know it will stay the same for a long time... But if I want that to change, the only way for me is to talk about it, so that people realize the interest of other stats :)

Quote:

The primer articles on swords linked above don't include pivot points simply because no one was really talking about them when the articles were written.

And I was not criticizing the articles either. In my opinion, what is found in this kind of articles should be the basis of what is generally understood inside the community. Pivot points clearly do not fit that yet... And their applications have not been discussed in sufficient depth.

Quote:

Way back when, weight was the most important stat. For a while after that, PoB and CoP were the hot stats to know. Then, distal taper rates were big. Now, some people want pivot points or other things. In 2 years, there will be other measurements. :)

Quite possible, but I have faith in the advances of science :D In fact what drives me in the subject is not the values down to the fifth decimal for any particular sword, but rather the better understanding of how all weapons work.

Quote:

For me, I have never made a purchase decision based on PoB or CoP or even pivot points. [...] The handful of dry stats seen in reviews mean much less to me than the paragraphs relating how the sword performed in hand. :)

Well I'm not proposing to reduce the reviews to five numbers either :D In fact, currently I act just as you do, I don't even read the stats but skip directly to the paragraphs below... The difference is that I don't have many occasions to handle the swords I fancy. If I had values for pivot points, I think I could build a semi-reasonable feel simulator that would give me an idea of how the sword behaves, which would be a step forward. Well that's an ongoing experiment of mine, I suppose I'll have more to say about that in the future...

Thanks for your work!

Dear Sirs,

I thank you very much for your precious information. My lack of technical knowledge of the matter is evident, beside my difficulties to approach the topic with my english.
The empiric method is enough for me and I was not going to unveil any magic formula.
My "argumentative" reply was not meant to contradict any of your competent contributes neither.

kind regards

Gianluca Zanini