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Swords, Cutting and Military History

Why a sword feels right

Why a sword feels right

photo by Brent Flanders

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Many readers will have had the experience of shopping for modern, practical cutting swords, both replicas of ancient swords and modern designs. One of the most common tips given to new sword-shoppers is to pick up and try out many different swords “until you find one that feels right for you”.  Rarely is any explanation given for precisely what this means.

Shoppers presume it has something to do with whether the hilt is the right size for their hand, or that it has something to do with the sword’s “balance”… whatever that is.

Some lucky few will have had the chance to handle high quality antique weapons.  Those who have are often shocked that these blades — often of the same weight and length as the modern replica blade they use at home — have a completely different “feel”.  Often master blades seem lighter than than their actual weight, with a sense of “liveliness” (easy to rotate in the hand), and with the feeling to make almost effortless cuts or thrusts.  This isn’t to criticize the sword makers of today — there are master swordsmiths around the world — but to demonstrate the skill and genius of the weapon makers of old.

The basic question then is why is there a difference between how these swords feel, and how can a sword practitioner use this knowledge to their advantage? There have been a number of papers, articles and discussion threads on this topic, often delving into physics formula to define and explain mathematically how and why a sword feels, moves and strikes as it does.

One of the main resources for this will be Dynamics of Hand-Held Impact Weapons by George Turner;  a fairly technical exploration of the physics behind why swords handle as they do (and an indispensable resource for those interested in designing good swords) .  There are also several other articles, plus web forum discussion threads, which explore this area which we’ll draw on.

Never fear though;  we’ll leave the calculations behind and focus on the practical applications.  Those who wish to see the maths can check the links in the Sources section

So, let’s start off with a few basics.  We’ll presume that the swords you’re looking at are well designed, have properly sized hilt grips, etc., so we can ignore the ergonomic factors.

A sword has several physical characteristics which can affect both its feel in the hand and how it handles.  Let’s take a look at these, along with examples of how you would check these while inspecting your blade:

 

Weight

Simply, how much does the sword weigh?  Much of the time, people will pick up a sword and hold it with blade straight up, or tipped forward at a very slight angle, then bounce their hand up and down several times.  This is an instinctive way of getting a feel for the weight of the blade as a whole (in physics, this is known as mass).

The only real consideration here is whether the blade the right weight for your strength.  Too heavy a sword means your cuts / thrusts will be slow and you will tire very quickly.  Too light a blade means that you’ll move like lightning, but the blade can’t transmit the full force of your strike.  Simply put, the movement of your strike (foot to body to shoulder to elbow to wrist to sword) is building and transmitting energy.  For our purposes, it’s best to think of a blade as a conduit for these forces;  it can only transmit a certain amount of this energy through to the target, and the amount of this energy is based on the mass (weight) of the blade.  So heavier blades are capable of delivering more energy (force) through to the target.

Thus, choose the weight of your blade carefully for your intended purposes.  If you want to cut targets, you’ll need a blade with more weight.  If you’re thrusting, where you don’t need to transmit so much energy, a lighter blade is fine.

 

Balance (or Center of Gravity / Center of Mass)

This is the point on a sword where there is equal weight distribution from the balance point to tip, and from balance point to hilt / pommel end.  Some people will try and locate a sword’s balance point by trying to balance the side of the blade on their finger.  Others will strike an en garde position, then make a series of practice parries, cuts, thrusts and other movements “to get a feel” for the sword.  What the first person is doing consciously, and the second instinctively, is to identify the center of gravity (mass) for the blade (along with another factor we’ll discuss later on).

Putting the math aside, the laws of physics say that when more than one point of force is exerted on a long object, the object will always rotate around its center of mass.  When considering how a sword moves, the laws of physics don’t look at the human hand as a single object pushing on the sword hilt;  it defines it as multiple points of force, with each finger being a point.  Thus when maneuvering a blade, the sword will tend to move around its center of mass.

Picture a fencer holding a blade, moving it through a variety of en garde and blocking poses;   hilt down and tip up, to hilt up and tip down, then side-to-side.  An observer would note that much of the time the fencer seems to be rotating the blade around an invisible point near the hilt – the center of mass

In most well-balanced blades the center of mass is a few inches / centimeters in front of the crossguard / tsuba.  If the center of mass were too far back, within the hilt, it would make the blade feel very heavy  and hard to move (as you lose the leverage effect of the balance point), and it would be a poor striker, as more mass at the back of the sword means a lot of your energy would stay there, rather than being in the blade end.   If the center of mass were too far forward, it would make the sword feel “tip heavy”.  With the weight being out on the end of the blade, it would take a lot of strength (more energy) to get the sword moving, and particularly to change its direction if in motion.  A great striker, but not a sword to easily fence with.

It may help to visualize it like this:  Take a 5 pound (2kg) weight and a three foot long rod.  Fix the weight about 1/3 from the hilt end, then hold the stick by that “hilt” and try to move it.  You’ll see the stick tends to move with the weight as the center-point of rotation.  Now move the weight behind your hand on the “hilt”;  you’ll find it takes a lot more energy to move the rod, and that while the hilt feels heavy, the rest of the rod seems very light; if you struck something it would tend to bounce off.  If you move the weight towards the tip end, the rod becomes very hard to hold out extended, and  hard to either start moving or change direction while moving.

So, what controls the position of the center of mass?   The skill of the swordsmith, who had several ways of controlling weight distribution and thus the center of mass.  Let’s say a smith has made a sword, but it’s tip-heavy, with the center of mass far forward.  One way to move the center back to where we want it is to remove material from near the tip of the blade.  This can be done by incising fullers (grooves) into the sides of the blade, but most often is accomplished by tapering the blade towards the edges and tip.  Often we see old sword blades with a lot of steel near the hilt, but with taper thinning out more as the edges / tip are neared.

Taper chart

From Dynamics of Hand-Held Impact Weapons
Showing how the center of mass moves back towards the hilt the more the edges and tip are tapered.

Another arrow in the smith’s quiver to control the balance point is the use of a pommel.  More than just an item to hold the hilt on, the pommel places more weight (mass) at the back of the blade, again moving the center of balance back towards the hilt.  Old European knightly one-handed swords often have large pommel as a counterweight.  You may have noted that two handed weapons have tiny pommels, and in the case of Japanese katanas, no pommels at all.  This is because the additional length of the tang/handle are acting as the counterweight, making a large pommel unnecessary.

Some blades are made hilt-heavy in balance because they’re meant to deliver lightning quick movement, but no force, like modern fencing epees. Other blades are designed to be tip heavy, taking a lot of energy to use, and are difficult to fence with, but deliver massive amounts of force;  examples would be classic falchions, machetes and cutlasses.

Then we have balanced swords, meant to be effectively used for both fencing and striking, though they can be slightly weighted towards hilt or tip, depending on the needs of the owner.

Precisely what kind of balance you’ll be looking for in your blade will depend on the use you want to put it to.

 

Rotational Inertia

We’ll have to touch on this somewhat complex topic for just a moment.  Rotational inertia (RI, or more technically known as the “moment of inertia”) is the amount of force it takes to rotate a sword, and is integral to how a blade feels as you move it.  Remember that example I gave earlier, of a fencer trying out a blade by moving it through a variety of postures?  The ease (or difficulty) with which a blade can be turned in the hand is based on the inherent inertia of the sword.  If the blade has a large RI, with extra weight spread out towards tip or pommel, you have to apply extra energy to overcome that weight and get it moving (think of picking up a barbell with all the weight at one end).  Moving the weight away from the ends lessens the inertia inherent in the sword.

This is one reason many replica blades don’t move as well as the antique weapon they’re copied from.  As Turner says:

“Your sword can have the exact weight and balance point as an original, but the little details like tip thickness can devastate its performance. If the last foot of your blade is 1/16” thicker than an authentic sword, it can increase the moment of inertia by 50%.”

If you’re trying to replicate sword techniques from centuries back, but your sword is moving like it is stuck in molasses, you’ll either think you’re doing it wrong, get frustrated and quit, or try to compensate for a poor weapon by improperly altering the techniques to fit.

What this means is that, when shopping for blades, you’ll need to try them out to see how well they turn in your hand.  Are they easy to move, or do you feel like you’re pushing weights?  If the latter, then that’s not the sword for you!

 

Center of Percussion and Point of Percussion

The final quality we’re going to examine doesn’t so much have to do with how a blade feels when you pick it up, but with how well it cuts and stays in your hand.  I think everyone has had the experience of taking a stick, hitting something solid, and ending up with a stinging hand at the shock of the strike at the tip end get transmitted down to your hand.  You may also have experienced this with baseball bats, golf clubs, or while cutting wood with an ax.

So, for swinging strikes (rather than thrusts) the center of percussion in the point where the sword rotates.  For most swords in the process of cutting, this will be the hand.  Picture holding a broadsword and making a cut;  while the elbow is extending your arm outward, it’s the rotation of your wrist which is moving the blade in an arc;  therefore, your hand will be the center of the percussive strike.

So what causes the painful shock to your hand?  Well, physics describes how the point of percussion — the section of the sword which strikes the target — has a relation to the center of percussion.  Remember how we talked about a sword just being a conduit for the energy you put into it?  Well, it turns out there are certain areas on a sword which deliver more energy than others, and one perfect place which delivers all the sword’s energy on contact with the target.   The further you are from this perfect point of percussion, the more stored energy remains in the blade… and, well, that energy has to go somewhere!  So it gets kicked back to the other end of the blade and delivered to it’s point of origin, the Center of Percussion.

And your hand.  Ouch!

In sword and sporting parlance, this point of perfect percussion is called the “sweet spot”;  the area of maximum delivery.  The reason that it’s not a single point on the blade goes back to physics;  it turns out that the percussion point is incremental;  you can be off the absolute point one way or another and still deliver 99.9% of your blade’s energy.  This is why the “sweet spot” is often described as an area some two or three inches in length.

It also turns out there is a relation in the size of a sweet spot to the length of a blade.  The longer the blade, the larger the sweet spot.

The percussion point is also related to the center of mass, thanks to the laws of physics.  The further out on a blade the balance point is, the further out the point of percussion will be, according to a mathematic formula I’m not going to trouble you with.   It’s entirely possible, on a very tip heavy blade such as a cutlass, for the point of perfect percussion to be past the tip of the blade.

This is why I’ve brought this up this somewhat esoteric point in an article on why a sword feels right.  A poorly balanced blade can end up being a very, very poor sword for practical cutting, with the sweetest spot appearing some distance past the end of the blade.  Luckily there are a few basic tests you can make right in the store to get an idea where the point of perfect percussion will be.

1) One of the oldest methods is described by Sir Richard Burton (1821-1890) in his The Book of the Sword (published 1884, Chapter VII), in which he describes how one can take a blade and make cuts, inch by inch, at different points down its length, until the spot at which there is the least — or no — feeling of kickback is found;  the sweet spot.

2) A common, more modern version is to hold the sword up straight in front of you, edge on.  You then tap the blade on the side; this creates an oscillation in the blade, like a shock wave.  You continue tapping, slowly and carefully looking up the length of the blade.  At some point, you may see a spot on the blade (nearer the tip end) which does not vibrate.  The section of blade below and above that point will oscillate with vibration, but that single point will not.  That is the point of perfect percussion.

 3) In his paper, George Turner describes an even simpler system.  In fact, he’s surprised no one had ever mentioned it to him before.  In order to conduct this test, grasp the hilt of your sword with only your thumb and forefinger, at the spot on the hilt where your forefinger would normally rest.  Make sure your thumb and finger are exactly opposite each other.  Now, holding the sword straight up, rapidly shift your hand right and left, watching near the tip of your sword, roughly at eye level;  make sure you don’t move your wrist at all.  You’ll see a point where there is zero movement in sword (see diagram at left).  This will be very near the center of perfect percussion, relative to the place you normally grasp on the hilt.  Note that if you move your point of grasp with finger and thumb, the percussion point will also change, so be exact.

 

With these simple tests — which, with practice, can be performed in a few seconds — you can not only make sure the sword you are shopping for weighs the proper amount for your purposes, but also ensure you have selected one with proper balance and proportions which make it easy to move, yet will deliver the amount of force you need,

 

Sources

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