the example of fig1 represents an alpine ski in a bottom view , where one can distinguish the lower surface or gliding sole 1 made of polyethylene , the side running edges 2 , the shovel 3 and tail 4 . the sole is cambered and rests on a front contact line 10 and a rear contact line 11 when the ski does not bear a load . the surface comprised between these two contact lines 10 , 11 meets the snow due to the weight of the skier and becomes the so - called &# 34 ; bearing &# 34 ; surface of the ski . the bearing surface comprises a central zone 5 called the &# 34 ; flat gliding &# 34 ; zone , having a width l2 in which substantial structuring is obtained . fig1 a shows , in an enlarged view , an example of substantial structuring as per the invention . this could be a plurality of short , rectilinear and discontinuous serrations 50 , arranged in rows that are meshed into one another . on either side of central zone 5 , the bearing zone comprises two lateral zones 6 , 7 for curved gliding , having a width l1 . these zones ( fig1 ) have a less pronounced structuring in which the serrations 70 are more spaced , shorter , less deep and less wide , for example , than the serrations in the central zone ( fig1 a ). the structuring can also be quasi non - existent and the surface can be as smooth as possible . fig2 shows the ski when a turn is taken and its inclination with respect to the terrain surface . a portion of the sole penetrates more or less deeply into the snow or ice , according to the prevailing conditions , by virtue of the cut exerted by the running edge 2 . one can understand the usefulness of reserving a zone 7 of width l1 of the bearing surface that has minimum friction with the snow , central zone 5 of width l2 being almost never in contact with the snow during the turn . in the example illustrated , as in all the embodiments of the invention , width l of the sole varies to follow the side cut of the ski . width l1 is always greater than 0 . 03 l , at all points of width l of the sole , measured between contact lines 10 , 11 . preferably , l1 is comprised between 0 . 03 l and 0 . 3 l , it being understood that l2 must remain greater than or equal to 0 . 45 l . as an example , in a traditional alpine ski , l1 can be comprised between 3 mm and 25 mm approximately , and l2 is greater than 27 mm . in the example illustrated , l2 is constant between contact lines 10 , 11 and thus , l1 varies progressively . in particular , width l1 of each lateral zone 6 , 7 increases progressively in the direction of contact lines 10 , 11 from the smallest l min width l , located in the vicinity of the center of the bearing surface . in snow trials , the following surface characteristics of central zone 5 are consistent with the aforementioned objects of the invention : the measurement of the roughness parameters is undertaken perpendicularly with respect to the longitudinal axis of the ski , as are the width measurements l , l1 , l2 . fig3 and 4 illustrate an advantageous example enabling a structuring of the sole of a ski as per the invention to be obtained , and more particularly in the case where one wants to obtain a constant width l2 for central zone 5 of the lower surface conferring a variable width l1 for the lateral zones 6 , 7 . for this , a device comprising a heated tool 8 is used , the tool having the shape of a roller that has a raised design on its surface of a constant edge width l , comprising a plurality of discontinuous ribs 80 . the surface of the roller is applied , under pressure , against the surface of sole 1 of the ski to be prepared , by undertaking a longitudinal run of the ski between several guide plates 9 . it can also be provided that the ski remains immobile and the tool moves with respect to the ski , as an entirely equivalent means . according to the nature of the material constituting the sole and the dimensions of the serrations to be obtained , the pressure and temperature conditions can be adapted without any special difficulties , so as to obtain satisfactory and reproducible results . as a non - limiting example , for a sole of the high density polyethylene type and in case the serrations are obtained as per the aforementioned characteristics provided as examples , it is preferable to work at a temperature comprised between 80 ° and 120 ° c ., and to exert pressure on the surface of the sole by the revolving tool 8 . in such a method , it is not necessary to undertake several passes of the tool over the surface . on the other hand , however , it is not forbidden to rework the surface by a finishing process enabling micro - structuring over the entire sole , for example , by using techniques that are well known to a person skilled in the art , as for example , by stone polishing , for example . fig5 shows a variation of the invention wherein width l1 of the lateral curved gliding zones is constant along the ski . due to the side cut of the ski , therefore , that confers a variable width l in the shape of a &# 34 ; wasp waist &# 34 ; to the lower surface , width l2 of the central flat gliding zone is also variable along the ski and increases from the vicinity of the width line l min in the direction of the contact lines 10 , 11 of the ski . in fig6 only the rear portion of central zone 5 comprised between the rear contact line 11 and the boot center line 12 is embellished with serrations . boot center line 12 is a line provided by each manufacturer and its position with respect to the center of the ski can vary from one type of ski to another . the front portion comprised between forward contact line 10 and the boot center line 12 is lightly or non - serrated such that the roughness parameters rtm and rku defined in the front portion are close to or equal to the respective values of the lateral curved gliding zones 6 , 7 . tests undertaken by the applicants have demonstrated that such a configuration can perform better in certain snow conditions . in the case of fig7 the structuring of central zone 5 has such a roughness gradient that the measured parameters rtm and rku decrease progressively , from the rear contact line 11 towards the forward contact line 10 . this gradient can be obtained , for example , by progressively reducing the number of serrations and their dimensions ( length , width and depth ), from rear line 11 towards forward line 10 . it can be provided that on certain short width portions δl of the lower surface , values rtm and rku can remain substantially constant in central zone 5 . thus , it can advantageously be provided that the decreasing progression of values rtm and rku be done by stages , i . e ., by a succession of short , adjacent portions in which values rtm and rku are substantially constant , but vary from one adjacent portion to the other . fig8 through 10 represent non - limiting examples of the positioning of the serrations on the gliding surface as per the invention . among the serrations equipping the surface , one can have : serrations 50 that are parallel and arranged in separate rows , oriented longitudinally ( fig8 ); parallel serrations oriented longitudinally 50a and others that are inclined 50b ( fig1 ). the length of the serrations can vary , generally between 4 mm and 50 mm , according to the type of snow and type of ski used . naturally , the invention is not limited to the embodiments described and represented hereinabove , but can also comprise all technical equivalents thereof as well as their combinations that can be included within the scope of the following claims .
a ski having a sole equipped with discontinuous serrations including a central flat gliding zone and on one side , at least , a lateral curved gliding zone bordering the central zone . the surface of the rear portion at least , of the central zone between the rear contact line and the center line of the boot of the ski is equipped with a pronounced structuring such that the measured values of the significant roughness parameters rtm and rku are greater than the values measured on the surface of the lateral curved gliding zone . the invention is more specifically related to an alpine ski whose ability to pivot and to glide are thus improved .