Sandal

Nov. 13, 1962

P. BLAVIER

SANDAL

3,063,166

Filed June 14, 1960

Sheets-Sheet 1

<img file="US3063166A_D0001.tif" />

FIG. I

<img file="US3063166A_D0002.tif" />

Nov. 13, 1962

P. BLAVIER

SANDAL

3,063,166

Sheets-Sheet 2

Filed June 14, 1960

<img file="US3063166A_D0003.tif" />

FIG. 3

<img file="US3063166A_D0004.tif" />

FIG. 4

INVENTOR.

Paul Blavibr

BY

A&lt;y£A/7United States Patent Office _Paienled resistance of the sole urges the instep strap downward against the instep. To provide reasonable foot comfort, the instep strap must be so loose that it does not severely bind the foot during flexing. A loose instep strap, however, is not capable of securing the sandal to the foot in the normal position in which the foot fully rests on the sole. If use of a rubber strap or the like is to be avoided, a compromise must be reached which is neither comfortable enough nor safe enough as far as fastening the sandal to the foot is concerned.

By merely altering the cross sectional shape of the instep strap and its relationship to the sole, I have overcome the necessity for such an undesirable compromise. An instep strap which completely circles the foot substantially in an O-shape gives the necessary freedom of movement without binding, and still holds the sandal to the foot when the latter is fully placed on the sole.

For theoretically best results, the length of a line drawn along the instep strap from one end thereof where it is 20 fastened to the sole around the contour of the foot to the other strap end fastened to the sole should change as little as possible when the foot is raised from a position flat on the sole to a flexed position. It can be readily shown by simple geometrical relationships that this length difference becomes a minimum if the free length of the instep strap becomes a maximum, that is, when one end of the strap is secured to the left lateral edge portion of the sole, passes between sole and foot toward the right, then upwardly arching over the instep and down towards the left edge portion of the sole, and again under the foot and over the sole to the right lateral edge portion of the sole to be fastened there.

In practical application of the principle of this invention, I prefer to fasten both ends of the instep strap in a single fastening approximately in the longitudinal median plane of the sole so that the strap forms a closed loop of approximate O-shape about the instep portion of the foot. Because of the normal forwardly downward slope of the dorsum of the foot, the instep strap, unless prevented from following the stresses exerted by the foot, will align itself at a slight forward slant so that the plane of the O-shape is approximately perpendicular to the slope of the dorsum.

Even with the loop of the instep strap limited to substantially a full circle, a decided improvement in the freedom of movement of the foot is achieved together with a very much firmer grip of the foot on the sandal. The natural resiliency of the foot is sufficient to permit the small amount of tightening of the strap which occurs as the foot is raised from the sole. The strap hugs a large 50 portion of the circumference of the foot in all operative positions of the sandal.

The advantages of a loop which is slightly shorter than the theoretical optimum are two-fold. Even the arch under the instep of a well-built foot provides only a very 55 limited amount of space in which to house the portions of the instep strap of the invention which extend inwardly beyond the customary points of anchorage. A single strand of strap under the outer edge of the foot is tolerated without discomfort. Passing two strands of strap 60 under the edges of the foot, and particularly under the outer edge, requires modification of the sole to minimize chafing. Fastening both ends of the strap at the same point of the sole is not only esthetically desirable, but also advantageous from the point of view of simple manu65 factoring operations.

Considerations of a technical or economic nature may, however, militate against a single fastening, and may outweigh the afore-described advantages of such an arrangement. When employing rope soles, for example, it is 70 more convenient to fasten the ends of the instep strap to two adjacent wales of rope rather than to a single wale in order not to weaken the sole unduly. Some of the basic

3,063,166 SANDAL

Paul Blavier, Brussels, Belgium, assignor to Sigmund Spiegel, Auburn, N.Y.

Filed June 14, 1960, Ser. No. 35,946 5

Claims. (Cl. 36—11.5)

The present invention relates to footwear, and more particularly to a novel sandal.

For the purposes of this application, a sandal will be io understood to be a piece of footwear consisting essentially of a sole and of one or several elongated straps of material for attaching the sole to the foot of a wearer.

Sandals are worn when it is desired to expose a large area of the foot to the atmosphere, yet to protect the foot 15 from contact with a traffic surface. It is often preferred that a sandal have as few straps as possible. Yet, sandals to be comfortable and to cling to the foot are generally found to require three U-shaped straps or their structural equivalents namely a vamp band connecting the two sides of the sole at the front of the foot and passing across the toes, an instep strap bi-laterally secured to the middle of the sole and somewhat longer than the vamp band, and a back strap or ankle strap usually fastened to the instep strap and forming a more or less horizontally extending 25 loop about the Achilles tendon of the foot. The instep strap and the vamp band may be connected by a linking strap, and the functions of the instep strap, the linking strap, and the vamp band may be combined in a pair of cross bands each of which has one end secured to one 30 side of the sole in the middle of the foot, and another end secured to the other side of the sole laterally of the toes. The two bands cross each other over the forward portion of-the instep.

Without a back strap or other uppers element retaining 35 the heel, a conventional sandal of the type described will not firmly hold to the foot of a wearer when he walks. A sandal equipped only with vamp band and instep strap is easy to slip into, but it tends to slide from the foot. This tendency can only partly be overcome by making the straps 40 of resiliently stretchable material such as rubber, but rubber and the like is neither a desirable material for a piece of light footwear, nor a durable one. Buckles and other means for tightening straps or bands on a sandal are neither effective nor comfortable.

It is a primary object of this invention to provide a sandal which does not require a back strap nor its functional equivalent, and is therefore readily put on and taken off, and which still firmly holds to the foot and will not unintentionally be slipped off during normal walking.

Another object is the provision of a sandal which is comfortable to wear without unduly binding the foot.

A further object is the provision of a sandal that will fit a relatively wide range of foot sizes.

I have found that the usual instep strap and its cooperation with the sole of the sandal are responsible for many of the afore-described shortcomings of the conventional sandals lacking a back strap, and analysis of the relevant geometrical relationships has shown the way to an improved instep strap arrangement. The conventional instep strap is secured to a lateral edge portion of the sole, intermediate the toe and the heel portion, rises substantially straight upward along the side of the foot, arches over the instep, and descends on the other side of the foot substantially straight down to the other lateral edge portion of the sole. When viewed in section transversely of the foot, it has an inverted U shape.

The instep strap cooperates with the sole which is relatively stiff, though resilient. When during walking the foot is flexed in the metatarsal joints, the sole does not readily follow the movement, and the foot is lifted from the sole in the region of the instep strap. The resilient

3,063,166 advantages of this invention are achieved as long as both, or at least one of the ends of the instep strap are secured to the sole at a point inward from the corresponding lateral edge portion by at least one-third of the width of the foot and the corresponding width of the sole.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 diagrammatically illustrates geometrical relationships relevant to this invention;

FIG. 2 shows a woman’s sandal according to a preferred embodiment of the invention in a perspective view;

FIG. 3 illustrates a plan view of a man’s sandal of the invention in the position assumed when the wearer of the sandal stands still; and

FIG. 4 shows the sandal of FIG. 3 in the position assumed when the foot of the wearer is flexed in the metatarsal joints.

Referring now to the drawings and initially to FIG. 1, the triangular figures shown represent geometrical relationships in the plane of the loop formed by an instep strap of the invention. The strap is fastened to the sole at the point B and extends therefrom to a point of contact C at the lateral edge of the foot. The point A is the vertical projection of the point C on the top face of the sole.

The point A defines the laterally outer boundary of the contact face of the sole, that is, of that portion of the top face of the sole which makes contact with the plantar face of the foot if the latter is placed flat against the sole. The edge portion of the sole extends outwardly from point A, that is, in a direction away from point B.

The sides of the triangle ABC are indicated by lower case characters a, b, c respectively correlated to the opposite corners of the triangle.

When the foot is flexed and its plantar face moves away from the contact face c of the sole, the contact point between the instep strap and the foot moves from C to C'. The triangle ABC' is bounded by the sides ο-|-Δα, Ζ&gt;+ΔΖ», and c. The following relationships then hold:

(1) c²=c²-[-Z&gt;² and by differentiation du_— ^{1 }} db~-J&lt;?+W·

Since a is the length of the instep strap between its fastening at B on the sole surface and point C on the foot, and b is the vertical distance of the foot from the sole, it follows from Equation 2 that the rate of increase da/db of the strap length with increasing distance between foot and sole is at a maximum when c=0; and that da/db approaches a minimum of zero when c approaches infinity.

Equation 2 thus indicates that for a given movement of the foot away from the contact face of the sole, the required elongation of the instep strap (or conversely the pressure exerted by the strap on the foot) is smaller the further the fastening point of the strap is displaced inwardly of the edge portion of the sole. The fastening point of the conventional instep strap is at or very near point A, and thus requires maximum elongation of the strap when the foot is flexed, or conversely, it exerts maximum binding force on the instep. By locating the fastening point inwardly of the edge portion, I succeed in reducing the elongation required of the strap at constant pressure against the foot, or in reducing the pressure against the foot assuming a strap of invariable length.

Under actual conditions, the strap has some measure of elasticity when it is made of leather or synthetic plastic material as is usual, and the foot will tolerate and absorb a moderate increase in strap pressure without noticeable discomfort. The margin of comfort is much wider with the instep strap of my invention than with the conventional strap arrangement because the strap elongation required for a given flexing of the foot is not only smaller in absolute size, but it is also distributed over a strap of greater length so that the percentage elongation in a strap of my invention under the conditions schematically illustrated in FIG. 1 is smaller by an order of magnitude than in the conventional strap. The pressure of the strap also is distributed over a larger surface of the foot because of the greater length of strap which hugs the foot.

FIGS. 2 to 4 illustrate practical applications of my invention to otherwise conventional sandals. FIG. 2 shows a so-called T-strap sandal for women in perspective view. It has a sole 1, and a heel 2 attached under one longitudinal end portion of the sole 1. A vamp band 3 is fastened to the two opposite lateral edge portions of the toe portion of the sole 1. An instep strap 4 which forms a closed loop about the instep of the wearer’s foot is pivotally mounted on the sole 1 by insertion of its two free ends in a slit 5 of the sole 1 in a manner known in itself and usual in shoemaking. The two halves of the instep strap 4 extend from their respective attached ends outward toward an edge of the sole, then upwardly away from the sole, and finally inward again at a distance from the sole. They are integrally connected by a common center portion. The vamp band 3 and the instep strap 4 are connected by a linking strip 6 which is fixedly attached to the vamp band 3 and is slidably mounted on the center portion of the instep strap 4.

FIGS. 3 and 4 show a cross-band sandal of the invention of a general type commonly worn by men, but also capable of being worn by women with or without minor modifications. The sole 1 of the embodiment of the invention illustrated in FIGS. 3 and 4 is not equipped with a heel.

The functions of the instep strap and of the vamp band are combined in two cross bands 7 and 8, each of which has one end fastened in a slit 5 of the sole 1 located in the median plane of the sole just forward of the heel portion. The cross bands extend laterally outward from the slit 5, and then arch upwardly and forwardly across the instep and the metatarsal region of the foot to a second fastening point at the opposite edge portion of the sole near or in the toe portion of the sole 1.

The mode of action of the sandals illustrated is evident from what has been explained above with reference to FIG. 1. In the rest position of the foot when its plantar surface is substantially flat against the top contact face of the sole 1, the attached ends of the strap of the invention and the adjacent outwardly extending longitudinal portions lie substantially flat against the contact face of the sole on the outer side of the foot, and follow the contour of the foot in the arch region of the plantar face on the inner side, still being very close to the contact face of the sole.

When the foot is flexed as in walking, the instep strap portions at and near the attached ends pivotally move about the slit 5 until they assume the positions illustrated in FIG. 4 in which the strap extends from a point in the contact face of the sole at the slit 5 slantingly upward and outward to a point of contact with the side of the foot. This change of configuration of the loop formed by the instep strap involves but a minimum of stretching of the strap, or of corresponding compression of the foot.

It is, therefore, possible to design a sandal of my invention in such a manner that the instep strap fits rather closely about the instep in the rest position of the foot without causing uncomfortable binding in the flexed position. The sandal is thus held securely to the foot in all positions, and cannot be slipped off unintentionally. Still, the absence of a back strap makes it easy to put the sandal on or to take it off if this is desired.

Because of the general wedge shape of the human foot

3,063,166 and the corresponding dimensions of the various straps which hold the foot in the sandals of this invention, the sandals fit a relatively wide variation of foot sizes. To accommodate feet of different wedge angle, I prefer to insert in the strap a short length of elastic material 9 such as a textile band partly woven of rubber threads. For esthetic reasons, the elastic material is best located under the foot at the slit S. The elastic strap section also contributes to some extent to the firm grip of the sandal on the foot. Contrary to conventional sandals equipped with elastic instep straps, the short length 9 of elastic material in my sandal is stretched only very slightly during walking, and thus is not subject to rapid deterioration.

While I have shown sandals without backstrap or upper heel portion to illustrate my invention, it is used to advantage in sandals equipped with an uppers portion around the heel. The invention also is not limited to the specific arrangement of the forward end of the sandal shown. The vamp band illustrated in FIG. 2 may be replaced by an open or closed conventional toe or vamp portion of the uppers which may be connected to the instep strap. These and other modifications of the sandal of the invention will readily suggest themselves to those skilled in this art without involving the exercise of inventive faculty.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.