Originally Posted by DWFII
Well, how do you determine whether your treadline width is identical with the foot? Or the heel seat width. Or forepart volume in relation to toe spread? If you don't take a footprint? Even if you use the 30° pencil technique?
Doesn't the footprint being a near photographic representation of the plantar surface of the foot make any sense?
Doesn't my observation about firm feet and flabby feet make any sense?
What good does it do to make the insole wider or narrower than the foot will use?
In the absence of a pedograph, the 30° technique is adequate, I suppose. I used it for years. But, ultimately, IMO, it's a guess. Perhaps that's good enough, I don't know but I have never been comfortable with "good enough."
Well, first of all, I never wrote that I don't use a pedograph....
However, if one makes a fitter, then one can compare the width of the insole, on the actual foot, at the joints, and then use that as guidance for modifying the last. In my personal case, I've made a last with the insole width (feather line to feather line, across the joints) corresponding to my pedograph; that has, in fact, turned out to be wider than I actually need, when my foot is in the shoe. Is that because the pedograph was flawed? It it because a pedograph is taken on a flat surface, but the bottom of the last is ovoid, and hence will be greater due to the radius of the curve being longer than the cord connecting those two points? I don't know exactly why, but I do know that the pedograph did not yield a perfect insole width for me. Moreover, we both know, the foot changes shape and dimension through the gait cycle; so, should one do a pedograph in a stationary manner, or having the client impact the pedograph while striding? In other words, I think a pedograph is helpful, but I don't find it to be absolutely conclusive or flawless.
Also, I recall you writing that you don't necessarily make the heel seat the same width as the heel print. So, if that is the case, aren't you too relying, in that context, on 'good enough'?
Much as I'd like to understand what you're saying, I am having trouble. I don't know whether I'm addressing anything you might consider relevant or not but in addition to the idea of a treadline, I have this notion of a tread area. Roughly analogous to the roll of tissue that surrounds the ball of the foot. The actual metatarsalphalangeal joint is in the middle of that roll of tissue. I see something very similar on the the lasts I use and every last I have ever inspected. Given that anatomical reality, however, I don't see how or why the last should be shaped such that it goes "up" from the treadline. I guess I'd have to see it in person...I'm from Missouri.
I would agree that the treadline is really a tread area, in that the foot isn't resting on the ground at only one exact location (when looked at from the side), but rather an area with width, since the flesh compresses under the body's weight. If you look at the earlier drawing, Pic 3, the treadline is perhaps 1/2" wide (front to back), and the treadline is reasonably flat or slightly convex when viewed from the side.
In terms of the last going 'up': for a shoe with a heel, the insole is higher at the heel than it is at the joint. So, the last bottom has to go 'up,' relative to the ground and the joint, in order to reach the heel height. Furthermore, the shape of the joint (and hence the tread line, per above), in profile from the side, is more or less oval in shape (perhaps a squashed oval right at the joint/across the treadline). So, if it is ovoid, and the ground describes the line of tangency (as is does, by definition, for the joint/treadline of a last), then the rest of the oval has to move away from that line of tangency, and hence, in my vernacular, 'up.' The rest of the last bottom is off the ground, hence is 'up' relative to the location of the joint.
Of course, the last bottom could do so in a constant manner (i.e. a straight line, angled from the joint up to the heel), but that is not how the foot is. If you look at the last profile (Pic 1) in my earlier post, the last goes 'up' relative to the ground on either side of the joint. Also, at some point, the profile of the last between the joint and the heel has to go from convex to concave in order to accommodate both the profile of the joint and the fact that the heel seat is more or less parallel to the ground. So, the bottom of the last has to have, at some point, a change in shape from convex to concave, thus a point of inflection (i.e. the second derivative of the curve crosses zero). That is how the foot is, and it is unavoidable in a last that more or less mirrors the foot. So, you have to have a location where that inflection occurs, it is unavoidable. This is true for factory lasts as well as hand made lasts.