The invention relates to an anti-theft device for bicycles, which comprise a bicycle frame and a steering column mounted on the bicycle frame for steering the bicycle, which steering column is provided for a rotationally fixed coupling of a bicycle handlebar with a fork for a front wheel, having a separating means for neutralising the rotationally fixed coupling. 
     BACKGROUND OF THE ART 
     Such anti-theft devices are known. Unlike a conventional U lock, for instance, which in a secured position is disposed in an exposed position on the bicycle and for this reason provides a thief with contact points for possibly forced opening, the above-mentioned anti-theft device enables improved protection of the bicycle against theft, since the thief, even if he is prepared to use force, cannot neutralise the action of the anti-theft device without damaging the bicycle. 
     In the case of a U-type lock, a classic security principle comes to be used, which consists in bringing parts which are constructed on the article to be secure and move in relation to one another into a more or less fixed or stationary position with respect to one another so that the movability of the parts is limited and the item cannot be used in the secured position. Provided that the item to be secured is a bicycle, as is known free rotatability of the rear wheel, for instance, is prevented by the U-type lock being placed both around a tube of the bicycle frame and also around a portion of the rim of the rear wheel and then being locked. 
     Alternatively, the U-type lock can also be positioned around the tube of the bicycle frame and around a fixed object, such as, for instance, a lamp post, so as to prevent the movability of the bicycle overall. 
     WO 92/10392 discloses a handlebar locking device for bicycles. The device comprises a locking mechanism in a handlebar front part, which brings the handlebar front part either in a non-secured position to engage with the front wheel fork or in the secured position separates it from said fork so that the steering column and the front wheel forks are freely rotatable with respect to one another. A steering column fixed to the front wheel fork and a tubular piece fixed to the handlebar front part are connected to one another in rotationally fixed manner in the non-secured position and can rotate together in a bicycle frame section. A cam actuated by means of a key moves a roller lock disposed in the steering column radially outwards and for releasing the safety device introduces it into a socket situated in the tubular piece so that the bicycle can be controlled. The cam also enables the introduction of the roller lock into an opening which is provided to separate the front wheel fork from the handlebar fore part, so that the bicycle cannot be steered and as a result is protected against theft. The known anti-theft device thus works in reverse to the safety principle described above in that to be precise a secure connection of two parts which is required for the operation of the bicycle is removed in the secured position. 
     The known anti-theft device offers no practical opportunity for producing a release of the lock by the application of force. The known anti-theft device is nevertheless disadvantageous inasmuch as a skilled thief may neutralise it to release the security device of the bicycle without causing any damage. This is of course a problem with respect to the protection of valuable bicycles, which are naturally of interest to a thief and with which, to obtain their resale value, an application of brute force is out of the question for the conscientious thief, so that the thief is prepared to study and learn the unauthorised nuetralisation of the anti-theft device in detail. 
     The object of the invention is to create an anti-theft device for bicycles, the release of which poses a thief a considerably more difficult task than known anti-theft devices. 
     SUMMARY OF THE INVENTION 
     This object is achieved with the device of the type mentioned at the beginning in that a coupling piece or a connecting element is provided as a separating means, which divides the steering column into two parts and which produces the rotationally fixed coupling of these steering column parts in a first state and neutralises it in the second state. 
     With the invention one obtains an anti-theft device which is clearly more difficult to release than known anti-theft devices when the coupling piece or the connecting element is removed by a user from the bicycle in a secured position. The anti-theft device according to the invention is highly integrated into the bicycle. 
     The coupling piece or the connecting element has an essentially reflection-symmetrical or point-symmetrical design. On either side of the plane of symmetry it has recesses and/or projections, which in the first state are in engagement with complementary means constructed on the steering column parts and at the same time produce a form-fit and/or frictional connection and in the second state are released from the complementary means. 
     In a first embodiment of the invention, the coupling piece is constructed with a column piece, which in the first state is inserted between the parts of the steering column and aligns with them and which in the second state is completely removed from the parts of the steering column. In the first state the bicycle is not secured. The column piece inserted between the parts of the steering column enables a rotationally fixed connection of the parts of the steering column with one another, so that the steering column with the coupling piece according to the invention is in practice not restricted in its function in comparison with a conventional steering column. The bicycle is secured in the second state. The parts of the steering column are disposed with coaxial spacing in relation to one another. The coupling piece, which is completely removed in the second state, is required to produce a connection of the parts of the steering column and thus the operability of the steering column again. Provided that a correct coupling piece is not inserted between the parts of the steering column, the use of the bicycle is prevented and is therefore of no interest to a thief. 
     Furthermore, the coupling piece comprises two coupling sleeves disposed axially next to one another. The column piece is housed in the coupling sleeves. The coupling sleeves comprise external threads, which are provided for an engagement with an internal thread in the respective adjacent portion of the bicycle frame. In the secured position the coupling sleeves are axially adjacent; in the non-secured position they are spaced from one another. Upon the transition from the secured position into the non-secured position, the coupling sleeves are screwed into the respective adjacent portion of the bicycle frame. Provided that the coupling sleeves have threads running in the same direction, the directions of rotation of the coupling sleeves are opposite during screwing in and unscrewing. It is advantageous if the coupling sleeves have threads running in opposite directions to one another, because then the directions of rotation of the two coupling sleeves agree when screwing in and unscrewing. The column piece can preferably be twisted coaxially in the coupling sleeves around its longitudinal axis and is also disposed to be axially displaceable. 
     In a preferred embodiment, the external threads on the coupling sleeves and the associated internal threads in the respective adjacent portion of the bicycle frame are individually matched to one another. As a result of the individual matching of the threads to be made to mesh with one another, only the external threads situated on the associated coupling pieces can be screwed into the respective adjacent portion of the bicycle frame. A different coupling piece to that individually provided in contrast has coupling sleeves whose external threads cannot be brought into engagement with the internal thread of the respective adjacent portion of the bicycle frame. With this individualisation of the coupling piece with respect to the particular bicycle frame, a particularly high degree of protection against theft is achieved. 
     The threads are preferably multiple-start, so that upon a rotation of the coupling sleeves a relatively large axial movement of the coupling sleeves is possible. Just one revolution of the coupling sleeves around the column piece is particularly preferably sufficient to achieve an axial movement by a predetermined insertion length of the respective coupling sleeve. With this preferred embodiment, a simple rotation of the coupling sleeves is sufficient to secure and release the safety device of the bicycle. 
     The column piece and the coupling sleeves are housed in a protecting tube. The protecting tube can be detached from the bicycle frame and comprises axially displaceable carriers. The carriers entrain the coupling sleeves upon a rotation of the protecting sleeve in relation to the bicycle frame. The carriers are preferably constructed as radial pins, which protrude outwardly through at least one axial slot in the protecting tube and of which one of the pins is engaged with one of the coupling sleeves. The protecting tube protects the user from dirt, which occurs, for instance, by lubricants from the connection between coupling sleeves and column pieces. The protecting tube also protects the column piece and the coupling sleeves from the entry of impurities and moisture, by which effective protection against corrosion in particular is achieved. If the device is in the non-secured position, the protecting tube also protects the adjacent portions of the bicycle frame from the entry of impurities and moisture. For the user the protecting tube reduces the danger of injury, since it preferably closes smoothly with the bicycle frame, so that the operation of the anti-theft device is particularly simple. Because the protecting tube is inserted into the bicycle frame, the anti-theft device according to the invention is disposed in a particularly inconspicuous manner in the bicycle. Alternatively, the protecting tube can be made conspicuous in order to display the anti-theft device on the bicycle and to represent it to the user as a desirable product. 
     In a second embodiment of the invention, the connecting element has an annular or frame-shaped construction. It comprises engagement bars which are provided for an engagement with claws of the steering column parts in the first state. Each engagement bar is grasped by the claw of the associated steering column part in the first state, the non-secured position. The engagement bars move from the first state into the second state by a first rotational movement of the connecting element around its centre axis at preferably right angles to the axis of the steering column. In the second state, the secured position, the connecting element is lodged freely between the steering column parts. The connecting element can be transferred from the second state, the secured position, into the first state, the non-secured position, by a second rotational movement in the opposite direction to the first. 
     In the second state, through an opening in the wall of the bicycle frame portion with the steering column, the connecting element can be removed from it and inserted into it. The safety device is particularly effective when the connecting element is completely removed from the steering column. For the thief it is a very difficult task to produce the connection between the steering column parts if the correct connecting element is not available to him. 
     On a section of the engagement bars facing the centre axis, the connecting element preferably comprises an individual profile or fitting profile, which can be brought into engagement with a complementary profile or fitting profile on the claws. Because of the complementary profile on the claws, the construction of the individual profile on the associated portion of the connecting element is necessary for releasing the anti-theft device specified by the invention. Thus unauthorised release of the safety device is made even more difficult. 
     According to one embodiment of the invention, the engagement bars of the connecting element are separated from one another possibly in the plane of symmetry and are disposed with radial displaceability with respect to the centre axis. The engagement bars can be brought into engagement with locking recesses in the steering column parts. 
     The axial sections of the locking bars can be brought into engagement with recesses in the end sides of the steering column parts. If in the non-secured position the expansion frame halves are in each case brought to engage in the end sides of the steering column parts, it is guaranteed that an uncontrolled backward rotation out of the non-secured position, in which the steering column parts are connected to one another in rotationally fixed manner, into the secured position, in which steering column parts are separated from one another and are freely rotatable with respect to one another, is prevented. Thus the operational safety of the bicycle is guaranteed when the anti-theft device is situated in the non-secured position. 
     In a further development according to the invention of the connecting element, the engagement bars form an inner ring which is surrounded by an outer ring. The outer ring comprises locking bars which are separated from one another with the formation of end faces preferably in the plane of symmetry of the connecting element. The locking bars are in each case displaceably mounted on the associated engagement bar of the inner ring. The locking bars are initially stressed with respect to one another. The outer ring also comprises one or more expansion elements for pushing apart the locking bars against the initial stress. The expansion elements are in each case rotatably mounted around the centre axis between the end faces of the locking bars. By rotating the expansion elements, the locking bars can be pushed apart. In this embodiment of the invention, the operational safety is particularly high because in the non-secured position the connecting element is grasped in frictional and/or form-fit manner both by the claws and also in the corresponding recesses in the steering column parts when the locking bars are pushed apart. 
     In a plane through which the centre axis passes at right angles, the cross-sectional profile of the expansion element is substantially rectangular with two wide and two narrow sides, which in each case lie opposite one another. If, in the secured position, the connecting element is inserted between the steering column parts, the expansion elements are disposed between the locking bars of the outer ring in such a manner that they press with the end faces of their free ends against the wide side faces of the expansion element. The oppositely disposed end faces of the locking bars have a distance between them which essentially corresponds to the narrow side of the rectangular cross-sectional profile of the expansion element. 
     If the connecting element is introduced between steering column parts and the expansion elements are rotated, the locking bars and the engagement bars follow a first quarter-circle rotation of the expansion elements. The engagement bars are grasped by the claws. The connecting element cannot follow a second quarter-circle rotation of the expansion elements around the centre axis, as the engagement bars are grasped by the claws. Therefore the second quarter-circle rotation results in the expansion element between the end faces of the locking bars being rotated by 90ø. The locking bars are pushed apart or wedged apart by the expansion element according to the difference in length between the wide side and narrow side of the cross-sectional profile of the expansion element. Upon the second quarter-circle rotation of the expansion elements, the outer ring of the connecting element is thus expanded. An unintentional twisting of the expansion element is avoided by the fact that in the course of the quarter-circle rotation the locking bars have to be pushed apart against the initial stress at first up to a maximum, more precisely over the length of the diagonals of the rectangular cross-sectional profile. 
     The cross-sectional profile of the expansion element preferably comprises rounded corners so that the rotatability of the expansion element between the end faces of the initially stressed locking bars is increased. 
     The sides of the cross-sectional profile of the expansion element preferably have a concave design. The cross-sectional profile of the locking bar in the portion of the free ends preferably has a convex design so that the locking bar fits with the convex end face against the concave side face of the expansion element. With this design of the outer ring, the danger of an unintentional twisting of the expansion elements is particularly low. Thus the operational reliability of the bicycle in the non-secured position of the anti-theft device according to the invention is particularly high. 
     The anti-theft device according to the invention which is described above is situated in a bicycle having a bicycle frame and a steering column for steering the bicycle which is mounted on the bicycle frame and which is provided for a rotationally fixed coupling of a bicycle handlebar with a fork for a front wheel. Although the anti-theft device according to the invention which is described above is particularly suitable for protecting bicycles from theft, the person skilled in the art may easily deduce further areas of application, such as the protection of motor-driven two-wheelers or other vehicles having a steering column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two exemplified embodiments of the invention are explained in further detail below by means of the drawings. Therein: 
     FIG. 1 shows a perspective view of a front section of a bicycle, which is equipped with a first embodiment of the anti-theft device according to the invention; 
     FIG. 2 shows a perspective view of a coupling piece according to the first embodiment of the invention; 
     FIG. 3 shows a view in section of the coupling piece inserted between two steering column parts according to the first embodiment of the invention; 
     FIG. 4 shows the same view as FIG. 3, in which the coupling sleeves are screwed into respectively adjacent sections of a head tube constructed on the bicycle frame; 
     FIG. 5 shows an exploded representation of a carrier locking device according to the first embodiment of the invention; 
     FIG. 6 shows a perspective view of a coupling piece according to a second embodiment of the invention; 
     FIG. 7 shows a perspective view of a steering column part constructed with a claw according to the second embodiment of the invention; 
     FIG. 8 shows a perspective view of a expansion element with abutting locking bars according to the second embodiment of the invention; and 
     FIGS. 9 to  12  each show a sectional view of the connecting element according to the second embodiment of the invention on the path of movement between a secured position and a non-secured position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The bicycle represented in FIG. 1 is, by way of example, a conventional ladies&#39; bicycle having a bicycle frame  2 , a front wheel  4 , a front wheel fork  5  and a bicycle handlebar  6 , which can be connected in rotationally fixed manner by means of a steering column  14  to the front wheel fork  5 . The steering column  14  is pivoted in a head tube  26  of the bicycle frame  2 . The ladies&#39; bicycle is provided with an anti-theft device as specified by the invention, in which in accordance with a first embodiment of the invention the separating means is constructed as a coupling piece  8  in such a manner that the coupling piece  8  can be inserted into a recess  10  in the head tube  26 . 
     FIG. 2 shows the coupling piece  8  in detail. The coupling piece  8  has a cylindrical construction and comprises a column piece  22 , two coupling sleeves  24  and a protecting tube  28 , which are disposed coaxially one within the other. At right angles to the longitudinal axis  19  of the coupling piece  8 , an engagement projection  18  is formed on the end faces of the column piece  22  and also on the corresponding end faces of the coupling sleeves  24 . The housing  42  of a carrier locking device  40  essentially designed as a mechanical switch is formed and attached to the shell face of the protecting tube  29  in such a manner that a portion of the protecting tube  28  forms the base of the housing  42 . 
     In the representation in FIGS. 3 and 4, the coupling piece  8  is inserted between two steering column parts  34  in such a manner that the column piece  22  connects the steering column parts  34  to one another and the unit comprising the steering column parts  34  and column piece  22  forms the steering column  14 . In this case the engagement projections  18  constructed on the end faces of the column piece  22  in each case engage with a groove  20 , which on the end side is constructed in the respective adjacent steering column part  34  as a complementary means to the engagement projection  18 , so that the steering column parts  34  are connected to one another in rotationally fixed manner. The protecting tube  28  of the coupling piece  8  inserted into the recess  10  of the head tube  26  aligns with the head tube  26 . 
     The coupling sleeves  24  comprise on their outer side a multiple-start external thread  25 , which is provided for an engagement with a corresponding internal thread  27  on the inner side of the respective adjacent portion of the head tube  26 . The threads are designed to run in opposite directions. Attached to the two coupling sleeves  24  of the coupling piece  8  is a carrier  30 , which is designed as a pin and extends radially through a slot  32  (cf. FIG. 5) in the wall of the protecting tube  28  into the interior of the carrier locking device  40 . As shown in FIG. 3, the coupling sleeves  24  abut one another, whereas, as shown in FIG. 4, the coupling sleeves  24  are screwed into the respective adjacent portion of the head tube  26  by rotating the protecting tube  28 . 
     The structure of the carrier locking device  40  is represented in FIG.  5 . The carrier locking device  40  comprises the housing  42  moulded in one piece or fixed to the protecting tube  28  (cf. FIGS.  2 - 4 ), a press button  44  and an abutment  46 , which in each case have a box-shaped design in such a manner that they can be inserted into one another. In the base  47  of the abutment  46  there are a central oblong first locking hole  56  and also two peripheral second locking holes  57  which are spaced from one another in the longitudinal direction of the coupling piece  8 . 
     The press button  44  and the abutment  46 —which are opposite one another—are displaceably connected to one another by means of articulated pincers  48  having two pincer arms  51 . Bearings  53  are pushed onto the pincer arms  51 . Each bearing  53  is disposed fixed with respect to the press button  44  and the abutment  46  by means of a pin  54  which is mounted moveably therein, and which is retained in corresponding bores  55  in the wall of the press button  44  or of the abutment  46 . In another embodiment, not shown, the pincer arms  51  are provided with slots through each of which a pin  54 , which is retained in two corresponding, mutually opposite bores  55  in the wall of the press button  44 , or in the wall of the abutment  46 , is placed in such a manner that each pincer arm  51  is mounted displaceably in the longitudinal direction on two pins  54 . The press button  44  and the abutment  46  are initially stressed away from one another by springs (not shown), which are disposed between the upper part  45  of the press button  44  and the base  47  of the abutment  46 , in such a manner that the base  47  of the abutment  46  lies on the base  43  of the housing  42  or the corresponding wall portion of the protecting tube  28  when—as explained below—the carriers  30  are in engagement with the locking holes  56  or  57 . 
     The articulated pincers  48  are connected to the housing  42  by the pincer articulation  49  being mounted on a shaft  50 , which in turn is mounted in bores  52  in the wall of the housing  42  and is thus fixed. 
     The carriers  30  protrude through the slot  32  in the base  43  into the interior of the housing  42 . If the distance between the carriers  30  corresponds to the distance between the second locking holes  57  in the base  47  of the abutment  46 , the carriers  30  are in a first locking position and are in each case housed by the associated second locking hole  57 . When the carriers  30  lie adjacent to one another, they are situated in a second locking position and are jointly housed by the central first locking hole  56  in the base  47  of the abutment. 
     If the coupling sleeves  24 —as represented in FIG.  4 —are screwed into the adjacent head tube portions  26  to a predetermined extent, the carriers  30  are situated on the coupling sleeves  24  in the first locking position. The carriers protrude through the slot  32  in the protecting tube  28  into the interior of the housing  42  and are in each case housed in an associated second hole  57  in the base  47  of the abutment  46 . Thus the coupling sleeves  24  are locked with the carriers  30 , since a movement of the carriers  30  along the connecting line between the second holes  57 , which runs in the longitudinal direction of the coupling piece  8 , cannot take place, as a result of which a movement of the coupling sleeves  24  accompanied by a reduction of their axial spacing is prevented. Since the column piece  22  connects the steering column parts  34  to one another in rotationally fixed manner, and furthermore the coupling piece  8  with the column piece  22  cannot be removed from the recess  10  in the head tube  26  as long as the coupling sleeves  24  are screwed into the respective adjacent portion of the head tube  26 , and finally the coupling sleeves  24  cannot be screwed into the protecting tube  28  and out of the adjacent portions of the head tube  26  because of the fixing of the spacing, the coupling piece  8  is locked in the first state, the non-secured position. 
     A transition from the non-secured position into the secured position takes place as follows: To neutralise the locking, the user actuates the press button  44 , by which the articulated pincers  48  fixed with respect to the housing  42  are closed. The articulated pincers  48  pull the press button  44  and the abutment  46  towards and into one another against the initial stress of the springs. At the same time the base  47  is raised from the housing base  43 . The carriers  30  are released from the peripheral locking holes  57  in the base  47  of the abutment  46 . If, in this state, the user twists the protecting tube  28 , the carriers  30  in the slot  32  in the wall of the protecting tube  28  are entrained and in turn entrain the coupling sleeves  24 . At the same time, depending on the direction of rotation, the coupling sleeves  24  are unscrewed from the adjacent portion of the head tube  26  or are screwed into it in such a manner that the coupling sleeves  24  change their mutual axial spacing in the course of the rotation by virtue of the oppositely directed threads. Accordingly the spacing of the carriers  30  from one another also alters. During the rotation of the protecting tube  28  between the first and the second locking position, the carriers  30  run along the lower and outer side of the base  47  of the abutment  46  and thus prevent a lowering of the abutment  46  onto the base  43  of the housing against the initial stress exerted by the springs. 
     When the coupling sleeves  24  are unscrewed from the respective adjacent portion of the head tube  2 , the carriers  30  are situated in the second locking position. If the press button  44  is now released, the press button  44  and the abutment  46  are pushed apart by virtue of the initial tension exerted by the springs. At the same time the two carriers  30  are together brought into engagement with the central locking hole  56  in the base  47  of the abutment  46 . Thus the carriers  30  are locked with the coupling sleeves  24  in the secured position. Since the coupling sleeves  25  are unscrewed from the adjacent portions of the head tube  26 , the coupling piece  8  can now be removed from the recess  10  and be brought into a secured position. 
     A second embodiment of the anti-theft device according to the invention is represented in FIGS. 6 to  12 . In the second embodiment of the invention, the separating means is constructed as a connecting element  60 . The connecting element  60  represented in detail in FIG. 6 (but without expansion elements) comprises an essentially cylindrical core  58 , which is provided with an axial bore  56 , and also two engagement bars  70 . Each engagement bar  70  has a roughly U-shaped design with two limbs  71 , which are spaced from one another by a transverse bar  75 . The free ends of the engagement bar  70  are formed on the core  58  in such a manner that the engagement bars  70  lie opposite one another in one plane and form a rectangle, the median line of which coincides with the longitudinal axis of the core  58  and forms the centre axis  64  of the connecting element  60 . 
     An individual profile  67  is constructed on the inner face of the transverse bar  75  of the engagement bar  70  which faces the core  58 . The engagement bar  70  is provided for receiving a claw  62 , which—as represented in FIG.  7 —is constructed on the free end of a steering column part  34 . The claw  62  in turn comprises a profile which is complementary to the profile  67  of the associated transverse bar  75 . 
     The connecting element  60  also comprises two locking bars  72 . The locking bars  72  have—like the engagement bars  70 —a roughly U-shaped construction each with two limbs  73 , which are spaced from one another by a transverse bar  77 . The locking bars  72  lie opposite one another in the plane of the engagement bars  70 , are separated from one another along the centre axis  64  and are initially stressed with respect to one another. The opposite end faces  90  at the free ends of the locking bars  72  have a convex construction. 
     The surface of the engagement bar  70  facing the limb  73  of the locking bars  72  is provided with a radially extending guide groove  92 . A radially extending rail  82 , which engages with the associated guide groove  92  of the engagement bar  70 , is formed on the surface of the locking bars  72  facing the limbs  71  of the engagement bars  70 . As a result the locking bars  72  are displaceably mounted radially to the centre axis  64  on the engagement bars  70 . 
     For better understanding, the mutually opposite locking bars  72  are represented without a core  58  and engagement bars  70  in FIG.  8 . Two roughly cuboidal expansion elements  78  are clamped between the locking bars  72  which lie opposite each other and are initially stressed in relation to one another. The expansion elements  78  are fixed to the free ends of a rod  86  in such a manner that the rod axis passes at right angles through the end faces  74  of the expansion elements  78 . The rod  86  is rotatably mounted in the axial bore  56  of the core  58  so that the rod axis coincides with the centre axis  64  of the connecting element. 
     The expansion element  78  comprises a substantially rectangular cross-sectional profile with two mutually opposite wide sides  81  and two narrow sides  80  disposed at right angles to the wide sides  81 . The corners of the cross-sectional profile are rounded and its sides each have a concave construction. 
     As the expansion elements  78  in each case are clamped between the locking bars  72  which are initially stressed against one another, there is a frictional connection between each expansion element  78  and the limbs  73  of the locking bars  72  pressing against this expansion element  78 . The frictional connection is supplemented by a form-fit connection, since in each case the convex end faces  90  of the locking bars  72  fit against the associated concave side faces of the expansion element  78 . 
     When the cuboidal expansion elements  78  abut with the face of the wide side  81  against the end faces  90 , the locking bars  72  are spaced from one another roughly by the length of the narrow side  80  of the expansion elements  78 . If, on the other hand, the expansion elements  78  abut the end faces  90  with the face of the narrow side  80  (cf. FIG.  8 ), the locking bars  72  are spaced from one another roughly by the length of the wide side  81 . Thus the rotational position of the expansion elements  78  between the locking bars  72  determines the spacing of the locking bars  72  from one another. 
     With the embodiment of the invention at present being described, a lock (not represented) is situated in the expansion element  78  for additional security. The lock can be operated by means of a key, which can be inserted into the expansion element  78  through an opening  88 . When the key is inserted into the lock and it fits, the user can rotate the expansion elements  78  connected to one another by the rod  86  around the centre axis  64  by means of the key. As a result of the frictional and form-fit connection of the expansion elements  78  with the locking bars  72 , the latter follow the rotational movement of the expansion elements  78 . During this rotational movement the locking bars  72  entrain the engagement bars  70  with the core  58 , as the rails  82  of each locking bar  72  engage in the respective associated guide grooves  92  of the engagement bars  70 . 
     The use of the connecting element  60  is described below. FIGS. 9-12 diagrammatically show individual steps of the transition from the secured position, in which the rotationally fixed coupling of the steering column parts  34  is neutralised, into the non-secured position, in which the rotationally fixed coupling of the steering column parts  34  is produced. Through an opening in the wall of the head tube  26  in the portion provided to receive the connecting element  60  (not represented in FIGS.  9 - 12 ), the connecting element  60  is inserted into the interior of the head tube  26  between the steering column parts  34 . As represented in FIG. 9, the connecting element  60  inserted between the steering column parts  34  lies in a plane through which the steering column axis  36  perpendicularly passes. The locking bars  72  clamp the expansion elements  78  (not represented in FIGS. 9-12) between the end faces  90  of the free ends of their limbs  73 . At the same time the expansion elements  78  abut the end faces  90  by the faces of the wide sides  81 . The distance between the free ends of the mutually opposite locking bars  72  is thus determined by the length of the narrow side  80  of the expansion elements  78 . 
     The user pushes the key (not represented) through the opening  88  into the lock and turns it—in the anti-clockwise direction in the representation. Upon turning the key—by 45ø in FIG.  10  and by 90ø in FIG.  11 —the connection element  60  is entrained in the manner described above. The engagement bars  70  are pushed over the claws  62 , which are constructed on the steering column parts  34 . The profile  68  constructed on the claw  62  in so doing moves into engagement with the profile  67  on the inner side of the engagement bar  70 . The rotational movement can only be completed when the profiles  67  and  68  are complementary, so that the engagement of the profiles  67  and  68  with one another succeeds. When the connection element is turned—as represented in FIG.  11 —by 90ø, the claws  62  hold the engagement bars  70 . The steering column parts  34  are connected to one another in rotationally fixed manner and the safety device for bicycle is released. 
     For the specially reliable release of the safety device of the bicycle, the user rotates the key by a further 90ø (cf. FIG.  12 ). The connecting element  60  cannot follow this further quarter-circle rotation, because the claws  62  retain the engagement bars  70 . If, however, the turning moment applied by the user is sufficient to neutralise the frictional and form-fit connection of the expansion elements  78  with the locking bars  72 , just the expansion elements  78  follow this further rotation of the key. As a result of this further rotation, the locking bars  72  are pushed apart slight further than before, as the expansion elements  78  are rotated between the locking bars  72  in such a manner that the mutual spacing of the locking bars  72  is now determined by the length of the wide side  81  instead of the narrow side  80  of the expansion element  78  (cf. also FIG.  8 ). The locking bars  72  are housed by locking recesses  84  constructed in the steering column parts  34  (cf. FIG. 7) so that an uncontrolled backward rotation of the connecting element  60  into the secured position is not possible. The connecting element  60  is thus reliably locked in the non-secured position. 
     For a transition of the connecting element  60  into the secured position, the user firstly has to turn back the expansion elements  78 —as shown in the representation of FIGS. 9-12 in the anti-clockwise direction—before he then rotates the connection element  60  into the secured position and removes it from the head tube, by which the rotationally fixed coupling of the bicycle handlebar  6  with the fork  5  is completely removed.
With the invention is presented an anti-theft device for bicycles, which comprise a bicycle frame ( 2) and a steering column ( 14) mounted on the bicycle frame ( 2) for steering the bicycle, which steering column is provided for a rotationally fixed coupling of bicycle handlebars ( 6) with a fork ( 5) for a front wheel ( 4), having a separating means for nuetralising the rotationally fixed coupling, wherein a coupling piece ( 8) or a connecting element ( 60) is provided as separating means, which divides the steering column ( 14) into two parts ( 34) and produces the rotationally fixed coupling of these steering column parts ( 34) in a first state and neutralises it in a second state.