Ski with improved three-dimensional running surface
BACKGROUND OF THE INVENTION 
     In cross-country or tour skiing, one aspect which is of considerable importance is the running surface of the ski, because such surface must be designed to reduce friction while gliding and yet provide sufficient control and stability with respect to kicking and climbing. Traditionally, tour-ski running surfaces, which engage the snow or ice, have had to be waxed carefully. A known alternative has been to modify structurally the running surface. Early attempts to modify such a surface were disclosed in U.S. Pat. No. 3,381,972. An improvement on this teaching was the utilization of a three-dimensional scale formation on the running surface, as exemplified in U.S. Pat. No. 3,408,086, hereby incorporated by reference. This patent discloses the employment of rows of protuberances with convexly curved edges along the running surface in a regular, repeating pattern over the gliding surface of the ski. Additionally, U.S. Pat. No. 3,858,894, hereby incorporated by reference, should be noted as an improvement on U.S. Pat. No. 3,408,086 in which protuberances, particularly trapezoidaltype formations, are nonharmonically spaced to eliminate or reduce the noise level during gliding movement of the ski. 
     While these variations of protuberances have been known for some time, they, alone, have not satisfied fully the combined requirements of skiers as to reducing friction while gliding, yet providing sufficient snow-gripping action during climbing and kicking maneuvers. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved, three-dimensional, ski base and to skis, particularly snow skis and more particularly cross-country skis, incorporating such an improved base. In particular, this invention envisions a cross-country or touring ski having a three-dimensional base or running surface, with a combination of scale-like and trapezoidal protuberances that have convex, straight and concave curves on their rearward edges, to provide a ski with significantly improved gliding and kicking properties. 
     The improved ski of this invention comprises a running surface having a plurality of differently shaped protuberances which vary longitudinally upon the surface from the ends of the ski to the center, so as to create, firstly, a specific, three-dimensional, surface configuration well-suited for gliding, secondly, another surface configuration that serves as a transition region, and, thirdly, at the center of the ski base, another surface configuration which optimizes kicking. 
     The gliding segment or region of the base comprises a plurality of transverse rows of scale-like protuberances, preferably some of which are longitudinally elongated near the ski ends, and all of which are characterized by having convex, rearward, drop-off edges. 
     The transitional region comprises a plurality of transverse rows of trapezoidal protuberances, preferably longitudinally shorter in comparison with the protuberances found in the glide region. The transition-region protuberances are characterized by straight-edge, rearward, drop-off edges. 
     The kick region also comprises a plurality of transverse rows of roughly trapezoidal protuberances. However, in this kick region, the protuberances are further defined, so as to have concavely shaped, rearward, drop-off edges. 
     During cross-country skiing, a skier&#39;s actions can be divided roughly into two parts: kicking and gliding. The kick occurs when the skier shifts his or her weight to one leg and pushes down and rearwardly. The most important glide occurs when a skier maintains nearly equal-weight distribution on each ski. Gliding properties in skis are often enhanced by adding a camber to the ski; that is, an arching or bowing up of the middle of the ski away from the snow; thus, distributing the skier&#39;s weight over a larger area of the running surface while gliding. To maximize the kick of the skier, it is necessary to increase the frictional forces on the running surface to the point where the force of the skier&#39;s leg propel the skier forward without backsliding; yet this must be done in a manner that does not appreciably increase the friction during the skier&#39;s gliding action. 
     It has been discovered that the efficiency of a skier&#39;s kick and glide can be improved by varying the shape of a series of three-dimensional protuberances or scales along the running surface of a ski, as disclosed herein, and that this invention is particularly well-suited for use with cambered skis. 
     The particular arrangement of gliding, transitional and kicking sections along the longitudinal axis of the ski base provides for unexpected improvement over any one of the particular structural features, alone. In addition, it has been found that varying the height of the drop-off edges provides an even better combination of ski properties. Thus, for example, the height of the rear, drop-off edge may vary from 0 to 1.0 mm or more, but preferably from 0.1 to 0.8 mm, with the kicking section having edges of the greatest height (for example, kicking --0.5 to 1.0 mm; transition--0.3 to 0.5 mm; and gliding--0.1 to 0.3 mm). Furthermore, it has been discovered that varying the size of the protuberances also provides a better combination of ski properties. For example, the gliding and kicking properties may be enhanced by using a larger number of small scales in the kick section and a lesser number of large scales in the glide section. The particular length of each section along the longitudinal axis of the ski may vary with the running surface, itself, but typically the kicking section extends directly under the length or slightly more of the ball of the skier&#39;s foot, while the transition region is short; for example, 2.0 to 10 cm in length, and while the gliding section extends, as desired, toward the tip and rear end of the ski. 
     The three-dimensional base is typically made from a polymer material, such as a thermoplastic like polyethylene or other plastic material, formed by an extrusion or molding operation. The plastic base is then secured to the ski to form the ski bottom or running surface. Alternatively, the polymer running surface may be formed as an integral part of the ski during manufacture, instead of being secured adhesively to the ski after manufacture. 
     This invention will be described in connection with the illustrated embodiments below; however, it should be recognized that various changes and modifications may be made by those persons skilled in the art, without departing from the intent and scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a single ski with the present invention embodied upon its base or bottom; 
     FIG. 2 is a bottom plan view of the section of the ski embodying the present invention; 
     FIG. 3 is an enlarged view of a segment of FIG. 2, showing the glide, transition and kick regions on the bottom surface; 
     FIG. 4 is a longitudinal, sectional view of FIG. 3; 
     FIG. 5 is an enlarged view of another segment of FIG. 2; and 
     FIG. 6 is a longitudinal, sectional view of FIG. 5. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     FIGS. 1 and 2 show the shifting pattern of scales as described herein. In this embodiment, a gliding region of scales begins a short distance from the ski tip 10 as elongated protuberances 16a having exposed faces inclined relative to the general plane of the bottom and terminating in a rearward direction in a convexly curved, drop-off edge, basically following the teaching of U.S. Pat. No. 3,408,086. Proceeding longitudinally along the running surface towards the middle of the ski 12, a region of shortened, gliding scales 18a, still terminating in a rearward direction in a convexly curved, drop-off edge, is encountered. 
     Continuing longitudinally, a region of trapezoidal scales 20a, having roughly straight, rearward, drop-off edges, is encountered, which is the transtion region. Following the transition region of straight-edged scales, a kicking region of concave scales 22 is encountered, such region being located approximately directly below the skier&#39;s foot. The pattern reverses itself after procession from the tip to the kick regions. Continuing longitudinally away from the middle region and towards the rear end 14 of the ski, another straight-edged, transition region 20b, then a shortened, convexly shaped, glide region 18b and finally an elongated, convexly shaped, glide region 16b are encountered. 
     FIG. 3 is a drawing of a segment from the foreward portion of the ski running surface, showing the gliding, transition and kicking regions more clearly. The ski base is divided by a groove 24 running longitudinally along the center of the ski. Protuberances 28 lie on both sides of the groove 24 and extend to the edge 26 of the ski. In this embodiment, the protuberances are set out in rows running perpendicular to the groove 24, and the protuberances of each row are misaligned with respect to the next row. 
     FIG. 5 is a drawing of a segment from the rearward portion of the ski base, showing how the pattern of protuberances reverses itself as it progresses from the center of the ski to the rear end. 
     FIGS. 4 and 6 show longitudinal, sectional views of the ski segment illustrated in FIGS. 3 and 5, respectively. It is pointed out that both FIGS. 4 and 6 are exaggerated, in order to show more clearly the construction and the action involved in this invention. As shown, the ski body 32 is connected to a base 34 having the protuberances incorporated thereon. The protuberances are each inclined with respect to the bottom plane of the base 34, by rising towards the rear end of the ski before dropping off. In this embodiment, three distinct aspects of each protuberance can be seen: firstly, a steadily inclining surface 36 beginning at the forward edge of each protuberance; secondly, a flattened surface or plateau 38 near the rearward end of the protuberance; and, thirdly, a drop-off edge 40. The variation in height of the dropoff edges, between the gliding and kicking regions, is clearly shown by the exaggerated form of FIGS. 4 and 6. 
     Preferably the kicking region of concavely shaped scales is about 10 to 20 cm in length. In operation, it is over this region, roughly 10 to 20 cm, that most of the skier&#39;s kicking force is transmitted. As a skier kicks, the skier shifts his or her weight to the ball of one foot and pushes down and rearwardly. An increasing force is applied by the skier, the camber of the ski is overcome and the bearing-force distribution along the running surface changes, so that most of the force is transmitted to the snow or ice by the region of the running surface directly below the skier&#39;s boot. In order for the skier to propel forward, it is necessary for the skier to exert a sufficient force normal to the snow or ice, so that resulting frictional forces prevent rearward slippage. If the skier is travelling up an incline as he or she attempts to kick, it is even more difficult to maintain sufficient frictional force or grip on the snow or ice surface to continue upward, as the skier can no longer rely on the full-force gravity transmitting his or her weight normally to the snow or ice surface. 
     The present invention discloses a means of increasing frictional forces acting contrary to slippage in the region of the ball of a skier&#39;s foot during the kick. The means are the concavely shaped scales shown in detail in FIGS. 3 and 4. By the utilization of concavely shaped, drop-off edges in this region, the rearward perimeter or edge of each scale is increased, and thus the contact area available to resist slippage is also increased. The use of such concavely shaped scales, solely in the region of the skier&#39;s foot, has the advantage of providing additional frictional forces only where needed (where most of the skier&#39;s force is transmitted via the running surface of the snow of ice). Use of shortened scales in this kick region is also beneficial, as it allows a greater number of concavely shaped, drop-off edges to be placed in the region; thus, further increasing the resistance to slippage. 
     It has been found that use of such concavely shaped scales, shortened or not, along the entire running surface, is not recommended, as the concave configuration does increase the coefficient of friction in both the forward and rearward directions, and such a uniform, concave, scale configuration would, therefore, interfere unnecessarily with the skier&#39;s gliding action. Additionally, it has been discovered that interference with gliding by the concavely shaped scales can be decreased by the use of transitional regions, such as the straight-edged scales and the shortened, convexly shaped scales described in the preferred embodiment. 
     As mentioned previously, the height of the drop-off edges may vary, so that the highest edges are found in the middle (longitudinally) of the ski. Such a variation in height compliments the invention disclosed herein by helping also to provide a greater frictional force acting against slippage in the kick region. 
     It should be noted that the embodiment disclosed herein includes a number of features that are not essential to the practice of the invention, itself. For example, a center groove, running longitudinally in the ski base, is not essential to the invention, nor is the misalignment of certain rows of the protuberances. Furthermore, while the preferred embodiment reveals protuberances having specific shapes, either scale-like or trapezoidal, with inclined surfaces, plateaus and rear drop-off edges, it should be obvious that other structures may be substituted. For example, a scale-like or trapezoidal protuberance could be constructed without a plateau, having, instead, only an inclined surface and rear drop-off edges. Similarly, a dome-shaped structure with a drop-off edge could be substituted for the incline and plateau.