A Basic Explanation of Asymmetry

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DavidPF

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A Basic Explanation of Asymmetry

Way back when I began sharpening I remember reading through many forum threads about asymmetry in double bevel knives and having no idea what was going on. All this talk about 70/30 and 80/20, "You should be doing this and shouldn't do that", but no real explanation for what was actually going on. So I ignored the whole thing and went on sharpening in my 50/50 way.

A few years (and many knives) later, I realized after spending hours grinding further back behind the edge, I had developed my understanding on kitchen knives to the point where I could not only understand, but explain it in simple-enough terms. It really boils down to 3 general rules which, when understood individually, combine to explain the whole thing.

So we'll start off with a diagram of a crappy knife, then introduce one rule at a time and improve it till we get something which represents a good knife (the rules are in random order and only numbered for ease of reading).

JZOu0Y0.png

Figure 1. A huge, fat, symmetrical knife

Rule 1 - Food releases off a curved surface better than it does off a flat surface

Most cuts we do in the kitchen are flat, and when a flat surface on the food comes into contact with a flat surface on the side of your knife, the large amount of surface area in contact between them increases drag and can be enough to hold them together. This is known as stickage, and it's a pain in the ass to put up with. We can minimize this nasty effect with the use of a curve instead of a flat on the side of the knife, because there is less surface area shared between a flat and a curve (the same effect can be had with a multi-faceted compound bevel) when compared to a flat-on-flat.
dvhsqEz.png

Figure 2. A curvy, fat, symmetrical knife

Rule 2 - The thinner the knife, the easier it moves through what it is cutting

This is pretty much self explanatory. Thicker knives have to push more out of the way on either side as they pass through food.

But here's a problem: The thinner you make a knife, the less curve you can fit on either side. This compromises Rule 1, and so we have to balance how thin the knife is with how much stickage we are willing to put up with.
The heat-treatment and kind of steel will determine how thin you can get your knife before its strength becomes compromised.
rjrwW3d.png

Figure 3. A curvy, thin, symmetrical knife

So here we are at a nice compromise of thickness vs. curvature. Cut food falls off on either side of the knife and it's not so wide as to get wedged in the food all too often. Perfect, right?
It turns out there's another way to improve the shape even further, and it has to do with how we use kitchen knives.
When cutting food, we generally hold the knife in our dominant hand and the foodstuff in the other. Lets say you're right handed and the food is held securely in your left. Do you really need the ability for food to drop off the left side of your knife if you're already holding it down? We may as well sacrifice some of the curve on that side and make the knife thinner overall. Like so:
JZ5O4mq.png

Figure 4. A curvy, thin, asymmetrical knife

This flatter surface on the left also has the added benefit of helping your left hand guide the up/down movement of the knife in a more predictable fashion.
Of course, using this little trick introduces a new problem: Steer.

Rule 3 - Asymmetry in the grind will determine the amount of steer the user experiences

A blade pushing through foodstuff will turn if it's getting deflected more on one side compared to the other. This asymmetry we introduced in Figure 4 has increased the amount of deflection on the right side, and as such the blade will steer towards the left (of the user).
P7Z9Los.png

Figure 5. Different amounts of deflection on either side of the blade (green arrows) will cause the knife to steer off-center (red arrow)

We can counter-act this somewhat by sharpening the knife in such a way to make it want to steer in the other direction.
1roGHWs.png

Figure 6. Asymmetrical sharpening. Note the right bevel is both shorter and more acute to the blue line when compared to the left

If we zoom into the sharpened area you can see that the size and the angles of the bevels are uneven. This is done on purpose for two reasons: It helps steer the knife to the right, which in turn cancels out the existing steer to the left. And it also helps keep the sharp point closer in line to the middle of the blade (the blue line), which is very important in the case of clad/san-mai knives.
ML4LsWb.png

Figure 7. The differing size and angle of the bevels introduce deflection in the opposite direction of Figure 5, helping to cancel out any steering overall

So that's basically it! :)
Hopefully it's not too hard to understand and helps alleviate the confusion that some people have on the topic.
I should point out that while these general rules do apply to single bevel knives, the sharpening of them is done differently to what I've shown here. No correction is done to steering on a single bevel knife.
Let me know if I've missed something.
I'm sure I'm just echoing others by now, but thank you very much for the clear explanation!
 

DavidPF

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The thing I don’t get is the mark up for left handed grinds.
It may partly depend on human efficiency. When you're doing repetitive tasks involving manual dexterity, you get into a groove and you get faster and better at it. When someone says "Please create a backwards version of the same result", you slow down and lose your groove, and you don't have all the same muscle memory to count on. I have no idea how much of an influence on making left-handed knives this actually is. A custom-made knife is less likely to need a surcharge, because it was already custom anyway. But a matching left-handed version of something that's "semi-production" seems like it might really be more work. For knives that are 100% machine-produced, I don't know about factory tooling etc.
 

Kippington

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It may partly depend on human efficiency. When you're doing repetitive tasks involving manual dexterity, you get into a groove and you get faster and better at it. When someone says "Please create a backwards version of the same result", you slow down and lose your groove, and you don't have all the same muscle memory to count on. I have no idea how much of an influence on making left-handed knives this actually is. A custom-made knife is less likely to need a surcharge, because it was already custom anyway. But a matching left-handed version of something that's "semi-production" seems like it might really be more work. For knives that are 100% machine-produced, I don't know about factory tooling etc.
Sounds right to me! Well written btw, I had similar thoughts but never could put them into words.
 

Jeff

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A Basic Explanation of Asymmetry

Way back when I began sharpening I remember reading through many forum threads about asymmetry in double bevel knives and having no idea what was going on. All this talk about 70/30 and 80/20, "You should be doing this and shouldn't do that", but no real explanation for what was actually going on. So I ignored the whole thing and went on sharpening in my 50/50 way.

A few years (and many knives) later, I realized after spending hours grinding further back behind the edge, I had developed my understanding on kitchen knives to the point where I could not only understand, but explain it in simple-enough terms. It really boils down to 3 general rules which, when understood individually, combine to explain the whole thing.

So we'll start off with a diagram of a crappy knife, then introduce one rule at a time and improve it till we get something which represents a good knife (the rules are in random order and only numbered for ease of reading).

JZOu0Y0.png

Figure 1. A huge, fat, symmetrical knife

Rule 1 - Food releases off a curved surface better than it does off a flat surface

Most cuts we do in the kitchen are flat, and when a flat surface on the food comes into contact with a flat surface on the side of your knife, the large amount of surface area in contact between them increases drag and can be enough to hold them together. This is known as stickage, and it's a pain in the ass to put up with. We can minimize this nasty effect with the use of a curve instead of a flat on the side of the knife, because there is less surface area shared between a flat and a curve (the same effect can be had with a multi-faceted compound bevel) when compared to a flat-on-flat.
dvhsqEz.png

Figure 2. A curvy, fat, symmetrical knife

Rule 2 - The thinner the knife, the easier it moves through what it is cutting

This is pretty much self explanatory. Thicker knives have to push more out of the way on either side as they pass through food.

But here's a problem: The thinner you make a knife, the less curve you can fit on either side. This compromises Rule 1, and so we have to balance how thin the knife is with how much stickage we are willing to put up with.
The heat-treatment and kind of steel will determine how thin you can get your knife before its strength becomes compromised.
rjrwW3d.png

Figure 3. A curvy, thin, symmetrical knife

So here we are at a nice compromise of thickness vs. curvature. Cut food falls off on either side of the knife and it's not so wide as to get wedged in the food all too often. Perfect, right?
It turns out there's another way to improve the shape even further, and it has to do with how we use kitchen knives.
When cutting food, we generally hold the knife in our dominant hand and the foodstuff in the other. Lets say you're right handed and the food is held securely in your left. Do you really need the ability for food to drop off the left side of your knife if you're already holding it down? We may as well sacrifice some of the curve on that side and make the knife thinner overall. Like so:
JZ5O4mq.png

Figure 4. A curvy, thin, asymmetrical knife

This flatter surface on the left also has the added benefit of helping your left hand guide the up/down movement of the knife in a more predictable fashion.
Of course, using this little trick introduces a new problem: Steer.

Rule 3 - Asymmetry in the grind will determine the amount of steer the user experiences

A blade pushing through foodstuff will turn if it's getting deflected more on one side compared to the other. This asymmetry we introduced in Figure 4 has increased the amount of deflection on the right side, and as such the blade will steer towards the left (of the user).
P7Z9Los.png

Figure 5. Different amounts of deflection on either side of the blade (green arrows) will cause the knife to steer off-center (red arrow)

We can counter-act this somewhat by sharpening the knife in such a way to make it want to steer in the other direction.
1roGHWs.png

Figure 6. Asymmetrical sharpening. Note the right bevel is both shorter and more acute to the blue line when compared to the left

If we zoom into the sharpened area you can see that the size and the angles of the bevels are uneven. This is done on purpose for two reasons: It helps steer the knife to the right, which in turn cancels out the existing steer to the left. And it also helps keep the sharp point closer in line to the middle of the blade (the blue line), which is very important in the case of clad/san-mai knives.
ML4LsWb.png

Figure 7. The differing size and angle of the bevels introduce deflection in the opposite direction of Figure 5, helping to cancel out any steering overall

So that's basically it! :)
Hopefully it's not too hard to understand and helps alleviate the confusion that some people have on the topic.
I should point out that while these general rules do apply to single bevel knives, the sharpening of them is done differently to what I've shown here. No correction is done to steering on a single bevel knife.
Let me know if I've missed something.


AMAZING EXPLANATION !!!
 

Fuzzy

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A Basic Explanation of Asymmetry

Way back when I began sharpening I remember reading through many forum threads about asymmetry in double bevel knives and having no idea what was going on. All this talk about 70/30 and 80/20, "You should be doing this and shouldn't do that", but no real explanation for what was actually going on. So I ignored the whole thing and went on sharpening in my 50/50 way.

A few years (and many knives) later, I realized after spending hours grinding further back behind the edge, I had developed my understanding on kitchen knives to the point where I could not only understand, but explain it in simple-enough terms. It really boils down to 3 general rules which, when understood individually, combine to explain the whole thing.

So we'll start off with a diagram of a crappy knife, then introduce one rule at a time and improve it till we get something which represents a good knife (the rules are in random order and only numbered for ease of reading).

JZOu0Y0.png

Figure 1. A huge, fat, symmetrical knife

Rule 1 - Food releases off a curved surface better than it does off a flat surface

Most cuts we do in the kitchen are flat, and when a flat surface on the food comes into contact with a flat surface on the side of your knife, the large amount of surface area in contact between them increases drag and can be enough to hold them together. This is known as stickage, and it's a pain in the ass to put up with. We can minimize this nasty effect with the use of a curve instead of a flat on the side of the knife, because there is less surface area shared between a flat and a curve (the same effect can be had with a multi-faceted compound bevel) when compared to a flat-on-flat.
dvhsqEz.png

Figure 2. A curvy, fat, symmetrical knife

Rule 2 - The thinner the knife, the easier it moves through what it is cutting

This is pretty much self explanatory. Thicker knives have to push more out of the way on either side as they pass through food.

But here's a problem: The thinner you make a knife, the less curve you can fit on either side. This compromises Rule 1, and so we have to balance how thin the knife is with how much stickage we are willing to put up with.
The heat-treatment and kind of steel will determine how thin you can get your knife before its strength becomes compromised.
rjrwW3d.png

Figure 3. A curvy, thin, symmetrical knife

So here we are at a nice compromise of thickness vs. curvature. Cut food falls off on either side of the knife and it's not so wide as to get wedged in the food all too often. Perfect, right?
It turns out there's another way to improve the shape even further, and it has to do with how we use kitchen knives.
When cutting food, we generally hold the knife in our dominant hand and the foodstuff in the other. Lets say you're right handed and the food is held securely in your left. Do you really need the ability for food to drop off the left side of your knife if you're already holding it down? We may as well sacrifice some of the curve on that side and make the knife thinner overall. Like so:
JZ5O4mq.png

Figure 4. A curvy, thin, asymmetrical knife

This flatter surface on the left also has the added benefit of helping your left hand guide the up/down movement of the knife in a more predictable fashion.
Of course, using this little trick introduces a new problem: Steer.

Rule 3 - Asymmetry in the grind will determine the amount of steer the user experiences

A blade pushing through foodstuff will turn if it's getting deflected more on one side compared to the other. This asymmetry we introduced in Figure 4 has increased the amount of deflection on the right side, and as such the blade will steer towards the left (of the user).
P7Z9Los.png

Figure 5. Different amounts of deflection on either side of the blade (green arrows) will cause the knife to steer off-center (red arrow)

We can counter-act this somewhat by sharpening the knife in such a way to make it want to steer in the other direction.
1roGHWs.png

Figure 6. Asymmetrical sharpening. Note the right bevel is both shorter and more acute to the blue line when compared to the left

If we zoom into the sharpened area you can see that the size and the angles of the bevels are uneven. This is done on purpose for two reasons: It helps steer the knife to the right, which in turn cancels out the existing steer to the left. And it also helps keep the sharp point closer in line to the middle of the blade (the blue line), which is very important in the case of clad/san-mai knives.
ML4LsWb.png

Figure 7. The differing size and angle of the bevels introduce deflection in the opposite direction of Figure 5, helping to cancel out any steering overall

So that's basically it! :)
Hopefully it's not too hard to understand and helps alleviate the confusion that some people have on the topic.
I should point out that while these general rules do apply to single bevel knives, the sharpening of them is done differently to what I've shown here. No correction is done to steering on a single bevel knife.
Let me know if I've missed something.
Thank you, very helpful. Had the idea now have the pictures. Thumbs UP
 

boblob

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A Basic Explanation of Asymmetry

Way back when I began sharpening I remember reading through many forum threads about asymmetry in double bevel knives and having no idea what was going on. All this talk about 70/30 and 80/20, "You should be doing this and shouldn't do that", but no real explanation for what was actually going on. So I ignored the whole thing and went on sharpening in my 50/50 way.

A few years (and many knives) later, I realized after spending hours grinding further back behind the edge, I had developed my understanding on kitchen knives to the point where I could not only understand, but explain it in simple-enough terms. It really boils down to 3 general rules which, when understood individually, combine to explain the whole thing.

So we'll start off with a diagram of a crappy knife, then introduce one rule at a time and improve it till we get something which represents a good knife (the rules are in random order and only numbered for ease of reading).

JZOu0Y0.png

Figure 1. A huge, fat, symmetrical knife

Rule 1 - Food releases off a curved surface better than it does off a flat surface

Most cuts we do in the kitchen are flat, and when a flat surface on the food comes into contact with a flat surface on the side of your knife, the large amount of surface area in contact between them increases drag and can be enough to hold them together. This is known as stickage, and it's a pain in the ass to put up with. We can minimize this nasty effect with the use of a curve instead of a flat on the side of the knife, because there is less surface area shared between a flat and a curve (the same effect can be had with a multi-faceted compound bevel) when compared to a flat-on-flat.
dvhsqEz.png

Figure 2. A curvy, fat, symmetrical knife

Rule 2 - The thinner the knife, the easier it moves through what it is cutting

This is pretty much self explanatory. Thicker knives have to push more out of the way on either side as they pass through food.

But here's a problem: The thinner you make a knife, the less curve you can fit on either side. This compromises Rule 1, and so we have to balance how thin the knife is with how much stickage we are willing to put up with.
The heat-treatment and kind of steel will determine how thin you can get your knife before its strength becomes compromised.
rjrwW3d.png

Figure 3. A curvy, thin, symmetrical knife

So here we are at a nice compromise of thickness vs. curvature. Cut food falls off on either side of the knife and it's not so wide as to get wedged in the food all too often. Perfect, right?
It turns out there's another way to improve the shape even further, and it has to do with how we use kitchen knives.
When cutting food, we generally hold the knife in our dominant hand and the foodstuff in the other. Lets say you're right handed and the food is held securely in your left. Do you really need the ability for food to drop off the left side of your knife if you're already holding it down? We may as well sacrifice some of the curve on that side and make the knife thinner overall. Like so:
JZ5O4mq.png

Figure 4. A curvy, thin, asymmetrical knife

This flatter surface on the left also has the added benefit of helping your left hand guide the up/down movement of the knife in a more predictable fashion.
Of course, using this little trick introduces a new problem: Steer.

Rule 3 - Asymmetry in the grind will determine the amount of steer the user experiences

A blade pushing through foodstuff will turn if it's getting deflected more on one side compared to the other. This asymmetry we introduced in Figure 4 has increased the amount of deflection on the right side, and as such the blade will steer towards the left (of the user).
P7Z9Los.png

Figure 5. Different amounts of deflection on either side of the blade (green arrows) will cause the knife to steer off-center (red arrow)

We can counter-act this somewhat by sharpening the knife in such a way to make it want to steer in the other direction.
1roGHWs.png

Figure 6. Asymmetrical sharpening. Note the right bevel is both shorter and more acute to the blue line when compared to the left

If we zoom into the sharpened area you can see that the size and the angles of the bevels are uneven. This is done on purpose for two reasons: It helps steer the knife to the right, which in turn cancels out the existing steer to the left. And it also helps keep the sharp point closer in line to the middle of the blade (the blue line), which is very important in the case of clad/san-mai knives.
ML4LsWb.png

Figure 7. The differing size and angle of the bevels introduce deflection in the opposite direction of Figure 5, helping to cancel out any steering overall

So that's basically it! :)
Hopefully it's not too hard to understand and helps alleviate the confusion that some people have on the topic.
I should point out that while these general rules do apply to single bevel knives, the sharpening of them is done differently to what I've shown here. No correction is done to steering on a single bevel knife.
Let me know if I've missed something.
@Kippington if i have a 50/50 grind westersn style knife is it possible to create this edge and convexity of the last photo ? figure 7
if so how
 

Kippington

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Yes it is possible, but you would have to remove a lot of steel.

It would be similar to the picture I posted earlier in this thread demonstrating how much steel you'd have to remove on a single bevel to get the same effect, although there would be less to remove on a 50/50 flat grind Western:

dM05F3Z.png
 

boblob

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Yes it is possible, but you would have to remove a lot of steel.

It would be similar to the picture I posted earlier in this thread demonstrating how much steel you'd have to remove on a single bevel to get the same effect, although there would be less to remove on a 50/50 flat grind Western:

dM05F3Z.png
thank you for the response i dont quite understand the illustration
 

Benuser

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It may partly depend on human efficiency. When you're doing repetitive tasks involving manual dexterity, you get into a groove and you get faster and better at it. When someone says "Please create a backwards version of the same result", you slow down and lose your groove, and you don't have all the same muscle memory to count on. I have no idea how much of an influence on making left-handed knives this actually is. A custom-made knife is less likely to need a surcharge, because it was already custom anyway. But a matching left-handed version of something that's "semi-production" seems like it might really be more work. For knives that are 100% machine-produced, I don't know about factory tooling etc.
I found an interesting explanation by Mr Koki Iwahara about the 50% premium on single bevels: one has to organise an entire batch, and the local demand is very, very low.
 
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