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Thread: A Basic Explanation of Asymmetry

  1. #1
    Kippington's Avatar
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    A Basic Explanation of Asymmetry

    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).


    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.

    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.

    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:

    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).

    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.

    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.

    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.


  2. #2
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    Excellent post OP

    Bumping this thread so more people see it.


  3. #3
    Senior Member milkbaby's Avatar
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    Great cartoons, a picture is worth a thousand words!

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    This should be stickied

  5. #5

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    Agreed - this is a very helpful post. Thank you Kippington

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    Senior Member Benuser's Avatar
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    Excellent!

  7. #7
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    Great job, thank you. Someone say that you should spend some time on right side and less amount of time on left side. Is it correct approach, or only angles are important?

  8. #8
    Senior Member Benuser's Avatar
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    I generally have the right side as convexed as I can, and I even keep the shoulder on the left one on purpose in some extreme cases. But steering is a very individual matter. Get used to it, loosen your grip and little by little you will compensate for it and need less correction of the geometry.
    A good trick to see how the blade itself behaves is by making a cut without any pressure, only with the blade's weight in the inverse sense of your normal cutting. If you normally push, try this pulling and vice versa. See under which angle cuts the best.

  9. #9
    Senior Member Mute-on's Avatar
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    I didn't even have to read any of the words to understand this. A perfect explantation of what took me ages to conceptualise earlier in my knife sharpening journey.

    Many thanks!

  10. #10
    Senior Member GeneH's Avatar
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    Awesome diagrams and explanation. That really makes steering issues with asymmetrical convex shapes easier to understand. And yep, not going there with any of my blades. I don't see intentional asymmetry in my future.


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