apparatus are known , which mainly consist of a structure , of a turntable with jaws between which the wheel is fixed , of a control element which permits the jaws to be moved and the turntable to be rotated , of a vertical column fitted upon said structure , of a horizontal arm mounted transversally in aforementioned column and having the possibility of sliding and being fixed therein , of a rod with tool for fitting and removing the tire , this rod having the possibility of being adjusted and fixed in height , of a relatively complicated means for adjusting the tool , both in a horizontal and in a vertical plane , to a predetermined distance from the fixed wheel , in such a manner as to avoid interference between the wheel and the tool . besides the fact that such an apparatus is relatively complicated and of costly construction , various manipulations have still to be carried out to retract and replace the tool in working position at each new operation . the object of the invention is to provide a mechanism of relatively simple construction , by means of which only a minimum of manipulations need be carried out for locating the tool at the desired distance from the wheel . according to a main characteristic of the invention , the arm which can be moved transversally in the vertical column is inclined and slants down towards the wheel , whereby a means is provided betweeen the arm and the vertical column which permits the moving and the locking of aforesaid arm . thanks to the oblique fitting of the arm and to the relatively simple means mentioned above , it becomes possible to adjust the tool to the desired distance from the wheel , both in the horizontal and in the vertical plane , by carrying out a minimum of manipulations . it is moreover not necessary to adjust the apparatus each time when several wheels and tires with the same dimensions have to be successively fitted or removed . merely as an example and without the slightest restrictive intention , a detailed description will be given hereinafter of a selected form of embodiment of the mechanism according to the invention . this description refers to the appended drawings in which : fig1 is a side view of the mechanism installed on an apparatus used for fitting and removing automobile tires ; fig2 is an enlarged side view of the mechanism in position of rest , with partially cut - away elements ; fig3 is an identical view to that of fig2 but in which the mechanism is in operating position . in fig1 we can see the structure 1 in which is housed the control mechanism of a well - known device 2 with jaws 3 for fixing and driving wheel 4 so as to make it rotate . the apparatus may be fitted with a known device 5 for detaching the tire . a hollow column 6 is fixed upon structure 1 . this column 6 carries a box 7 at its upper end . in this box an axially movable arm 8 is fitted obliquely and slanting down towards the wheel , and is guided between two guide rollers 9 fitted diagonally and two studs 10 fitted diagonally . due to this arrangement , the arm 8 can be rolled in box 7 and fitted at a fixed angle . the end of arm 8 which is directed towards wheel 4 is provided with a block 11 in which a rod 12 with handle 13 is suspended vertically . this rod is adjustable in height and can , thanks to a blocking means 14 and handle 15 , be fixed in block 11 . between handle 13 and block 11 a pressure spring l6 is fitted , which balances the weight of rod 12 . the tool which serves to fit and remove the tire from the wheel is fixed to the lower end of this rod 12 . as parts of this tool we have the pressure roller 17 which is fixed on support 18 of rod 12 , this pressure roller cooperating with the bead of the tire to be fitted . also forming of this tool is , the guide roller 19 , the purpose of which is to guide the bead of the tire outside the circumference of the wheel rim when the tire is being fitted . at the rear end of arm 8 , a freewheeling cogwheel 20 is fitted which can rotate freely clockwise , but not in the other direction . this freewheel is fitted between two disc shaped flanges 21 - 22 . by means of a shaft 23 , the end of a toothed segment 24 is fitted to box 7 , and by means of a handle 25 attached to the other end of aforesaid segment , the latter can be moved into mesh with freewheeling cogwheel 20 . for reasons of safety , flanges 2l - 22 also cover the meshing portion of toothed segment 24 . shaft 23 of toothed segment 24 is made up of a bolt with a nut , a spring washer being provided between the toothed segment and the nut , so as to exert a pressure on the segment and to be able to place and maintain the toothed segment in a desired position . in order to keep slanting arm 8 in balance , a counterweight 26 is fitted inside column 6 and is connected via a cable 27 , which passes over a small cable pulley 28 , to a small block 29 on arm 8 . cable 28 fits in a groove 30 , provided in the bottom part of arm 8 . the toothed segment 24 shall preferably be provided with two stops which cooperate with fixed parts of box 7 , so as to limit the two end positions of the segment . in order to fit a tire on a wheel , arm 8 is rolled back in box 7 in such a manner that the tool , which is used for fitting and removing the tire , is located at a distance from device 2 for fixing the wheel ( fig2 ). toothed segment 24 is then rotated around shaft 23 to its extreme forward position ( fig2 ). a wheel 4 is placed between jaws 3 of abovementioned device 2 and is then secured to this device , which may be of any appropriate type . the tire is next deposited slanting on the wheel in the well known manner . subsequently arm 8 is rolled towards wheel 4 by means of handle 13 of rod 12 and the latter is released by means of handle 15 in order to lower the tool , and such untill the extreme upper edge of the wheel is in contact with the tool in the angle formed between pressure roller 17 and its support 18 . next , rod 12 is fixed in its position by means of handle 15 . due to the slanting position of arm 8 , the slightest movement of the latter in the direction opposite to that of the wheel automatically assures a clearance of the tool 17 - 19 , both in the vertical and in the horizontal plane , with respect to wheel 4 . in order to locate both the pressure roller 17 and the guide roller 19 at the desired distance from the wheel , in such a manner that no interference occurs between the wheel 4 and the rollers 17 - 19 , toothed segment 24 is pivoted around shaft 23 in the direction of the freewheeling cogwheel 20 , in such a manner that these two elements mesh with each other . by continuing the pivoting movement of segment 24 , cogwheel 20 will rotate clockwise and arm 8 will be moved , and such until the tool 17 - 19 has taken up the desired position with respect to the wheel ( fig3 ). due to the fact that the freewheel is unable to rotate counterclockwise , it is impossible in that position to move arm 8 towards wheel 4 , whereas it is however possible to move arm 8 with tool 17 - 19 away from wheel 4 , for instance when the wheel is somewhat out of round and consequently exerts a pressure on the tool . similarly , tool 17 - 19 can be entirely moved away from wheel 4 by rolling arm 8 backward , and be brought back towards the wheel without having to start the adjustment over again . when the bead of the tire is located under pressure roller 17 and close against guide roller 19 , control device 2 , which firmly grips the wheel , is started , in such a manner that the driven wheel carries out a rotating movement and that the tire is entirely fitted on the wheel , and such without the fitting tool or the removing tool of the tire fouling the wheel . it is quite obvious , that the relative locations of the elements , the shape and the dimensions of the elements as described above may vary , and that even the parts as defined here may be replaced by others which are capable of being used for the same purpose .