Rotational Balance

[Ed Schempp]

I believe this concept was not covered adequately in the thread with the topic Balance.

This aspect of balance is somewhat subtle yet extremely important when you are working a blade quickly and fairly hard. This aspect increases in importance as the length and the weight increases. The target cut is not stagnant and requires many quick changes of the blade vector. These circumstances often occur in two main environments. The first being a natural setting where you might be cutting trail through brush or doing a task like limbing a fallen tree. The second condition is combat.

I usually drop my handle to align the edge of the blade with my forarm. This gives much less moment arm of force torqueing the blade, therefore less energy to start and stop to change rotational direction.

To achieve this rotational balance you can use two methods; diamond shape the blade to move the mass of the blade toward the edge or fuller the back of the blade to remove weight from the back and move the center of gravity toward the edge.

During the golden age of edge weapons this rotational balance was very evolved and incorporated into combat blades. The Japanese Katana and the multi-fullered sabers of western Europe are good examples. Some of the better early American bowie knife makers were all over this aspect of balance.

When using a big triangular cross section blade change of target and rotational force will often torque the blade and the flat of the grind will slap the target rather than making the cut.

Diffferent applications are designed to cut at different parts of the blade, rotational blace can be tuned to the function....Take Care...Ed

[TooSharp]

Ed,
Very good info. This site rock!!

TooSharp

[angusW]

Good info. So what you are saying is, there's a line between being able to move the blade in different directions easily without the knife being too heavy yet not too light so the blade has built in kinetic energy for impact.

[phaust]

Some great info. I have one question about something I'm not 100% sure I understand. When you say, "I usually drop my handle to align the edge of the blade with my forarm," do you mean you design the handle so that when using it, you start off the cut with your wrist bent up or horizontal rather than bent down? To say it another way, having the blade aligned with your forearm, your wrist is not angled down (so that you can angle your wrist downward to make the cut, rather than having to move your whole arm). I am picturing the Khukuri as the epitome of this, as far as Spyderco knives go anyway.

Assuming I understand that correctly, is that also why the Khukuri is thicker at the far end from the blade (where it's gripped by the ring and pinky fingers)? It seems like that being thicker there also makes it feel natural to have your wrist angled upward for the blade to be horizontal.

[Ed Schempp]

Twisting is the rotational force that this edge to spine balance avoids.

There are many parts of a long edge that can be designed as the working part of the blade. A Katana works with the tip, the belly of the blade leading the stroke and aligning the following edge cut. With a heavy cut or chop it is beneficial to cut near the handle. Consider the material to be cut as the fulcrum, the more mass on the opposide side of the fulcrum the harder you can swing and the deeper you cut. If you try heavy cuts with the tip, it will just bounce off the fulcrum.

Changing directions of a cut to a new target each time requires that you twist the knife one way or the other. The direction of the rotation is usually the side of the blade that lays flat and slaps the cut. A bad rotationally balanced blade generates mometum, that tries to twist the blade in your hand.

Many missed cuts are because the blade, tip to handle failed to stay square with target. The tip will bounce back and the blade will slide off the target. Picture a nail with a string tied to it. Stretch the string out tight and proceed in circles around the nail. You can consider the string as the line of force. You need to keep the edge of the blade on the string if the tip falls back the blade will not purchase and cut the target it will slash the target. I cock my wrist for cuts, but my released position is neutral not forced through a neutral position in order to square the blade. That is why I drop my handles on my knives.

Every part of a knife relates to every other part of a knife. Grips and hand position are very important to rotational balance, and I have a hard time talking about one without the other...Take care...Ed

[Shenmue728]

Very interesting. How would this effect a folder with a spyder hole? Is it too small to do any significant effect?

[Dr. Snubnose]

Very good explanation Ed! I not only understand what you are saying but it is a very important concept to understand when using a large blade for combat....you said it perfectly...in a way I would have found very difficult to explain to others....Doc :D

[Michael Janich]

Awesome information Ed--thank you for sharing.

Based on your explantion, would a kukri provide good rotational balance because, rather than a dropped handle, it has a dropped handle and half a blade? Also, I've always found its "boomerang" shape to make tactile edge orientation a lot easier. Do you feel the less pronounced boomerang of a dropped-handle knife does the same thing?

Thanks again for sharing your insights!

Stay safe,

Mike

[Ed Schempp]

Yes Michael a well made khukuri can have good rotational balance. Weight distribution and edge geometry also have alot to do with this very interesting piece. The khukuri has a positive rake or a negative blade angle which will purchase or capture a medium in a cut or chop.

Angled knives like the khukuri can generate a torsional cut because part of the edge is following the centerline of the mass or the piece. A cut in, cut out motion. Knives like this follow the force of the cut giving good response to changing direction.

Take a dagger for example. It is a very straight piece. Any torsion on the piece has to be accommodated by an anti twist grip. Any forward movement on the handle has to be prevented by a substantial guard. Compare that to a pust dagger, that when thrusted has the mass of the palm in line with the forearm all behind the line of the blade. On a straight dagger the force is parallel but not behind blade. Now consider putting torque on the line of entry. The push dagger is always aligned with the mass of your hand. Look at the hand and arm on the straight dagger always has the power and the grip rotating around the line of entry.

There are a lot of ways to skin a cat...Take Care...Ed

[Michael Janich]

Thank you for your response, Ed.

Your insights into the dynamics of cutting are fascinating. The more I read, the more I learn--and the more I understand and appreciate your designs.

Stay safe,

Mike

[CanisMajor]

Very good explanation and info Ed, thank you.

Canis