The razor-design aspect that is usually referred to as the "span" is the blade-bar span, or, perhaps more precisely, the guard span. I'm just going to call it the "span."
Although in most DE razors with a safety bar there is some kind of visible open space between the blade edge and the safety bar, this open space has little to do with a razor's shaving character. This is not the blade-bar span that influences the shaving behavior of the razor. I will refer to this relatively-easily-measured opening the blade-bar gap, the blade-guard gap, or simply the gap.
Like the blade exposure, which is a measurement between a hard point (the blade edge) and an imaginary point (the shave plane), the blade span is also a measurement between a hard point (the blade edge) and an imaginary point (the imaginary line along the safety bar where it touches the shave plane; and I will refer to this line as the ISPDL -- the Imaginary-Shave-Plane-Determining Line).
This is a very important point.
The gap, which is between two hard points on the razor, may be similar to a given razor's span, but is almost always different. The gap measurement isn't useful to compare one razor to another either because the degree of variation between gap and span measurement on a given razor may be large, will also vary greatly from one razor design to another.
So to be somewhat redundant: I can use some kind of instrument to measure the blade-bar gap, and I may be able to do that very accurately right on the actual razor. However, despite the accuracy of this measurement, it isn't necessarily a measurement of the blade-guard span. As a result, it (the gap) likely will not explain the shaving behavior of the instrument.
Take the Lord L.6 razor head for example. This is the razor that months ago caused the light bulb to go on over my head, when I recognized the difference between blade-guard span and gap. Because of the cross-section design of its safety bar, there is a large difference between the two measurements. The cross section of the L.6 is such that it slopes inward from the ISPDL down toward the actual hard point that defines the blade-guard gap.
Because the blade-guard span has only one physical hard point against which to measure (that is, the blade edge) and its other measuring limit is an imaginary line along the highest point of the safety-bar curve (that is, the ISPDL, the line along the safety bar that first touches the face when the razor is in its optimal shaving position against the plane of the cheek), this cannot be accurately done on the razor. The span is perhaps best measured on an enlarged side view of the razor with the shave plane drawn in. Its distance is from the razor edge to the ISPDL.
For emphasis:
The span is always measured along the shave plane (pretty much -- depending on blade exposure). This cannot be done accurately on the razor itself with any measuring device. At best, on-razor measurements of the span are only approximations, which may vary greatly in validity and reliability.
The ultimate conclusion is that, given a better understanding of the difference between blade-bar gap and span, gap measurements are not terribly helpful to understand the absolute or relative performance of a razor; it is the span that is much more significant, though much more difficult to obtain.
The ultimate conclusion is that, given a better understanding of the difference between blade-bar gap and span, gap measurements are not terribly helpful to understand the absolute or relative performance of a razor; it is the span that is much more significant, though much more difficult to obtain.
Happy shaving -- and measuring!
Where did you get these definitions?
ReplyDeleteFrom my two sources (see below, one is from Gillette), what you are calling Blade Gap is usually called "Span" or "Guard Span". I believe what you are calling "Blade Void" is the "Blade Gap" ... unless I'm missing something?
http://wiki.badgerandblade.com/Image:SHAVE_ANGLE4_jpg
http://badgerandblade.com/vb/attachment.php?attachmentid=467002&d=1404065507
Also, I don't recall ever reading the term "Blade Void" before your article ... and a quick Google search seemed to yield no results for it....
Yep, derived the term this morning as I wrote the article. Anyway, a rose by any other name...
ReplyDeleteTerminology aside, the concept is the important thing: span(Gillette)/gap(Grandad) is a significant factor, gap(Gillette)/void(Grandad) is not when it comes to understanding the shave character of a razor head, IMO.
Thanks for the information on terminology.
Guard Span is usually not measured ... I've never seen a list of guard span measures ... probably because like Blade Exposure, Guard Span is also harder to precisely measure. Blade Span is rarely discussed. And ... for razors with positive Blade Exposure the Guard Span measure would not come into play with standard shallow angle shaving.
ReplyDeleteRegardless, could you possibly correct the terminology in your article above? Helping to prevent the obscuring of terminology is a significant goal of mine as a hobbyist consumer advocate on razors....
CORRECTION: "Blade Span" above should be "Guard Span"
DeleteReasonable request. Done.
DeleteThank you!
Delete