Cross country ski, especially for the practice of alternating steps
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 7/747,952, filed on Aug. 21, 1991, now U.S. Pat. No. 5,292,147, issued on Mar. 8, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to cross country skis and, especially, to those that are used for practicing the traditional step, called alternating step, as opposed to the sliding step. 
     2. Description of Background and Relevant Information 
     A cross country ski generally comprises an arched central portion when at rest, with an upward convexity, demarcated by two lines of contact with the ground, front and rear, respectively, separating this central portion respectively from the spatula and the heel of the ski. The support zone of the foot of the skier is located at a medium position with respect to this central part, at least approximately. 
     While practicing the alternating step, this support of the foot manifests itself between the sole of the ski and the ground, mainly at two very restricted, and localized points or zones. 
     During impulsion for forward advance, the foot of the skier takes support at the level of the metatarsal-phalanges joint, from which the position of the support point on the ski or the support point of the impulsion is defined. It is at the level of this point that maximum pressure is exercised in order to flatten the ski by elasticity against the slight bend of the central portion. In the sliding phase or advance of the ski, it is by the heel that the foot provides support on the ski. Due to the original camber of the ski, and the relative position of the foot with respect to it, this support of the heel leaves a residual arched zone in front of its application point, which thus corresponds to zero pressure of the ski on the ground, and which is commonly called &#34;waxing chamber&#34;. Indeed, it is in this zone that it is wisest to apply to the sole of the ski, a retention wax or any other means resisting backward movement, such as scales, seal skin, anti-backward movement chemical coating, etc . . . This retention wax, or any other means, is only effective when in contact with the ground, that is, during the impulsion phase, stopping the ski from sliding back by the positive reaction that the ground can then exercise on the ski. 
     This distribution along the sole of the ski is of considerable importance because it conditions the efficiency of the impulsion transmitted by the metatarsal-phalanges support in the impulsion or impulse phase and the quality of the sliding during support of the heel. 
     For a traditional ski, basically adapted to alternating steps, one notes, as is illustrated in FIG. 1, that this pressure distribution along the ski is not ideal. This diagram represents sequenced contact pressure, in accordance with the position on the ski in abssicus, between spatula S and the heel of the ski Ts, respectively in the impulsion phase, in dotted and dashed lines, and in the sliding phase, in non-broken lines. The support points in impulsion and of the heel are respectively referenced by Pi and T. One notes that if, in the sliding or heel support phase, the waxing chamber 8 or the zero pressure zone does in fact exist, during the impulsion phase (dotted and dashed lines), the pressure, which is maximum at the level of impulsion point Pi, is exercised on either side of this point such that it is very diffused and wide on the one hand, and has relatively little intensity in the zone of the waxing chamber 8 on the other hand, substantially hindering the efficiency of impulsion in this way. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome this disadvantage, or at least, significantly reduce this disadvantage, by concentrating the impulsion pressure on the zone of the waxing chamber 8. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will become apparent from the description and the various embodiments that follow, and will be further explained with reference to the annexed drawings, in which: 
     FIG. 1, mentioned above, is a diagram explicitly representing the known state of the art; 
     FIG. 2 is a longitudinal sectional view of a ski at rest wherein the present invention is implemented; 
     FIG. 3 is a top plan view of the same ski; 
     FIG. 4 is a longitudinal sectional view of the same ski during the sliding phase; 
     FIG. 5 is also a longitudinal sectional view of the same ski, but during the impulsion phase; 
     FIGS. 6 and 7 are two embodiments similar to FIG. 1, showing the distribution of contact pressure in accordance with two embodiments of the invention; 
     FIGS. 8 and 9 illustrate in a partial transverse section of a traditional ski and of a so-called trapezoidal ski respectively, two embodiments of the invention; 
     FIGS. 10 and 11 represent a partial longitudinal section of a detail of two embodiments of the invention; 
     FIGS. 12 and 13 illustrate in a partial longitudinal section two manufacturing methods of a ski respectively according to the invention; 
     FIG. 14 is a longitudinal sectional view of the ski according to a preferred embodiment; 
     FIG. 15 is a transverse sectional view of the ski according to the embodiment of FIG. 14; 
     FIGS. 16a and 16b are a longitudinal sectional and bottom plan views showing an embodiment of the removable anti-backward movement element; and 
     FIGS. 17a, 17b, and 17c are longitudinal and transverse sectional views, as well as an elevational view showing a second embodiment of the removable anti-backward movement element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An object of the invention is to eliminate, or at least to considerably reduce the disadvantages mentioned above, by arranging in the thickness of the ski, in the central portion corresponding at least approximately to the area of support of the foot of the skier, an open portion, or transverse slit crossing the ski to define two upper and lower beams, respectively. During an impulsion force, the upper beam, by flexing, comes into contact with the lower beam which flexes in turn so as to generate contact pressure with the ground which enables impulsion. 
     In the drawings, a cross country ski is represented by a eel T S , its spatula S, and its front 2 and rear 3 contact lines with the ground respectively, demarcating the central portion 1 between them. A boot 4 is represented schematically, and the force application points of the heel support T and of the impulsion or metatarsal-phalanges support Pi are highlighted. In the thickness of the central portion 1 and in the more restricted zone corresponding especially to that covering the assembly of boot 4, an open space is arranged, for example in the shape of a slit 5 of small thickness. This slit 5 defines a sharing zone by separating the ski, between the two upper 6 and lower 7 beams respectively. This longitudinal slit, transversely extending, is located at the rear of the so-called point of equilibrium P1 of the ski, Corresponding to the localization of the front end of the boot 4. Moreover, it extends towards the rear to a point located in the vicinity of point T where, in the sliding phase, the force of the heel support of boot 4 is exercised. Preferably, slit 5 is located in the thickness of its ski at the level of the neutral fiber. 
     As can be seen better in FIGS. 4 and 5, the slit 5 fulfills two distinct functions. In the sliding phase (FIG. 4), during support of heel T, it opens the waxing chamber 8 upwardly, this element having been introduced hereinabove in the introduction, by a scissoring phenomenon introduced between the upper beam 6 and the lower beam 7. On the other hand, in the impulse phase (FIG. 5), it enables the waxing chamber 8 to be optimally flattened, that is, with maximum intensity, of the waxing chamber 8 on the ground, by support during impulsion Pi of the upper beam 6 on lower beam 7. 
     Advantageously, the impulsion point Pi located in the area of slit 5 will be localized in front of the center of the slit area, and preferably two-thirds toward the front of it. Indeed, if the impulsion point Pi is centered longitudinally with respect to slit 5, optimal distribution of contact pressures between the sole of the ski and the ground during impulsion efforts is not achieved. This is illustrated in the diagram of FIG. 6, which is similar to the diagram represented in FIG. 1, where it can be seen on either side of the impulsion point zone Pi, there is a peak of intensity, i.e., two ranges of pressure whose intensity still remains substantial. The pressures exercised correspond to a load equivalent to the normal weight of a skier, that is, approximately 60-85 kgs. 
     The diagram of FIG. 7 represents the distribution of contact pressure when the impulsion point Pi is located, as has been mentioned above, at two-thirds of slit 5 in the direction of the spatula S of the ski. 
     The length of slit 5 can of course vary, and is a function of the length of the ski and the size of boot 4 of the wearer. It is clear that the longer the slit 5, the greater the tendency of the waxing chamber 8 to be curved, and thus, the contact pressure will increase in this zone during exertion of impulsion force Pi. For an average ski, the length of slit 5 may vary, for example, between 150 and 300 mm according to the rigidity in flexion of the upper beam 6. In a preferred range of 210 to 250 mm, only the impulsion pressure Pi varies and increases significantly, the pressure in the waxing chamber zone 8, remaining approximately constant. 
     For a very demanding skier, one can adjust the length of slit 5 in accordance with the size of boot 4 and the weight of the skier. This may be done by placing in slit 5 a wedge that is in contact with upper and lower beams 6 and 7, this wedge being able to be equipped with a cursor, displaceable longitudinally in slit 5, and possibly latchable in the adjusted position. Also, one can provide an adjustment of the thickness of the slit by means of longitudinal wedges of more or less substantial thickness. 
     Moreover, it must be noted that the thickness of slit 5, if it is to be adequate, must remain relatively small, or else the ski may become too fragile. A thickness of about 2-4 mm gives excellent results. 
     If slit 5 remains laterally opened towards the outside, involuntary entries of snow, mud, gravel or other undesirable foreign material may enter. As such, it can be provided that slit 5 be filled with an elastic material, very easily deformable in compression, such as for example, a plastic foam. The slit 5 may also be laterally blocked by a tongue or any other equivalent means. In either case, the slit can be regarded as being &#34;core-free&#34;. 
     The slit 5 that has just been described may also be used in any other type of skis, such as traditional skis as illustrated in a transverse section in FIG. 8, or trapezoidal section skis, curvilineal or not, as is shown in FIG. 9, or even in completely asymmetrical sections (not represented). 
     The slit 5, in the variants that have just been mentioned, may also be arranged obliquely, rectilinear or not, in the thickness of the ski, as is represented in FIG. 10 in a partial longitudinal section, beginning at the heel zone T towards the point of impulsion Pi. This device induces a gradation in the resistance to flexion of the upper beam 6, this flexion being greater in the zone of the impulsion point Pi, by favoring the flattening of waxing chamber 8, but this tends, however, to make upper beam 6 more fragile in this zone. 
     Also, as is illustrated in FIG. 11, the slit 5 may be provided with variable thickness, and can cross heel T towards the impulsion point Pi. This arrangement has approximately the same advantages as those that have been described immediately above. 
     For the provision of slit 5 in the thickness of the ski, several methods can be envisioned. 
     One can use traditional machining by lifting material from the finished ski. This solution, however, has a disadvantage that the core of the ski, generally made of rigid plastic foam (such as polyurethane), does not have adequate intrinsic resistance to adequately withstand scissoring forces. 
     Another solution that does not have this disadvantage comprises independently duplicate molding each of the two upper and lower beams 6 and 7 on a thickness of reinforcement 9, constituted for example of fiber and shaped in a mold to define the upper and lower walls of the slit 5. The beams 6, 7 are then assembled by adhesion, as is illustrated in FIG. 12. In this case, it is possible to make a single beam (shaped in a mold), preferably the upper 6, the second 7 being planar on the side of its adhesion surface. 
     It is also possible to first proceed with the coating of a full molding core 10, defining the future slit 5, by two reinforcements 9 similar to those mentioned previously, and to thereafter proceed with a duplicate molding of the remainder of the ski, especially beams 6 and 7, around a core 10 coated by reinforcements 9. The molding core 10 is then removed, thus definitively releasing slit 5. These different consecutive operations, traditional for a person of ordinary skill in the art, are illustrated in FIG. 13. 
     Finally, as is illustrated in FIGS. 14 and 15, it is possible to make a slit 5 by constructing a lower beam 7 and assembling all the mechanical reinforcement elements, that is, the core, the sole, the reinforcements, etc., and an upper beam 6 made of a molded plastic material covering the slit on a length at least equal to it. 
     This beam may advantageously be provided with a longitudinal rib 61 in which a complementary groove 41 cooperates, the latter being provided in the sole of boot 4. 
     The assembly of the upper beam 6 to lower beam 7 may be done by any means such as adhesion, vibration welding, etc. 
     As has been mentioned above, different means exist that oppose backward movement of the ski (scales, wax, seal skin) corresponding to different types of snow, and that are more or less long and complex to position under the ski. 
     FIG. 16a and 16b illustrate an advantageous solution to resolve this problem: under the sole of the ski, at the level of waxing chamber 8, a removable plate 63 supporting the anti-backward movement system 64 may be positioned and removed quickly. The lower beam 7 comprises, at the level of waxing chamber 8, a recess 61 that may attain a width that is equal to the ski. This recess 61 acts as housing for a casing 62 fixed in recess 61 by any means such screwing, adhesion, welding. This casing may also be obtained directly during manufacture of the ski by molding or machining. A removable plate 63, advantageously made of a plastic material, is embedded in casing 62 and remains in place by a friction or snap fit. On this removable plate, an anti-backward movement system 64 is fixed, that one wishes to use under the ski. All that the skier need to do is have several removable plates 63, with, on each of them, one or several anti-backward movement systems so as to choose, at the last moment, the system that is best adapted to the circumstances. It is also possible that a removable plate 63 does not possess an anti-backward movement system on all or a part of its surface. 
     Another embodiment of casing 62 and of the removable plate 63 is represented in FIG. 17a, 17b, 17c. The casing 62, represented here on the entire width of the ski, comprises two grooves 65, their removable plate 63 sliding in these grooves 65. In order to enable the positioning of the removable plate 63 by sliding in casing 62, the groove 65 is eliminated in one end 66 so as to be able to deform in flexion the removable plate 63, as has been represented in dotted and dashed lines 67, such that the removable plate 63 may pass above edge 68 of casing 62, both during its positioning, and during the extraction of removable plate 63. A cut-out 69, provided in removable plate 63 (or an edge 68) facilitates the extraction manoeuver of this plate. 
     The instant application is based upon French patent application 90.10747, filed on Aug. 24, 1990, the disclosure of which is hereby expressly incorporated by reference in its entirety thereto, and the priority of which is hereby claimed. 
     Finally, although the invention has been described with reference of particular means, materials and embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims.