How to convert knife edge angle "ratios" into degrees?
European-style Japanese knives seem to be described (also here and here, and in my most recent post) in terms of "ratios" that always add up to 100. It's not exactly clear what this means in terms of angles off the center plane of the blade.
It appears that, even on knife forums, there is some confusion as to what these numbers mean. My first thought was that they were actually angles, since a 30 degree angle is common on European knives. Upon reflection, though, that can't be--when you're sharpening a 70/30 knife, dragging across a stone at a 70 degree would be a catastrophe. What, then, is that magic number, and how does it vary in relation to these "ratios"?
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I found this on chefknivestogo and I think it explains it quite well.
RAY <> A "50/50" usually references an edge. So on the cutting edge, it is an even 50/50 "V". It can be 50/50 at 12 degrees or 50/50 @20 , but each sides angle is equivocal.
A double bevel is a knife design created by grinding. So from the spine to the cutting edge, there is a blade face that has been ground (most of the time). It can be a flat grind, a convex grind, a hollow grind, and any of these grinds can be symmetric (50/50) or asymmetric. So for instance, my Ginsanko Hiromoto Western Deba has an asymmetrical semi convex grind. It has a flat ground left blade face at around 30% of the total included angle and the right side blade face has a distinctly convex grind that is the other 70% of the total included angle, but the actual cutting edge has an asymmetry, as well. The actual cutting edge does not look like a "V" as it is, in fact, a 60/40 right-handed bias. It's a particularly unique blade, but exemplifies your point, quite well. :mrgreen:
In short, the numbers like 50/50 or 70/30 represent the percentage of the included angle on each side of the blade. So, for a 50/50 knife with an included angle of 50°, it would be ground to a 25° edge angle on each side of the blade. See diagram below:
You can't determine edge angle just from bevel ratio
You need to know what the height and width of the bevel is.
Here's a simple diagram that explains why:
Clearly, the edge angles for deep bevels are much smaller than the angles for shallow bevels, even at the same bevel ratios.
Once you have the bevel dimensions (w and h in the diagram below), you can calculate the edge angle using simple geometry:
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