Patent Publication Number: US-10328331-B2

Title: Mechanical traction for a cross-country ski

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to mechanical traction enhancements for snow skis. 
     2. Description of Related Art 
     In the classic style of cross-country skiing, as opposed to the skating style, traction is conventionally effected through the use of wax. During gliding the ski slips on a microscopic film of water created by friction between the ski and snow. When the ski stops, ice crystals in the snow embed in the wax, thereby creating traction. The type of wax used varies with snow temperature and texture. The wax wears off and must be re-applied. Improper waxing can inhibit gliding, give insufficient traction, or both. Changes in snow conditions can make a particular wax ineffective. Thus, ski wax cannot yield optimal results in all snow conditions at all times. 
     Another common means of giving traction to a ski is the practice of molding or inscribing a fish-scale pattern of scales in the base of the ski. The scales are low-inclined planes which present perpendicular biting surfaces towards the rear of the ski. These waxless skis require less learning-time, skill and effort to use, but give less satisfactory glide and traction in comparison to waxed skis. Both traction types, wax and fish-scale, will fail on steeper grades and the skier then resorts to climbing with skis in a herring bone pattern; side-stepping with skis transverse to the grade; or to removal of the skis at the bottom of the slope, walking up, and remounting the skis at the top. 
     With the touring or mountaineering style of nordic or alpine skis, traction is conventionally obtained by the use of natural or synthetic seal skins fixed over the underface of the ski. The skins need to be manually removed for gliding downhill. Self-adhesive skins lose adherence when wet and can leave adhesive residue on the gliding surface when removed. Clip-on skins must be manually attached and removed as needed. 
     Some mechanical traction devices are manually engaged and locked in a traction position when needed, and manually disengaged when no longer needed. Examples of such manually engaged traction devices can be found in U.S. Pat. No. 4,148,500 to Nidecker; U.S. Pat. No. 4,564,210 to Case; and U.S. Pat. No. 4,898,401 to Champagnac and in US patent application publication No. 2007/0246913 to Coulbourn. When engaged, such manually locked traction devices cannot provide tractionless gliding. 
     Some mechanical traction devices mechanically engage traction as the skier&#39;s heel lifts off the ski or as the ski is unweighted, and disengage traction as the heel is lowered or the ski is weighted. Examples of such heel-lift solutions can be found in U.S. Pat. No. 4,717,167 to Adam; U.S. Pat. No. 5,577,754 to Hwu; and U.S. Pat. No. 8,333,403 to Popel. Such heel-lift solutions have the disadvantage that the engagement and disengagement of the traction mechanism are not synchronized with the push-off and gliding phases defined by the classic style of cross-country skiing. In the classic style, the push-off phase begins before the heel lifts, and the gliding phase begins while the skier&#39;s heel is still raised. Accordingly, such heel-lift solutions will result in a lack of traction at the beginning of the push-off phase when it is needed most, and will result in undesirable drag when the ski is contacting the snow at the beginning of the gliding phase. 
     Some mechanical traction devices include a freely moveable traction element disposed along a side edge of a ski. Such traction element is freely moveable under the force of gravity and by mechanical contact with the snow surface. When the ski is moved forward along the surface of the snow, such as during forward gliding, the traction element is intended to move to a disengaged position in response to its contact with the snow surface. When the ski is lifted above the snow, the traction element is intended to move to an engaged position in response to the force of gravity acting on the traction element. When the ski is moved rearward along the surface of the snow, the freely moveable traction element is intended to move into an engaged position penetrating into the snow to provide traction for a forward push-off. Examples of such traction devices can be found in U.S. Pat. No. 4,674,764 to Miesen; U.S. Pat. No. 5,221,104 to Bejean et al.; U.S. Pat. No. 6,092,828 to Schumacher; and U.S. Pat. No. 4,844,501 to Lekhtman. However, such freely moveable traction element does not engage until after the ski has moved sufficiently rearward to complete the movement of the traction element into its engaged position. Thus, such freely moveable traction devices have the disadvantage that no traction is provided at the beginning of a push-off when it is needed most. In the case of hard-packed snow or ice, such freely moveable traction element may not move to its engaged position, thereby causing a loss of the device&#39;s traction. When the ski is lowered onto the snow just before forward ski motion, the possibility remains that such freely moveable traction element, instead of remaining in an engaged position and penetrating the snow, may move to its disengaged position causing a loss of the device&#39;s traction. Lowering the ski with a simultaneous rearward motion would increase the probability of the traction element remaining engaged, but doing so has the disadvantage of requiring the ski to be lifted and moved rearward along the snow surface with every forward step. 
     OBJECTS OF THE INVENTION 
     An object of the invention is to address the above shortcomings. Further objects may include providing a mechanical traction device for skis that can be engaged without dismounting the ski; that is actuated by means other than the lifting of a skier&#39;s heel; and that does not use the force of gravity to move a traction element of the device into an engaged position. 
     SUMMARY 
     The above shortcomings may be addressed by providing, in accordance with one aspect of the invention, a ski for traveling on snow-covered ground, the ski having a longitudinal body defining a sole for contacting the snow-covered ground. The ski includes: (a) a platform slidably coupled to the body for sliding longitudinally relative to the body; and (b) at least one gripping element coupled to the body and the platform for extending in a direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. 
     The at least one gripping element may be operable to extend in a first perpendicular direction perpendicular to the sole in response to the platform sliding in a first longitudinal direction relative to the body. The at least one gripping element may be operable to retract in a second perpendicular direction opposite the first perpendicular direction in response to the platform sliding in a second longitudinal direction opposite the first longitudinal direction. The body may define a forward direction toward a front section of the ski. The body may define a downward direction through the body perpendicularly toward the sole. The first perpendicular direction may be the downward direction. The first longitudinal direction may be a rearward direction opposite the forward direction. The second perpendicular direction may be an upward direction opposite the downward direction. The second longitudinal direction may be the forward direction. The at least one gripping element may be dimensioned for extending beyond the body when the platform is slid in the rearward direction sufficiently relative to the body. The at least one gripping element may be dimensioned for extending beyond the body in the downward direction so as to penetrate into snow of the snow-covered ground when the platform is slid in the rearward direction sufficiently relative to the body. The at least one gripping element may be dimensioned for not extending beyond the body when the platform is slid in the forward direction sufficiently relative to the body. The at least one gripping element may be dimensioned for not extending beyond the body in the downward direction when the platform is slid in the forward direction sufficiently relative to the body. The at least one gripping element may be rotatably coupled to the platform. The at least one gripping element may be slidably coupled to the body. The at least one gripping element may be rotatably coupled to the body. The at least one gripping element may be slidably and rotatably coupled to the body. The ski may include a first bumper attached to the body for limiting the sliding travel of the platform. The ski may include a second bumper attached to the body for limiting the sliding travel of the platform. The ski may include the first and second bumpers attached to the body for limiting the sliding travel of the platform. The ski may include a front bumper attached to the body for limiting the forward sliding travel of the platform. The ski may include a rear bumper attached to the body for limiting the rearward sliding travel of the platform. The ski may include the front and rear bumpers attached to the body for limiting the forward and rearward sliding travel of the platform. The platform may be coupled to the body at an upper side of the body opposite the sole. The platform may be operable to resist sliding relative to the body in response to receiving pressure in the downward direction toward the body. The platform may include a binding for receiving a ski boot. The platform may be operable to receive the pressure from the ski boot. The at least one gripping element may include first and second elements disposed on opposing sides of the body. The first element may be operable to extend in the direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. The second element may be operable to extend in the direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. The ski may further include a locking mechanism for locking the platform so as to prevent longitudinal sliding of the platform when the locking mechanism is engaged. The locking mechanism may be operable to lock the platform in a locked position selected from the group consisting of: a forward position, one or more intermediate positions and a rearward position. 
     In accordance with another aspect of the invention, there is provided a ski-traction kit for retrofitting a ski, the ski being operable to travel on snow-covered ground and having a longitudinal body defining a sole for contacting the snow-covered ground. The kit includes: (a) a platform dimensioned for being coupled to the body such that the platform becomes longitudinally slidable relative to the body; and (b) at least one gripping element dimensioned for being coupled to the body and the platform such that the at least one gripping element becomes extendable in a direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. 
     The kit may further include instructions for coupling the platform to the body. The kit may further include instructions for coupling the at least one gripping element to the body and the platform. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of embodiments of the invention in conjunction with the accompanying figures and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate by way of example only embodiments of the invention: 
         FIG. 1  is a perspective view of a portion of a ski for traveling on snow-covered ground, showing a traction device according to a first embodiment of the invention in a disengaged position; 
         FIG. 2  is a perspective view of the portion of the ski shown in  FIG. 1 , showing the traction device in an engaged position; 
         FIG. 3A  is a perspective view of a lesser portion of the ski shown in  FIG. 1 , showing an exploded view of a C-shaped sliding platform and a U-shaped rail; 
         FIG. 3B  is a perspective view of the lesser portion shown in  FIG. 3A , showing the C-shaped sliding platform assembled to the U-shaped rail; 
         FIG. 4A  is an exploded view of a variation of the sliding platform and rail shown in  FIGS. 3A and 3B , showing a C-shaped rail, a lower plate and an upper plate; 
         FIG. 4B  is a perspective view of the variation shown in  FIG. 4A , showing the variation in its assembled form; 
         FIG. 5  is a perspective view of the lesser portion shown in  FIG. 3B , showing a binding; 
         FIG. 6A  is a perspective view of a rectangular U-shaped claw for use according to embodiments of the invention, showing the obverse side of the rectangular U-shaped claw; 
         FIG. 6B  is a perspective view of the rectangular U-shaped claw shown in  FIG. 6A , showing the reverse side of the rectangular U-shaped claw; 
         FIG. 7  is a perspective view of a portion of a ski for traveling on snow-covered ground, showing a traction device according to a second embodiment of the invention in a disengaged position; 
         FIG. 8  is a perspective view of the portion of the ski shown in  FIG. 7 , showing the traction device in an engaged position; 
         FIG. 9  is a perspective view of a portion of a ski for traveling on snow-covered ground, showing a traction device according to a third embodiment of the invention in a partly engaged position; 
         FIG. 10  is a perspective view of a wrap-over plate for use with the portion shown in  FIG. 9 , showing threaded pins; 
         FIG. 11  is a perspective view of an angled U-shaped claw for use according to embodiments of the invention, showing the obverse side of the angled U-shaped claw; and 
         FIG. 12  is an exploded view of the traction device shown in  FIG. 9 , showing an aperture in a head of a C-shaped sliding platform. 
     
    
    
     DETAILED DESCRIPTION 
     A ski for traveling on snow-covered ground, the ski having a longitudinal body defining a sole for contacting the snow-covered ground, comprises: (a) platform means for sliding longitudinally relative to the body; and (b) snow-gripping means for extending in a direction perpendicular to the sole in response to the platform means sliding longitudinally relative to the body. 
     Referring to  FIGS. 1 and 2 , a portion of the ski according to a first embodiment of the invention is shown generally at  1 . The ski  1  is useable for traveling on snow-covered ground (not shown). The ski  1  includes a mechanical traction device  21  according to the first embodiment of the invention. 
     The ski  1  includes a rail, such as the U-shaped rail  2  shown in  FIGS. 1 and 2  attached to an upper surface of the longitudinal main part or ski body  22  of the ski  1 . A platform, such as the C-shaped sliding platform  3  shown in  FIGS. 1 and 2 , is shown coupled to the ski  1  via the rail  2 . In the first embodiment, the sliding platform  3  is slidably coupled to the rail  2 , thereby allowing the platform  3  to slide longitudinally along the ski body  22 .  FIG. 1  shows the sliding platform  3  in its longitudinally forward position toward a front section (not shown) of the ski  1 , and  FIG. 2  shows the sliding platform  3  in its longitudinally rearward position toward a rear section (not shown) of the ski  1 . 
       FIGS. 3A and 3B  show a lesser portion of the ski  1  for ease of illustration. In the first embodiment, the rail  2  is mounted to the ski body  22  and does not move relative to the ski body  22 . The rail  2  has a transverse cross-section that is substantially U-shaped, and the sliding platform  3  has a transverse cross-section that is substantially C-shaped. Also, the sliding platform  3  is dimensioned to wrap around or otherwise engage the rail  2  so that the sliding platform  3  is operable to move longitudinally relative to the ski body  22  by sliding along the rail  2 . Typically, the sliding platform  3  is restricted from moving along any axis other than the longitudinal axis defined by the longitudinal body  22 , including being restricted from rotating about any axis. 
     Referring to  FIGS. 4A and 4B , the sliding platform  3  may be implemented in any suitable manner including as the I-shaped sliding platform  41  shown in its assembled state in  FIG. 4B . The rail  4  shown in  FIGS. 4A and 4B  is mounted to the ski body  22  and has a transverse cross-section that is substantially C-shaped. The I-shaped platform  41  includes a lower plate  5  that is step-notched along its longitudinal side edges to give its transverse cross-section the shape of an inverted “T”. The inverted-T-shaped lower plate  5  at its side edges is dimensioned for fitting under the lip defined by the C-shaped rail  4 . The I-shaped platform  41  also includes an upper plate  6  having six sides and a transverse cross-section that is substantially rectangular. In its assembled form shown in  FIG. 4B , the lower plate  5  and the upper plate  6  are attached to each other to define the “I” shape of the sliding platform  41  with 180-degree rotational symmetry. The sides of the C-shaped rail  4  wrap into the opposing gaps of the sides of the I-shaped platform  41  to permit the sliding platform  41  to move longitudinally relative to the ski body  22  by sliding along the rail  4 . Typically, the sliding platform  41  is restricted from moving along any axis other than the longitudinal axis defined by the longitudinal body  22 , including being restricted from rotating about any axis. 
     While  FIGS. 3A, 3B, 4A and 4B  show two variations of a sliding platform, many other variations of sliding platforms can be implemented in accordance with embodiments of the invention, such as using a linear glide or a platform with longitudinal perforations which slide along rods. In some embodiments, the rail  2 , rail  4  or other rail is integrally attached to the ski body  22 . In some embodiments, the rail is implemented as a groove cut into the ski body  22 , and the sliding platform is dimensioned to include a longitudinal projection for engaging the groove to permit the sliding platform to move longitudinally relative to the ski body  22 . Generally, any variation of sliding platform that suitably provides for reciprocating motion longitudinally along the ski body  22  is within the scope contemplated by the present invention. 
     Referring to  FIGS. 1, 2 and 5 , a binding  7  is attached to the sliding platform  3  in accordance with the first embodiment. The illustrated binding  7  is for ski boots having a single lateral pin at the toe, but any binding of any configuration or type suitable for cross-country or touring skis will work in conjunction with any embodiment of the present invention. In variations, various parts or the whole of the binding  7  may be removably attachable to the sliding platform  3 , attached by fasteners to the sliding platform  3 , integrally attached to the sliding platform  3 , or any combination thereof for example. In some embodiments, it is preferable to mold or print plastic component(s) of the binding  7  together with the sliding platform  3 . Doing so might increase strength and help lower the total profile of the traction device  21  above the ski body  22 . A lower profile can advantageously give better overall lateral stability, reduce the tendency for the ski  1  to roll, and provide more control in the snowplow style of descent. In general, however, any binding operable to attach a ski boot to the ski  1  may be suitably employed. 
     Referring back to  FIGS. 1 and 2 , the ski  1  in accordance with the first embodiment includes a gripping element, such as the claw  17  shown in  FIGS. 1 and 2  as having the approximate shape of a blunted paisley. The paisley-shaped claw  17  is operable to retract above the bottom surface or sole  23  of the ski body  22  to a disengaged position of the traction device  21  when the sliding platform  3  is in its forward position, as shown in  FIG. 1 . The claw  17  is also operable to extend below the ski body  22 , such as in a downward direction defined as extending through the ski body  22  perpendicularly toward the sole  23 , to an engaged position of the traction device  21  when the sliding platform  3  is in its rearward position, as shown in  FIG. 2 . 
     The claw  17  includes an aperture, such as the pivot hole  10  in  FIGS. 1 and 2 , that is dimensioned to receive a projection, such as the platform pin  19  attached to the side of the sliding platform  3  as shown in  FIGS. 1 and 2 , such that the claw  17  becomes rotatably coupled to the sliding platform  3  via the platform pin  19  and its pivot hole  10 . In some embodiments, the platform pin  19  is fastened to the sliding platform  3 . However, in the first embodiment the platform pin  19  is integrally attached to the sliding platform  3 , such as by being molded or machined as part of the sliding platform  3  for example. 
     The claw  17  also includes an elongated aperture, such as the slot  11  shown in  FIGS. 1 and 2 , that is dimensioned to receive a second projection, such as the ski pin  20  attached to the side of the ski body  22  as shown in  FIGS. 1 and 2 , such that the claw  17  becomes slidably and rotatably coupled to the ski body  22  of the ski  1 . In some embodiments, the ski pin  20  is integrally attached to the ski body  22 , such as during manufacturing of the ski  1  for example. However, in the first embodiment, the ski pin  20  is fastened to the ski body  22  in any suitable manner, including transversely passing through the ski body  22  so as to form opposing ski pins  20  on opposing sides of the ski body  22  for example. 
     The rotational coupling between the claw  17  and the sliding platform  3  and the rotational and sliding coupling between the claw  17  and the ski body  22  advantageously causes the claw  17  to move between the disengaged and engaged positions in response to longitudinal movement of the sliding platform  3  relative to the ski body  22 . 
     In the first embodiment shown in  FIGS. 1 and 2 , the claw  17  includes a rear edge  18  projecting in a transverse direction away from the side of the ski body  22 . Typically, the rear edge  18  is formed by a bend, such as a 90-degree bend, in the material of the claw  17 . The rear edge  18  advantageously provides a transverse surface to push against snow when the claw  17  is penetrating into the snow. 
     Referring to  FIGS. 6A and 6B , the gripping element may alternatively be formed as a rectangular U-shaped claw  40 . The U-shaped claw  40  may be made of any suitable material, such as sheet metal for example. Both the paisley-shaped claw  17  of  FIGS. 1 and 2  and the U-shaped claw  40  of  FIGS. 6A and 6B  include an aperture such as the pivot hole  10  and an elongated aperture such as the slot  11 , such that the paisley-shaped claw  17  and the U-shaped claw  40  are advantageously interchangeable in embodiments of the invention. The U-shape of the claw  40  is formed by a rear edge  12  and a front edge  13 . Typically, a pair of U-shaped claws  40  is opposedly oriented such that the obverse sides of each U-shaped claw  40  is directed outwardly from a respective side edge of the ski body  22  ( FIGS. 1 and 2 ). The U-shaped claw  40  advantageously provides low cost and ease of manufacturing. 
     Referring back to  FIGS. 1 and 2 , the ski  1  in accordance with the first embodiment includes a heel platform  14  dimensioned to support a heel portion of a ski boot (not shown) during use of the ski  1 . In the first embodiment, the top of the heel platform  14  is generally at the same height from the ski body  22  as the top of the sliding platform  3 , thereby advantageously supporting a ski boot at a level position for enhanced user comfort. Typically, the heel platform  14  includes ridges  15  for gripping the sole of the ski boot (not shown). In variations, any desired style of ridges  15  may be suitably employed. 
     In the first embodiment shown in  FIGS. 1 and 2 , the heel platform  14  abuts a rear face of the rail  2 , thereby serving to limit the rearward travel of the sliding platform  3 . The forward travel of the sliding platform  3  is limited in the first embodiment by a front bumper  16  abutting a front face of the rail  2 . Preferably, the front bumper  16  is made of a resilient material, such as rubber for example, that renders the front bumper  16  operable to resiliently absorb impacts by the sliding platform  3  and consequently reduce the noise of such impacts. The forward and rearward limits of travel of the sliding platform  3  correspondingly limit the rotation of the claw  17  or similar about the platform pin  19  and the rotation and sliding of the claw  17  or similar about the ski pin  20 , thereby defining the placement of the claw  17  in the disengaged and engaged positions, respectively. Additionally or alternatively, the forward and/or rearward travel of the sliding platform  3  may be limited by the length and position of the slot  11  in conjunction with the placement of the ski pin  20 . In the first embodiment, the forward travel of the sliding platform  3  is limited to coincide with the claw  17  being raised above the sole  23  of the ski  1 , and the rearward travel of the sliding platform  3  is limited to coincide with the claw  17  maximally extending vertically in the downward direction. 
     Second Embodiment 
     Referring to  FIGS. 7 and 8 , a ski  1  and a traction device  21  in accordance with a second embodiment of the invention includes a sliding platform  24  according to the second embodiment. The sliding platform  24  has elevated front corners  30  for supporting a platform pin  32  that passes through the eye of an eye-rod claw  31 . The pin  32  may also extend transversely between the front corners  30  to pass, at opposing ends thereof, through the respective eyes of a pair of opposed eye-rod claws  31  (the far claw  31  not being visible in  FIGS. 7 and 8 ). 
     In the second embodiment, each eye-rod claw  31  is rotatably coupled to the sliding platform  24 . In variations, the eye-rod claw  31  may be rotatably coupled to the pin  32 , the pin  32  may be rotatably coupled to the sliding platform  24  at its front corners  30 , or both the eye-rod claw  31  may be rotatably coupled to the pin  32  and the pin  32  rotatably coupled to the sliding platform  24 . 
     The pin  32  is loosely held by a bracket  33  of the second embodiment, which permits the pin  32  to rotate as it slides through the bracket  33  in response to longitudinal movement of the sliding platform  24 . The bracket  33  is preferably inclined slightly to facilitate full movement of the eye-rod claw  31 . In the second embodiment, the elevated front corners  30  advantageously permit a greater vertical distance between the two axes of rotation of the eye-rod claw  31 , namely the axis of rotation at the pin  32  and the axis of rotation at the bracket  33 .  FIG. 7  shows the sliding platform  24  moved fully forward to place the traction device  21  in its disengaged position with the eye-rod claw  31  resting above the sole  23  of the ski  1 ; and  FIG. 8  shows the sliding platform  24  moved fully rearward to place the traction device  21  in its engaged position with the eye-rod claw  31  extending downward below the sole  23  of the ski  1 . 
     While  FIGS. 7 and 8  show the sliding platform  24  having a substantially C-shaped cross-section where it engages the U-shaped rail  2 , in the manner of the first embodiment&#39;s sliding platform  3  shown in  FIGS. 1 and 2, 3A, 3B, and 5 , in some embodiments the I-shaped sliding platform  41  and C-shaped rail  4  shown in  FIGS. 4A and 4B  are suitably employed in accordance with the second embodiment, with necessary changes being made to the sliding platform  41  to incorporate the elevated front corners  30  according to the second embodiment. In general, any suitable variation of sliding platform, including sliding platform variations not shown in  FIGS. 3A, 3B, 4A and 4B , may be employed in accordance with either or both of the first and second embodiments of the invention. 
     The traction device  21  in some embodiments includes a locking mechanism, such as the platform lock  25  shown in  FIGS. 7 and 8 , which permits the skier to selectively lock the traction device  21  in the engaged position, with the eye-rod claw  31  down; in the disengaged position, with the eye-rod claw  31  up above the sole  23 ; and in one or more intermediate positions between the engaged position and the disengaged position. The platform lock  25  is moveably coupled to the sliding platform  24 , such as by being hingedly connected to the sliding platform  24 . Receiving members, such as the apertures  26  shown in  FIGS. 7 and 8 , on the ski body  22  are dimensioned to receive a first locking member  27  of the platform lock  25 , thereby locking the sliding platform  24  to a single longitudinally locked position. Placing the sliding platform  24  to a locked position advantageously locks the eye-rod claw  31  to a corresponding locked position. One aperture  26  is also provided on the sliding platform  24  to receive a second locking member  28  of the platform lock  25  for stabilizing the unlocked position of the platform lock  25  when not in use. An actuation member  29  is operable to receive mechanical pressure, such as from a ski pole, to move the platform lock  25  between its unlocked position and each of its locked positions. 
     In variations, the locking mechanism may, for example, selectively lock the traction device  21  only in the engaged position; in both the engaged and disengaged position; or in the engaged, disengaged and one or more intermediate positions. Locking the traction device  21  in the engaged position advantageously facilitates skiing long ascents, and advantageously facilitates skiing steep or otherwise challenging descents in which the locked traction device  21  can provide continuous braking for enhanced safety and control. Locking the traction device  21  in its disengaged position can be useful when the skier wishes to perform quick jumps and turns, for example, without risking unintended engagement of the traction device  21 . Locking the traction device  21  in its disengaged position can also be useful when applying wax to the sole  23  by preventing the claws  31  from crossing the plane of the sole  23 . 
     While  FIGS. 7 and 8  show the platform lock  25  applied to the traction device  21  of the second embodiment, the platform lock  25  is operable in accordance with any embodiment of the present invention. While  FIGS. 7 and 8  show a locking mechanism implemented as the platform lock  25 , the locking mechanism may be implemented using any suitable means, including locking the eye-rod claw  31  or other gripping element directly to the bracket  33  (or similar) and/or the ski body  22  for example. Locking mechanisms that can be actuated without dismounting from the ski  1 , such as locking mechanisms actuated by hand or by a ski pole, are preferable to locking mechanisms that require dismounting from the ski  1  for actuation. 
     Third Embodiment 
     Referring to  FIGS. 9 to 12 , a ski  1  and a traction device  21  in accordance with a third embodiment of the invention includes a rail  50  having a transverse cross-section that is substantially T-shaped. The rail  50  is mounted to the ski body  22  of the ski  1 . The sliding platform  42  according to the third embodiment has a transverse cross-section that is C-shaped, and the sliding platform  42  is dimensioned to wrap around the T-shaped edges of the rail  50  to permit the sliding platform  42  to slide longitudinally while restricting its movement along other axes, including restricting rotation about any axis. 
     While  FIGS. 9 and 12  show the sliding platform  42  having a substantially C-shaped cross-section, in the manner of the first embodiment&#39;s sliding platform  3  shown in  FIGS. 1 and 2, 3A, 3B, and 5 , in some embodiments the I-shaped sliding platform  41  and C-shaped rail  4  shown in  FIGS. 4A and 4B  are suitably employed in accordance with the third embodiment, with necessary changes being made to the sliding platform  41  to incorporate the head  51  according to the third embodiment. In general, any suitable variation of sliding platform, including sliding platform variations not shown in  FIGS. 3A, 3B, 4A and 4B , may be employed in accordance with any of the first, second and third embodiments of the invention. In general, the U-shaped rail  2  shown in  FIGS. 1, 2, 3A, 3B and 5  is interchangeable with the T-shaped rail  50  shown in  FIGS. 9 and 12 , with any necessary changes being made to the sliding platform at its underface. 
     The sliding platform  42  includes or has attached thereto a binding  43  for receiving a ski boot (not shown). The binding  43  may be implemented in any suitable manner, including being similar or identical to, or different from, the binding  7  shown in  FIGS. 1, 2 and 5 . 
     The ski  1  according to the third embodiment includes a gripping element, such as the angled U-shaped claw  61  shown in  FIGS. 9, 11 and 12 . The U-shaped claw  61  may be made of any suitable material by any suitable manufacturing process, such as machined aluminum for example. The paisley-shaped claw  17  of  FIGS. 1 and 2 , the rectangular U-shaped claw  40  of  FIGS. 6A and 6B , and the angled U-shaped claw  61  of  FIGS. 9, 11 and 12  include an aperture, such as the pivot hole  10 , and an elongated aperture, such as the slot  11 , such that the paisley-shaped claw  17 , the rectangular U-shaped claw  40 , and the angled U-shaped claw  61  are advantageously interchangeable in embodiments of the invention. As best seen in  FIG. 11 , the U-shape of the claw  61  is formed by a rear edge  12  and a front edge  13  that together provide rigidity to the angled U-shaped claw  61 . Typically, a pair of angled U-shaped claws  61  is opposedly oriented such that the obverse sides of each angled U-shaped claw  61  is directed outwardly from a respective side edge of the ski body  22 , as shown in  FIG. 9 . The lower ends of the front edge  13  and the rear edge  12  of the angled U-shaped claw  61  are tapered to advantageously reduce drag when gliding while maximizing traction during push-off. 
     Still referring to  FIGS. 9 to 12 , the sliding platform  42  is operable to rotatably couple a gripping element such as the angled U-shaped claw  61 , and includes a head  51  for supporting a platform pin such as the machine screw  65  as is best seen in  FIG. 12 . The head  51  is preferably integrally formed with the remainder of the sliding platform  42  and extends transversely across the sliding platform  42  at its forward end. Typically, the head  51  includes a threaded aperture  64 , best seen in  FIG. 12 , for receiving the machine screw  65 . When the machine screw  65  is threaded into the threaded aperture  64 , a first washer  60  is typically disposed on the machine screw  65  between the side of the head  51  and the inner face of the angled U-shaped claw  61 , and a second washer  60  is typically disposed on the machine screw  65  between the outer face of the angled U-shaped claw  61  and the head of the machine screw  65 , thereby advantageously reducing friction therebetween, respectively. 
     In the third embodiment, the angled U-shaped claw  61  is slidably and rotatably coupled to the ski body  22  of the ski  1  via a wrap-over plate  57  having a pair of pin holes  59  in which threaded pins  58  are attached, such as by welding. Typically, a lock nut  63 , a tube-shaped bushing  62  and a pair of washers  60  are employed to secure the coupling between the angled U-shaped claw  61  and the wrap-over plate  57 . The tube-shaped bushing  62  typically rotates freely on the threaded pin  58  and is dimensioned to be received into the slot  11 , thereby advantageously reducing friction between the threaded pin  58  and the angled U-shaped claw  61 . The washer  60 , which may be made from nylon or other similar material, advantageously reduces wear and friction between the vertical face of the wrap-over plate  57  and the inner face of the angled U-shaped claw  61 . 
     The wrap-over plate  57  includes a horizontal face dimensioned to be received into a recess cut into the bottom side of the rail  50 , with the rail  50  and the wrap-over plate  57  at its horizontal face having aligned apertures through which fasteners, such as screws, pass when the rail  50  and the wrap-over plate  57  are attached to the ski body  22  of the ski  1 . By use of the wrap-over plate  57 , the third embodiment advantageously provides coupling between a claw, such as the angled U-shaped claw  61 , and the ski body  22  without requiring attachment of any component at or into the side edges of the ski body  22 , which is particularly advantageous in the case of foam-core skis for example. 
     Still referring to  FIGS. 9 to 12 , the forward travel of the sliding platform  42  is limited by the placement of the front bumper  16  and its accompanying front cushion  52 . In the third embodiment, the front cushion  52  is disposed between the front bumper  16  and a front face of the rail  50 . The rearward travel of the sliding platform  42  is limited by the placement of a rear bumper  53  and its accompanying rear cushion  52 . In the third embodiment, the rear cushion  52  is disposed between a rear face of the rail  50  and the rear bumper  53 . The front and rear cushions  52  are typically made of a resilient material, such as rubber, for resiliently absorbing impacts from the sliding platform  42  and thereby reducing the noise of such impacts. The front and rear bumpers  16  and  52  and their accompanying cushions  52  may be attached to the ski body  22  in any suitable manner, including by the use of fasteners, adhesive or both fasteners and adhesive for example. Typically, the front and rear bumpers  16  and  53  and their accompanying cushions  52  are mounted on the ski  1  such that the sliding platform  42  has sufficient longitudinal play to engage and disengage the traction device  21  but no more. 
     In the third embodiment, the ski  1  includes a two-piece heel plate made of a front heel plate  54  and a rear heel plate  55 , each of which is attached to the ski body  22  by fasteners, such as screws, passing through a single transverse row of screw holes in each of the plates  54  and  55 . The front and rear heel plates  54  and  55  are typically separated from each other by a spacing in the range of one to two millimeters. Such spacing and the transverse orientation of the rows of fasteners advantageously avoid restricting ski flex about a transverse axis. The respective edges of the front and rear heel plates  54  and  55  that face each other are configured in mortise-and-tenon fashion, thereby advantageously restricting ski flex about a vertical axis (i.e. yaw). 
     Optionally, embodiments of the invention may include ball springs  66  ( FIGS. 9 and 12 ) that are at least partly embedded into the rail  50  near the front end of the rail  50 . Such ball springs  66  are dimensioned to make frictional contact with the sliding platform  3 ,  41 ,  24  and/or  42  when it is sufficiently forward along its longitudinal track. Such frictional contact provides a measure of hindrance against longitudinal movement of the sliding platform  3 ,  41 ,  24  and/or  42  relative to the ski body  22 , which may provide enhanced comfort or greater useability for some skiiers, especially when the ski  1  is lifted after gliding. 
     Method of Assembly 
     Referring to  FIG. 12 , a method of assembling the angled U-shaped claw  61  to the sliding platform  42  in accordance with the third embodiment of the invention involves: (a) placing a first washer  60  onto the threaded pin  58  against the wrap-over plate  57 , the threaded pin  58  having been welded into the pin hole  59  of the wrap-over plate  57 ; (b) placing one tube-shaped bushing  62  onto the threaded pin  58 ; (c) placing the angled U-shaped claw  61  onto the threaded pin  58  such that the threaded pin  58  and the tube-shaped bushing  62  passes through and becomes disposed within the slot  11 ; (d) placing a second washer  60  onto the threaded pin  58  against the angled U-shaped claw  61 ; (e) threading one lock nut  63  onto the threaded pin  58 ; (f) tightening the lock nut  63 ; (g) placing a third washer  60  onto a machine screw  65 ; (h) placing the machine screw  65  through the pivot hole  10 , through a fourth washer  60 , and into the threaded aperture  64 ; and (i) tightening the machine screw  65 . 
     This method of assembling the angled U-shaped claw  61  to the sliding platform  42 , or similar, is typically employed to assemble a second claw  61  on the opposing side of the sliding platform  42 . 
     Variations of Embodiments 
     While the claws  17 ,  40 ,  31  and  61  can be rotatably coupled to the sliding platforms  3 ,  41 ,  24  and  42 , in various combinations thereof, and can be slidably and rotatably coupled to the ski body  22  by the exemplary techniques described and illustrated herein, any suitable technique for converting the reciprocating motion of the sliding platform  3 ,  41 ,  24  or  42  into extension and retraction of the claw  17 ,  40 ,  31  or  61  may be suitably employed. By way of example only, some embodiments may include a plurality of claws  17 ,  40 ,  31  and/or  61  on one or both sides of the ski body  22 . Some embodiments include flaps, louvres or the like along the sole  23  that are lowered and raised in response to reciprocating motion of a top-mounted sliding platform  3 ,  41 ,  24  or  42 . However, the specific embodiments described in detail and illustrated herein advantageously do not modify the gliding surface or sole  23  of the ski  1 . Other variations of the embodiments described and illustrated herein are possible. 
     The claws  17 ,  40 ,  31  and  61  may be made of any suitable material, such as metal, fiberglass, wood, plastic, other related materials, or any combination thereof. The claws  17 ,  40 ,  31  and  61  may be formed by casting, molding, extruding, forging, rolling, machining, printing, other similar manufacturing processes, or any combination thereof for example. Specific examples of materials for the claws  17 ,  40 ,  31  and  61  include sheet metal, iron, carbon steel, stainless steel, forged steel, cast aluminum, and molded plastic. 
     Generally, the claws  17 ,  40  and  61  are interchangeable with each other, and may be swapped during use for example. Different claws  17 ,  40 ,  31  and/or  61 , or a related variation, of different dimensions, including having different lengths, may be substituted according to snow conditions for example. When skiing on deep virgin snow, a skier may wish to use a longer claw  17 ,  40 ,  31  and/or  61  that is operable to penetrate deeper into the snow and/or a wider claw  17 ,  40 ,  31  and/or  61  that is operable to push rearward against snow with a wider sweep, for example. In contrast, a skier may wish to use a shorter and/or narrower claw  17 ,  40 ,  31  and/or  61  when skiing on hard-packed snow for example. While particular shapes of the claws  17 ,  40 ,  31  and  61  are shown in the Figures for ease of illustration, any suitable shape of claw  17 ,  40 ,  31  and/or  61  providing similar functionality is within the scope contemplated by the present invention. 
     A particular advantage of the embodiments described and illustrated herein is that ski-traction enhancement is obtained without unduly increasing the height above the ski body  22  of the binding  7  ( FIGS. 1, 2, 5, 7 and 8 ), the binding  43  ( FIGS. 9 and 12 ), the heel platform  14  ( FIGS. 1, 2, 7 and 8 ), the front heel plate  54  ( FIGS. 9 and 12 ) and the rear heel plate  55  ( FIGS. 9 and 12 ). The low profile of the traction device  21  gives excellent lateral stability, minimizes rolling, and provides excellent control in the snowplow style of descent. Further variations of embodiments may further improve this particular advantage by lowering the overall profile height of the traction device  21 , thereby minimizing the tendency for the ski  1  to roll and put rolling stress on a skier&#39;s ankle. By way of example only, a lower portion of the traction device  21  may be molded into the ski body  22  of the ski  1  to maintain a low profile. Additionally or alternatively, the sliding platform  3 ,  41 ,  24  and/or  42  and the rail  2 ,  4  and/or  50  may be partly or completely disposed at either side of the ski body  22 , with the claws  17 ,  40 ,  31  and/or  61  being disposed at the front of the sliding platform  3 ,  41 ,  24  and/or  42  and the rail  2 ,  4  and/or  50 , for example. 
     Thus, there is provided a ski for traveling on snow-covered ground, the ski comprising a longitudinal body defining a sole for contacting the snow-covered ground, the ski comprising: (a) a platform slidably coupled to the body for sliding longitudinally relative to the body; and (b) at least one gripping element coupled to the body and the platform for extending in a direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. 
     Method of Operation 
     After mounting a pair of the skis  1  according to a selected embodiment of the invention, a skier wishing to fully engage the traction device  21  on a given ski  1  (e.g. left or right) lifts their foot so that the ski  1  becomes unweighted and then moves their foot rearward with a jerking motion to cause the sliding platform  3 ,  41 ,  24  and/or  42  to slide rearward relative to the ski body  22  until the sliding platform  3 ,  41 ,  24  and/or  42  is moved fully rearward within the limit of its travel. Such rearward movement of the sliding platform  3 ,  41 ,  24  and/or  42  causes the claw(s)  17 ,  40 ,  31  and/or  61  to extend downwardly below the sole  23  of the ski body  22 . The traction device  21  can be partly engaged by placing the sliding platform  3 ,  41 ,  24  and/or  42  to an intermediate position between the limits of its travel in which the claw(s)  17 ,  40 ,  31  and/or  61  extend downwardly below the sole  23  of the ski body  22  less than their maximum downward extension. 
     After fully or partly engaging the traction device  21 , the skier lowers the ski  1  onto the snow-covered ground such that the claw(s)  17 ,  40 ,  31  and/or  61  penetrate into the snow for enhanced traction when the skier pushes their body forward against the force of traction between the ski  1  and the snow. The selected engagement of the traction device  21  is maintained by at least friction between the sliding platform  3 ,  41 ,  24  and/or  42  and the ski body  22 , especially when the skier&#39;s weight is bearing down onto the sliding platform  3 ,  41 ,  24  and/or  42 . Also, pushing forward against the force of traction between the traction-engaged ski  1  and the snow encourages the traction device  21  to remain engaged, even if the skier&#39;s weight is not bearing down onto the sliding platform  3 ,  41 ,  24  and/or  42  during the push-off phase. 
     The traction-enhanced push-off propels the skier forward on the skier&#39;s other ski  1  that is arranged for its traction device  21  to be in its disengaged state. Such other ski  1  glides along the snow with the skier, and allows the skier to bring the foot previously used for the push-off forward such that the sliding platform  3 ,  41 ,  24  and/or  42  slides forward relative to the ski body  22  so as to move the traction device  21  to its disengaged position. Thereafter, both skis  1  glide forward with their respective traction devices  21  in disengaged positions such that the claws  17 ,  40 ,  31  and/or  61  are positioned above the soles  23  to avoid inhibiting the forward gliding motion. The skier may at any time thereafter decide to perform a further push-off with either ski  1 . When making the next push-off, the skier has the option of engaging the traction device  21  then lowering the ski  1  onto the snow as far forward as can be comfortably reached by the skier, such that during the subsequent push-off phase the skier is initially pulling him or herself forward while bearing no or minimal weight down onto the ski  1 . Such traction-enhanced pulling is not possible with conventional skis absent sufficient weight bearing down onto the conventional ski. 
     The present invention makes the ascent of steep slopes possible without side-stepping or herring-boning. By engaging the traction device  21  with each step, steep slopes can be climbed in a fashion close to natural walking because engaging the traction device  21  provides excellent positive traction even when the skis  1  are both facing directly uphill. While it is not necessary to manually lock the traction device  21  in its engaged position because the skier can readily engage the traction device  21  with every step, some skiers may wish to lock the traction device  21  in an engaged position for long ascents. 
     The present invention makes a skate-boarding style of propulsion possible, even on level ground, by permitting the skier to perform repeated push-offs using one ski  1 , with its traction device  21  engaged, and glide the other ski  1  while its traction device  21  is disengaged. The enhanced-traction skate-boarding style of propulsion is advantageously more forceful, faster and less tiring than the conventional “marathon skate” style of cross-country skiing in which one ski glides forward while skating outwards to the side repeatedly with the other ski. Also, the traction-enhanced skateboard effect cannot be achieved with nearly as much kick force using traditional waxed or waxless skis when the skis are positioned parallel to each other in the forward direction of travel. 
     The present invention is advantageously compatible with the classical, skate, free-style, Nordic, mountaineering and other styles of cross-country skis, touring skis, and other types of skis. The present invention is advantageously compatible with cross-country and other forms of skiing. 
     Retro-Fitting Kit 
     The traction device  21  in accordance with any embodiment of the invention is suitable for installation on conventional skis, such as cross-country skis, touring skis or other types of skis. In particular, the traction device  21  can be retro-fitted to previously manufactured skis. A retro-fitting kit in accordance with embodiments of the invention typically includes the components described and illustrated herein other than the ski body  22 . Some kits do not include a binding, while other kits include the binding  7  and/or  43 . 
     Typically, the kit includes assembly instructions for installing the traction device  21  to a pair of previously manufactured skis, including installing the rails  2 ,  4  and/or  50 ; the sliding platforms  3 ,  41 ,  24  and/or  42 ; the claws  17 ,  40 ,  31  and/or  61 ; and other related components for example. Retro-fitting kits in accordance with the third embodiment of the invention preferably include, in part, instructions corresponding to the method of assembly provided herein above. 
     Thus, there is provided a ski-traction kit for retrofitting a ski, the ski being operable to travel on snow-covered ground and comprising a longitudinal body defining a sole for contacting the snow-covered ground, the kit comprising: (a) a platform dimensioned for being coupled to the body such that the platform becomes longitudinally slidable relative to the body; and (b) at least one gripping element dimensioned for being coupled to the body and the platform such that the at least one gripping element becomes extendable in a direction perpendicular to the sole in response to the platform being slid longitudinally relative to the body. 
     While embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only. The invention may include variants not described or illustrated herein in detail. Thus, the embodiments described and illustrated herein should not be considered to limit the invention as construed in accordance with the accompanying claims.