A Basic Explanation of Asymmetry

Discussion in 'Kitchen Knife Knowledge' started by Kippington, Aug 26, 2017.

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  1. Aug 26, 2017 #1

    Kippington

    Kippington

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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).

    [​IMG]
    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.
    [​IMG]
    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.
    [​IMG]
    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:
    [​IMG]
    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).
    [​IMG]
    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.
    [​IMG]
    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.
    [​IMG]
    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. Aug 27, 2017 #2

    HRC_64

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    Excellent post OP

    Bumping this thread so more people see it.
     
  3. Aug 27, 2017 #3

    milkbaby

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    Great cartoons, a picture is worth a thousand words! :doublethumbsup:
     
  4. Aug 27, 2017 #4

    James

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    This should be stickied
     
  5. Aug 27, 2017 #5

    txtrqdrt

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    Agreed - this is a very helpful post. Thank you Kippington :thumbsup:
     
  6. Aug 27, 2017 #6

    Benuser

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    Excellent!
     
  7. Aug 27, 2017 #7

    cyberbaton

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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. Aug 27, 2017 #8

    Benuser

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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. Aug 27, 2017 #9

    Mute-on

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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. Aug 27, 2017 #10

    GeneH

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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.
     
  11. Aug 27, 2017 #11

    Anton

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    thanks for sharing and the time put into this
     
  12. Aug 27, 2017 #12

    Benuser

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    Please be aware that good Western knives aren't either strictly symmetric. Often the left side is flatter, and the right one more convexed. See good Sabs or Herders. But off-centering the edge is indeed a change the Japanese have introduced.
     
  13. Aug 27, 2017 #13

    StonedEdge

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    Just wanna says thanks to the OP for this, I think one of the Daves had a thread on assymetry which I can't for the life of me find again. The illustrations are spot on
     
  14. Aug 27, 2017 #14
  15. Aug 27, 2017 #15

    StonedEdge

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    Benuser, that's the one! Thanks to you as well!
     
  16. Aug 27, 2017 #16

    Benuser

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    My pleasure!
     
  17. Aug 28, 2017 #17

    Kippington

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    Thanks everyone, it feels good to give a little back to the crew that taught me so much. :doublethumbsup:

    Gene, you're closer to this than you think!

    The important thing: The overall effect that each bevel and curve has on the performance of the knife.
    It's up to you if you want to change it or keep it the same.
     
  18. Aug 29, 2017 #18

    pd7077

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    Perfect explanation! Thanks for taking the time to write this up.
     
  19. Sep 1, 2017 #19

    Drosophil

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    This is so well explained. Thank you!
     
  20. Sep 1, 2017 #20

    Mute-on

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    On a double bevel knife, as illustrated here, the angles are more important. The amount of time is a byproduct of however long it takes to get your angles right :thumbsup:
     
  21. Dec 5, 2017 #21

    jbeng77

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    This is good stuff. I fear that implementing this is going to be easier said than done. I'll work on it.
     
  22. Dec 5, 2017 #22

    StonedEdge

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    It sounds daunting but in practice it's pretty straight forward...that said it's good to know that some j-knives don't have assymetrical bevels
     
  23. Dec 5, 2017 #23

    Boynutman

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    So I have a question related to this. When applying a small micro bevel (a hair's width, lets say 0.1-0.2mm), would this count as a significant asymetry?
    I think not... doesnt feel like it... but what are your thoughts?
    I am a righty, apply my micro bevels on the rh side out of habit, but ideally these should perhaps be on the lh side. Or am I exagerating here?

    To be complete, I am talking about double bevelled gyutos here.

    Thanks!
     
  24. Dec 5, 2017 #24

    StonedEdge

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    No..a micro bevel doesn't really factor into a knife's assymetry as it is at the very edge of the edge. Assymetry is mostly dealt with above the edge. Think of it as part of the primary bevel while the micro bevel is the very end part of the secondary bevel. Make sense?
    Primary bevel: shoulders of the knife
    Secondary: the taper leading to the actual cutting edge
     
  25. Dec 5, 2017 #25

    Kippington

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    Actually, this is a damn good question which touches on something I forgot to cover in my initial post.
    The short answer is no, it doesn't have much of an affect. But if it does, you'll feel the knife steer off center as you cut through certain foods.

    For a more detailed explanation:
    Lets imagine you buy a brand new gyuto and take it out of the box. Assuming the makers have done their job correctly, the knife should not steer due to asymmetry... and importantly it should be thin behind the edge.

    [​IMG]

    When a knife is thin behind the edge, any bevel you set through normal sharpening will be so small as to have a tiny effect on the overall steering of the knife.
    This is very important when you consider: If the maker needed to correct a problem with steer using only the tiny bevel available to them along the cutting-edge, they may have had to put an extreme angle on it to get the desired counter-steer effect.

    As the end user, you might go along happily using the knife and sharpening it every so often. Lets say you don't do any thinning to the areas behind the edge - The edge bevel gains a slight increase in size each time you sharpen the knife, and as a result that initially small bevel grows larger and begins to have more and more of an affect on the performance of the knife. Some people don't see this as a problem and allow it to spiral out of control.

    [​IMG]

    What happens if you stick to the extreme asymmetric sharpening angle that was on the knife out of the box? As you can imagine, the magnified counter-steer effect of the larger bevel grows disproportionately to the asymmetry behind the edge, resulting in an over-correction to the steer.
    The best way to deal with this is to THIN YOUR KNIFE! Fat edges are bad, mmmkay? :spankarse:
    But if this is too advanced for you to attempt (or maybe you don't have the time or equipment), you'll need to make up for it by adjusting the asymmetry of the edge bevels. You can do this if you feel the knife starting to steer off-center during normal use by slightly changing the sharpening angles the next time you take the knife to the stones. Just be aware that if you get the knife thinned later on, you may have to revert back to the original sharpening asymmetry.
     
    DainBramage1 likes this.
  26. Dec 5, 2017 #26

    HRC_64

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    mods really need to sticky this thread !
     
  27. Dec 6, 2017 #27

    Bolek

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    In practice how sharpen a 50/50 knife to a asymmetry of fig 7 ?
     
  28. Dec 6, 2017 #28

    Boynutman

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    @kippington and Stonededge, thanks! That makes a lot of sense, never really considered the thickness behind the edge issue and how growing bevels may magnify steering.
     
  29. Dec 6, 2017 #29

    Kippington

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    No problem, glad to help.

    It would be cool if they could do that! :biggrin:

    When you sharpen the left side, lift the spine a little higher off the stone. Have the spine closer to the stone on the right side, and use slightly fewer strokes.
     
  30. Dec 7, 2017 #30

    Bolek

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    Thanks. Will try.
     

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