Abstract:
A frame for a child&#39;s stroller which includes two rear arms and a single front arm which are joined to one another with the two rear arms spaced on opposite sides of a front to rear median vertical plane (P) of the frame and wherein wheels are pivotally mounted to each of the arms so as to be movable between first deployed positions wherein an axes of rotation of the wheels extend transversely to the plane (P) and a second compact storage position wherein the axes of rotation of the wheels extend parallel with respect to the plane (P), and wherein the wheels connected so as to be moved simultaneously between the two positions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stroller frame. It relates in particular to the field of strollers for transporting a very young child. 
     2. Brief Description of the Related Art 
     To push a stroller on the ground, its frame is typically equipped with wheels or casters. The invention deals in particular with the so-called “three-wheel” stroller frames, that is to say, stroller frames provided, at the front, with a single wheel or a single pair of twinned wheels, whereas, at the rear of the frame, there are distinctly provided two wheels or two pairs of twinned wheels, respectively on the left and right sides of the frame. This type of stroller frame is highly appreciated because it gives the stroller a particularly pleasing overall appearance, with a sporty look, which is particularly attractive to a young clientele, as well as for the fact that having the frame press on the ground in three areas forming the vertices of a forward-pointing triangle makes the frame extremely easy to handle. This type of stroller frame is thus recognized for its good rolling behavior on country paths and bumpy ground, particularly in cities where strollers have to negotiate sidewalks, staircases, etc. 
     However, the existing three-wheel stroller frames have a major drawback associated with their bulk: this type of frame makes it essential, for comfort and safety reasons, to provide for the wheels to have a relatively large diameter. Given these conditions, some users forego the purchase of this type of stroller frame because it is so difficult to stow away and transport when the stroller is not in use, and to load in and unload from transport vehicles. 
     In the field of so-called conventional strollers, unlike the three-wheel strollers, that is to say, in the field of strollers with a “four-wheel” frame, frames are known that allow for the two front wheels and the two rear wheels to be retracted, particularly for the purposes of stowing away the stroller. For example, in the recent document WO-A-2007/025551, the front and rear wheels of one and the same left or right side of a stroller can be simultaneously moved, relative to the frame of the stroller, from an extended service position, in which the wheels are extended vertically to roll on the ground, to a retracted storage position, in which the wheels are extended horizontally to be flattened against the frame. Other examples of four-wheel stroller frames that are retractable in this way are provided by the older documents FR-A-525 797, U.S. Pat. No. 2,429,763 and U.S. Pat. No. 4,659,096. 
     In practice, the teaching relating to four-wheel stroller frames cannot, however, be transposed to a three-wheel stroller frame because of the specific structure of this frame essentially consisting of a single front arm and two rear arms, respectively left and right. Thus, WO-A-03/097466 proposes a stroller frame of which the three wheels can be moved from an extended service position to a retracted storage position, but only in a way that is both successive, that is to say one wheel after the other, and complicated, that is to say after several manual actions on the part of the user in several different areas of the stroller frame. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to propose a “three-wheel” stroller frame of the type discussed hereinabove, of which all the wheels can be retracted quickly, simply and easily. 
     To this end, the subject of the invention is a stroller frame particularly for transporting a child, comprising:
         two rear arms and a single front arm, which are fixedly joined to one another and respectively provided with at least one wheel bearing on the ground,   means of moving the rear and front wheels relative to their respective arms, suitable for simultaneously moving all the wheels between, respectively, an extended service position, in which the rotation axes of all the wheels extend substantially perpendicularly to the same antero-posterior median plane of the frame, and a retracted storage position, in which the rotation axes of all the wheels extend substantially parallel to this plane, the antero-posterior distance between the rear wheels and the front wheel or wheels then being smaller than when the wheels are in their extended position, and   a specific handle for manually driving the movement means, arranged at the front end of the front arm.       

     The idea on which the invention is based is to exploit the particular structure of the “three-wheel” stroller frame to provide the user with a practical control handle, that specifically makes it possible to retract all the wheels. By virtue of this handle, the user is able to easily and quickly apply a driving force that the movement means, incorporated in the frame, transmit mechanically, in the front region of the frame, to the front wheel or to the twinned front wheel train, and, in the rear region of the frame, to the two rear wheels or to the two twinned rear wheel trains, respectively left and right. This handle enables the user to preferentially apply a driving torque to the input of the movement means advantageously with one hand. The mechanical nature of the arms, of the movement means and of the handle makes the stroller frame robust and reliable, the relative movement between the wheels and the arms being precise, rapid and reproducible. 
     When the wheels are in their retracted storage position, the bulk of the frame is significantly reduced, both in the direction in which the bodies of the wheels and the frame are then arranged in superposed planes, and in the antero-posterior direction of the frame because of the convergence toward the front of the rear wheels. 
     By positioning the control handle at the front end of the frame, this handle can be grasped manually by the user, quickly and easily. This handle then forms, with respect to the user, a reference element with regard to the mechanical positioning of the moving components of the stroller: depending on the drive action that the user imposes on this handle, all the wheels of the stroller are retracted or extended at the same time, being respectively accompanied by a “shortening” or an “elongation” of the frame, that is to say a bringing-together or a distancing of the rear wheels relative to the front wheel, by virtue of the action of the movement means. 
     Furthermore, the front handle makes it possible, when the wheels are in their retracted storage position, to lift the frame from the ground, for example to load it in a car trunk, or to pull it on the ground. Moreover, according to an advantageous feature of the stroller frame according to the invention, when the wheels are in their retracted position, the rear wheels extend substantially in the same plane perpendicular to the antero-posterior median plane of the frame and are able to roll on the ground in this same plane. This way, when the wheels are retracted, the rear wheels are able to rest in line on the ground, which enables the stroller to be stabilized vertically, to be very easily pulled or pushed laterally by virtue of the handle which is then situated at the top of the frame, particularly for the purposes of stowage in a closet or similar, in the manner of a baggage trolley. In practice, the user can then control the retraction of the wheels so that the rear wheels remain pressed on the ground while they are being retracted, whereas it progressively straightens the frame vertically, with its front end directed upward. In this way, the retraction of the wheels automatically results in the frame being converted to a stable vertical stowage position. 
     Other advantageous features of the stroller frame according to the invention, taken in isolation or in all technically possible combinations, are specified in the dependent claims  2  to  15 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description, given solely as an example, and with reference to the drawings in which: 
         FIG. 1  is a diagrammatic view in side elevation of a stroller frame according to the invention, in a service configuration; 
         FIG. 2  is a partial cross section along the line II-II of  FIG. 1 ; 
         FIG. 3  is a view similar to  FIG. 1 , illustrating the stroller frame in a storage configuration; 
         FIG. 4  is a partial cross section along the line IV-IV of  FIG. 3 ; 
         FIG. 5  is a larger scale view of the detail V of  FIG. 4 ; 
         FIG. 6  is a view similar to  FIG. 5 , illustrating the front end of the stroller frame in an immobilized state compared to the free state illustrated in  FIG. 5 ; 
         FIG. 7  is a perspective view of the front end of the stroller frame; 
         FIGS. 8 and 9  are respectively larger scale views of the details VIII and IX of  FIGS. 2 and 4 ; 
         FIG. 10  is a perspective view of the left rear end of the frame; 
         FIG. 11  is a diagrammatic view in elevation according to the arrow XII of  FIG. 2 , of a portion of the frame; 
         FIG. 12  is a view similar to  FIG. 11 , illustrating the portion of the frame observed when the latter is in its configuration of  FIGS. 3 and 4 ; 
         FIG. 13  is an exploded view in perspective and partially in cross section, illustrating a first variant embodiment according to the invention; 
         FIG. 14  is a perspective view of the components represented in  FIG. 13 , in an assembled configuration; 
         FIG. 15  is a diagrammatic view partially in cross section, roughly similar to  FIG. 8 , illustrating a second variant embodiment according to the invention, which is also combined with the variant of  FIGS. 13 and 14 ; 
         FIG. 16  is a diagrammatic cross section, in a plane parallel to that of the partial cross section of  FIG. 15 , illustrating a third variant embodiment according to the invention, relating to the portion of the frame involved in  FIGS. 11 and 12 , this third variant also being combined with the variant shown in  FIG. 15 ; 
         FIG. 17  is a view in elevation according to the arrow XVII of  FIG. 16 , showing only certain components of  FIG. 16 ; 
         FIG. 18  is a diagrammatic cross section along the line of  FIG. 17 ; and 
         FIG. 19  is a view similar to  FIG. 18 , illustrating the components of  FIG. 17  in an operating configuration that is different from that shown in  FIG. 18 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The figures show a frame  2  of a stroller intended to transport a very young child, by being pushed on the ground S by an adult. As represented very diagrammatically in  FIG. 1  only, this stroller comprises, in addition to the frame  2 , a mounting  4  to which is, in this case, added and fixed a seat  6  in which the transported child is sat, it being noted that, in the interests of clarity of the drawing, the child is not shown. In a variant that is not represented, the seat  6  is replaced by a bed, making it possible to transport the child lying down, or by another similar transport element that is nonlimiting of the present invention. In practice, the mounting  4  can have extremely diverse structures, including or not including components that are articulated relative to one another, it being understood that each of these structures is supported by the frame  2  in an articulated and/or removable manner, in particular for the purposes of folding and/or storing the stroller in periods during which said stroller is not used to transport the child. 
     For convenience, the rest of the description is oriented toward the stroller frame  2  when the latter is in its service configuration illustrated in  FIGS. 1 and 2 , so that the terms “upper” and “top” designate a direction directed away from the ground S and corresponding to the top portion of  FIG. 1 , whereas the terms “lower” and “bottom” designate an opposite direction. Similarly, the terms “front” and “rear” should be interpreted relative to the normal direction of advance of the stroller, that is to say the direction in which this stroller is normally pushed, so that the front corresponds to the right hand portion of  FIG. 1  and the top portion of  FIGS. 2 to 4 . Similarly, the terms “left” and “right” are defined relative to the direction of advance of the stroller, so that they respectively designate the left-hand and right-hand portions of  FIGS. 2 and 4 . 
     The frame  2  comprises a non-articulated rigid framework which includes a single front arm  12 , two left  14 G and right  14 D rear arms and a central section  16 . The arms  12 ,  14 G and  14 D each have an essentially tubular shape extending lengthwise overall in the antero-posterior, or front to rear, direction of the frame  2 . The front arm  12  is rectilinear over its entire length, whereas the left  14 G and right  14 D rear arms converge toward one another toward the front so that each of these arms consists of a rectilinear rear portion  14 G 1 ,  14 D 1 , centered on a longitudinal axis X 14G -X 14G , X 14D -X 14D  and extended toward the front by a bent portion  14 G 2 ,  14 D 2  in the direction of the section  16 . The left  14 G and right  14 D rear arms are advantageously symmetrical to one another relative to an antero-posterior, or front to rear, median vertical plane P of the frame  2 , this plane passing through the central longitudinal axis X 12 -X 12  of the front arm  12 . 
     The central section  16  comprises a partially hollow rigid main body to which are joined the rear end of the front arm  12  and the respective front ends of the rear arms  14 G and  14 D by leading inward. In the example considered in the figures, the arm  12  is made of a single piece with the body of the section  16  whereas the rear arms  14 G and  14 D are added and fixed to this body, with an external capping fitted, particularly for esthetic purposes. 
     To push the stroller  1  on the ground S, the frame  2  is provided with three wheels bearing on the ground, namely a front wheel  20  and two rear wheels, respectively left  22 G and right  22 D, designed to rotate about respective rotation axes Y 20 -Y 20 , Y 22G -Y 22G , Y 22D -Y 22D . In practice, each of these wheels can have various embodiments, as much regarding its outer peripheral portion  20   1 ,  22 G 1 ,  22 D 1  intended to roll on the ground, which may, for example, consist of a rubber sheath or a tubed tire, as with regard to its discoid central body  20   2 ,  22 G 2 ,  22 D 2 , possibly open-worked or spoked, the central geometrical axis of which corresponds to the rotation axis Y 20 -Y 20 , Y 22G -Y 22G , Y 22D -Y 22D . 
     As can be clearly seen in  FIGS. 1 and 4 , and as represented in more detail in  FIGS. 5 to 7 , the front wheel  20  is connected to the front arm  12  by, in succession, a rigid leg  30  and a mechanical articulation assembly  32 . More specifically, the wheel body  20   2  is mounted to rotate about the axis Y 20 -Y 20  on a hub formed by a longitudinal end  30   1  of the leg  30 , said leg extending lengthwise in a direction radial to this axis. The arm  12  is provided internally with a shaft  34  which extends over the entire length of the arm, being centered on its axis X 12 -X 12 . For mechanical strength reasons, the outer diameter of this shaft  34  is fitted to the inner diameter of the arm  12 . The front end  34   1  of the shaft  34  is prolonged forward, outside the arm  12 , by an end fitting  36  supporting the articulation assembly  32 . Here, the end fitting  36  is rigidly linked to the shaft end  34   1 , being of a single piece with this end, and is conformed in a hollow sphere truncated both on the side from which the leg  30  extends and on the opposite side. Inside this sphere formed by the end fitting  36 , a pivot articulation  38  is arranged between this end fitting and the end  30   2  of the leg  30 , opposite the wheel  20 : this articulation  38  allows free pivoting of the end  30   2  relative to the end fitting  36  and therefore relative to the shaft  34  about an axis Z-Z that is substantially perpendicular to the axis X 12 -X 12 . In this way, in particular when the stroller frame  2  is rolling on the ground S, the leg  30  and, by that, the front wheel  20 , can pivot about the axis Z-Z, the top edge of the end  30   2  then sliding against the bottom face of the end fitting  36 , whereas, in the longitudinal direction of the leg  30 , said leg and the end fitting  36  are fixedly linked to one another. 
     The pivot articulation  38  can be neutralized inasmuch as it can be mechanically immobilized. In the exemplary embodiment illustrated in detail in  FIGS. 5 and 6 , this immobilization is obtained by a pin  40  that can be accommodated in a complementary recess  42  delimited at the end  30   2  of the leg  30 , in the top edge of this end: as long as the pin  40  is outside the recess  42 , as in  FIG. 6 , the leg  30  and the end fitting  36  are free to pivot relative to one another about the axis Z-Z, whereas, when the pin  40  is inserted into the recess  42  as in  FIG. 5 , the leg  30  and the end fitting  36  are rigidly linked to one another, so that the rotation axis Y 20 -Y 20  then extends orthoradially to the axis X 12 -X 12 , as in  FIGS. 1 to 4 . 
     The movement of the pin  40  is controlled by a push finger  44 , with the interposition of an overtravel spring  46  partially housed in a bush  48  mounted to slide in the end fitting  36  in the direction of movement of the pin  40 . Under the action of the spring  46 , the push finger  44  is pressed, unlike the pin  40 , against a plate  50  fixedly joined to the end fitting  36 , in this case by screws, while extending overall in its truncation plane opposite the leg  30 . The finger  44  is rigidly linked to a trim  52  mounted on the end fitting  36  so as to rotate about an axis Z 52 -Z 52  both perpendicular to the plate  50  and passing through the center of the sphere formed by the end fitting  36 . 
     By rotating the trim  52  about the axis Z 52 , the user drives, over a corresponding travel, the finger  44  which, because of its variable thickness in a direction peripheral to this axis, acts on the immobilizing pin  40 : in the configuration illustrated in  FIG. 5 , the position of the trim  52  about the axis Z 52 -Z 52  is such that a significant thickness of the finger  44  is interposed between the plate  50  and the bush  48 , whereas, in the configuration of  FIG. 6 , the position of the trim is such that a lesser thickness of the finger  44  is interposed, a spring  54  then returning the pin  40  in the direction opposite to the recess  42 . 
     To facilitate handling of the trim  52 , and for esthetic purposes, this trim has an external cap  52   1 , contained within the spherical jacket defined by the end fitting  36 . Similarly, as can be clearly seen in  FIGS. 5 and 6 , it will be noted that the end  30   2  of the leg  30  is conformed in a spherical cap contained within the abovementioned jacket, so that the assembly of this end  30   1  of the end fitting  36  and of the trim  52  gives an overall ball shape, with a visual “signature” effect, specific to the stroller frame  2 , as can be clearly seen in  FIG. 7  in which the external cap  52   1  is represented by faint lines to reveal the interior of the trim. 
     Advantageously, a light source is arranged between the plate and an open-work internal cap  52   2  of the trim  52 , so as to have a visual warning lamp. Detailed modalities for producing such a lamp are given in WO-A-2006/111656 to which the reader can refer. 
     Moreover, as can be clearly seen in  FIGS. 1 to 7 , the end fitting  36  is prolonged, forward, by a handle  60  rigidly linked to the end fitting, while being, for example, directly of a piece with the end fitting. Advantageously, this handle  60  is in the shape of a bow, the elongate main body  62  of which is situated on the front side of the end fitting  36 , extending along the front end portion of the end fitting and in a direction that is overall peripheral to the end fitting while providing between them sufficient space for the user to be able to insert his fingers therein in order to grasp the body  62 . At its longitudinal ends, the body  62  is rigidly linked to the lateral sides of the end fitting  36  by legs  64  converging toward one another in the direction of the end fitting  36 . 
     At the back of the frame  2 , the bodies  22 G 2  and  22 D 2  of the wheels  22 G and  22 D are respectively connected to the rectilinear parts  14 G 1  and  14 D 1  of the arms  14 G and  14 D according to arrangements described hereinbelow in detail for the right side of the stroller, it being understood that the arrangements of the left side can be deduced by symmetry relative to the plane P and incorporate components which, in the figures, notably  FIG. 10 , bear the same numeric references as the right side, but followed by the letter G instead of the letter D. 
     As represented in more detail in  FIGS. 8 and 9 , the body  22 D 2  of the right wheel  22 D is mounted to rotate about the axis Y 22D -Y 22D  on a hub formed by a longitudinal end  70 D 1  of a leg  70 D extending lengthwise in a direction radial to this axis. Its end  70 D 2  opposite to the wheel body  22 D 2  is, in the longitudinal direction of the leg, fixedly linked to a sleeve  72 D which both coaxially surrounds the rectilinear part  14 D 1  of the arm  14 D and extends partially protruding behind this part. Advantageously, the link between the leg  70 D and the sleeve  720  is articulated about a tilting axis  74 D parallel to the rotation axis Y 22D -Y 22D , thus enabling the leg  70 D to oscillate about this axis relative to the sleeve, with interposition of a visco-elastic pad  76 D for damping purposes. The rear wheel  22 D is thus suspended relative to the framework by the oscillating leg  700 , which makes the rolling of the stroller very comfortable for the child being transported, even if the surface condition of the ground S is poor. This suspended structure of the rear wheels  22 G and  22 D also makes it possible to avoid the need for a rigid axle directly linking these two rear wheels through the space between the rear of the arms  14 G and  14 D. The adult pushing the stroller can then walk comfortably, without the risk of banging his leg when striding. 
     The sleeve  72 D is rigidly linked to a link block  78 D arranged coaxially inside both the sleeve  72 D and the rectilinear part  14 D 1 . In the exemplary embodiment considered here, the sleeve  72 D and the link block  78 D are linked to one another at their rear end. The link block  78 D is mounted to move inside a tubular slide  80 D interposed coaxially between the link block and the rectilinear part  14 D 1 . The link block  78 D is provided with helical external ribs  78 D 1  which wind around the longitudinal axis of the link block, whereas the internal face of the slide  80 D delimits complementary helical grooves  80 D 1  in which the ribs are received. This way, the link block  78 D is able to move inside the slide  80 D by a helical movement centered on the axis X 14D -X 14D . 
     The driving of the link block  78 D relative to the slide  80 D is controlled by a pulling and pushing cord  82 D. The rear end of this cord  82 D is provided with a small end fitting  82 D 1  which is essentially spherical and thus able to rotate freely on itself with the rest of the cord  82 D inside a head  84 D that is both fixed to the front end of the link block  78 D and fixedly linked to the cord  82 D in the longitudinal direction of this cord. 
     The cord  82 D extends toward the front inside the arm  14 D, the running part of this cord being received in a sheath  86 D fixedly wedged inside the arm. In particular, the rear end of this sheath is wedged in a fixed front end fitting  80 D 2  of the slide  80 D, which is passed through from side to side by the cord  82 D. 
     As represented in more detail in  FIGS. 11 to 13 , the respective front ends  82 D 2  and  82 G 2  of the cords  82 D and  82 G are fixed to one and the same control wheel  88  rigidly assembled at the rear end  34   2  of the shaft  34 . More specifically, the control wheel  88  is centered on the axis X 12 -X 12 , whereas the rear ends  82 D 2  and  82 G 2  of the cords  82 D and  82 G are fixed in areas of the control wheel  88  that are diametrically opposite relative to the axis X 12 -X 12 . 
     The handle  60  controls the movement of the wheels  20 ,  22 G and  22 D relative to the framework  2 . More specifically, if we consider that the wheels are initially in their service position of  FIGS. 1 and 2 , that is to say, in an extended position relative to the framework, enabling them to roll on the ground S, with their rotation axis Y 20 -Y 20 , Y 22G -Y 22G  and Y 22 -Y 22D  all perpendicular to the plane P, driving the shaft  34  to rotate on itself about the axis X 12 -X 12  by virtue of the corresponding manipulation of the handle  60  by a user gripping the body  62 , as indicated by the arrow R in  FIGS. 1 and 2 , and this over an angular travel of 90° C., results in the combined retraction of the three wheels as explained hereinbelow. In other words, to do this, the user grasps the handle  60  and applies a driving torque of a quarter turn about the axis X 12 -X 12 . 
     At the front of the frame  2 , rotating the front end  34   1  of the shaft  34  results in a corresponding rotation, over an angular travel of 90°, of the end fitting  36  and, through that, of the leg  30  and of the wheel body  20   2 . The wheel  20  then switches from its position of  FIGS. 1 and 2  to its position of  FIGS. 3 and 4 , in which its rotation axis Y 20 -Y 20  extends parallel to the plane P, it being noted that, during this retraction of the wheel, the pivot articulation  38  may be immobilized by virtue of the immobilizing pin  40  previously inserted into the recess  42 , as explained above. 
     At the rear of the stroller frame  2 , rotating the rear end  34   2  of the shaft  34  results in a 90° rotation about the axis X 12 -X 12  of the control wheel  88 , which then switches from its position of  FIG. 11  to that of  FIG. 12 . The cords  82 D and  82 G are then pulled over an identical travel, causing the link blocks  78 D and  78 G to be pulled forward inside their slide  80 D and  80 G, fixed relative to the arm  14 D and  14 G. The link blocks  78 D and  78 G then describe respective helical travels, combining a forward translation movement and a 90° rotation respectively about axes X 14D -X 14D  and X 14G -X 14G . The result of this is that the rear wheels  22 D and  22 G change from their position of  FIGS. 1 and 2  to their retracted position of  FIGS. 3 and 4 , in which the rotation axes Y 22D -Y 22D  and Y 22G -Y 22G  extend parallel to the plane P, it being noted that the rear wheels remain symmetrical to one another relative to the plane P throughout their retraction. 
     The bodies  22 D 2  and  22 G 2  of the rear wheels  22 D and  22 G then extend overall in one and the same plane perpendicular to the plane P, allowing the frame  2  to roll on the ground S by rotation of the two rear wheels about their axis Y 22D -Y 22D , Y 22G -Y 22G . If necessary, the stroller frame  2  is then locked in this configuration by an ad hoc immobilizing means, such as a locking pin. The stroller frame  2  can then be moved stably in the manner of a baggage trolley with its front arm  12  and the handle  60  directed upward, so that this handle can be grasped by the user to move the stroller frame in order to stow it vertically, by rolling it into a closet for example. In a particularly practical manner, it will be understood that, when the user grasps the handle  60  when the wheels are extended and he operates this handle to retract the wheels as described above, the user concomitantly uses this handle to straighten the frame of the stroller vertically so as to automatically bring the frame to its stable position bearing on its rear wheels. 
     Furthermore, because of the helical travel of the retraction movement of the rear wheels  22 G and  22 D, it will be understood that the distance Δ between the rotation axis Y 20 -Y 20  of the front wheel  20  and the rotation axis Y 22D -Y 22D , Y 22G -Y 22G  of each rear wheel  22 D,  22 G varies by the antero-posterior dimension of the part of the link blocks  78 D and  78 G retracted inside their corresponding slide  80 D,  80 G, which may in practice be as much as 40 mm. Consequently, the overall antero-posterior dimension of the stroller frame  2  is reduced by Δ when the wheels are retracted. 
     Obviously, the control by the handle  60  of the 90° rotation of the shaft  34  in the reverse direction to that considered above, when the wheels  20 ,  22 D and  20 G are initially retracted, results, through a transmission of reverse movements, in the concomitant extension of these three wheels. 
     Optionally, a longitudinal part of each cord  82 D,  82 G is advantageously made of a metal alloy with shape memory, such as the alloy based on nickel and titanium, marketed by the company NIMESIS (France), so that this cord part is able to be twisted, or even folded without damage, by transverse elastic deformation, when the user controls the retraction or the extension of the wheels when said retraction is inadvertently prevented, for example, following the wedging of a wheel against a wall. In this way, damage to the rest of the cord is avoided, which could have damaging consequences for the correct transmission of the pulling and pushing forces that must normally be provided by this cord. 
       FIGS. 13 and 14  show a variant embodiment of the link blocks  78 D and  78 G and the slides  80 G and  80 D of the stroller frame  2 . The corresponding arrangements of this variant are described in detail hereinbelow for the left side of the stroller frame  2 , it being understood that the arrangements of the right side can be deduced by symmetry relative to the plane P. 
     Thus, according to this variant of  FIGS. 13 and 14 , the link block  78 G and the slide  80 G are replaced respectively by a link block  78 G′ and a slide  80 G′. The arrangement, within the stroller frame  2 , and the function of this link block  78 G′ and of this slide  80 G′ are similar to the arrangement and the function of the link block  78 G and of the slide  80 G: in particular, the link block  78 G′ is arranged coaxially inside the sleeve  72 G as diagrammatically represented in  FIG. 15 , while being rigidly linked to this sleeve  72 G at their rear end, whereas the slide  80 G′ is fixedly arranged inside the rectilinear part  14 G 1  of the arm  14 G, also as shown in  FIG. 15 . 
     The link block  78 G′ and the slide  80 G′ are mainly distinguished from the link block  78 G and from the slide  80 G by arrangements relating to the guiding of the link block  78 G′ inside the slide  80 G′ by an overall helical movement centered on the axis X 14G -X 14G  of the arm  14 G. In practice, by replacing the helical ribs and grooves used in the embodiment of  FIGS. 1 to 12 , the tubular wall of the slide  80 G′ is passed through, from side to side, by two diametrically opposite slots  80 G′ 1 , symmetrically relative to the axis X 14G -X 14G . Each groove  80 G′ 1  includes, along its length, a helical front end part  80 G′ 2 , centered on the axis X 14G -X 14G , and a rectilinear rear end part  80 G′ 3 , parallel to this axis. When the link block  78 G′ is assembled inside the slide  80 G′, each of these grooves  80 G′ 1  receives one of the longitudinal ends of a transfixing locking pin  78 G′ 1  rigidly linked to the link block  78 G′, extending perpendicularly to the axis X 14G -X 14G , one of the longitudinal ends of this locking pin  78 G′ 1  being visible in  FIG. 14 , with the reference  78 G′ 2 . The width of each groove  80 G′ 1  is fitted to that of the ends  78 G′ 2  of the locking pin  78 G′ 1 , so that, during the relative movement between the link block  78 G′ and the slide  80 G′, the corresponding movement successively consists of a first helical sub-movement, when the ends  78 G′ 2  of the locking pin  78 G′ 1  pass through the helical part  80 G′ 2  of the grooves  80 G′ 1  and a second rectilinear sub-movement when the abovementioned ends pass through the rectilinear part  80 G′ 3  of the grooves. 
     Thus, by cooperation between the locking pin  78 G′ 1  and the helical parts  80 G′ 2  of the grooves  80 G′ 1 , the antero-posterior guidance between the link block  78 G′ and the slide  80 G′ by a helical movement, substantially as for the embodiment of  FIGS. 1 to 12 , is restored. Furthermore, by cooperation between the locking pin  78 G′ 1  and the rectilinear parts  80 G′ 3  of the grooves  80 G′ 1 , the relative angular position between the link block  78 G′ and the slide  80 G′ is fixed: this configuration corresponds to the extreme rear placement of the link block  78 G′ relative to the slide  80 G′, as shown in  FIG. 14 , that is to say to the configuration in which the rear wheels  22 G and  22 D are extended. The mechanical strength of these wheels in the extended configuration is thus enhanced. 
     Advantageously, this mechanical strength is reinforced by the presence, on the outer surface of the link block  78 G′, of a crenellated ring  78 G′ 3  comprising a plurality of axial ribs  78 G′ 4  which are distributed on the outer periphery of the link block and that are conformed to be received in a complementary manner in axial grooves  80 G′ 4  hollowed out in the internal face of the slide  80 G′, in this case at the rear end of this slide. By appropriate dimensioning, these ribs  78 G′ 4  and these grooves  80 G′ 4  are progressively axially geared to one another when the locking pin  78 G′ 1  passes through the rectilinear parts  80 G′ 3  of the grooves  80 G′ 1 , from the front end of these rectilinear parts to their rear end. The forces then applied to an ad hoc locking means, immobilizing the stroller frame  2  with its wheels in extended configuration, thus remain limited and supported by this immobilizing means, without damage. 
     By way of optional advantageous arrangement, not represented in detail in the figures, the helical parts  80 G′ 2  of the grooves  80 G′ 1  are prolonged forward by rectilinear groove parts that do not need to be as long as the rear rectilinear parts  80 G′ 3  and that make it possible to stabilize the relative angular position between the link block  78 G′ and the slide  80 G′ when the corresponding rear wheel  22 G is retracted, by being folded back against the framework  10  as explained above. The resistance to the forces then applied to the duly folded stroller frame  2  is enhanced. 
       FIGS. 15 and 16  represent another variant embodiment of the stroller frame  2  which, apart from what has just been described with regard to  FIGS. 13 and 14 , is distinguished from the embodiment described with regard to  FIGS. 1 to 12  essentially by the structure, detailed hereinbelow, of the driving means of the link blocks  78 D and  78 G or  78 D′ and  78 G′ relative to their corresponding rear arm  14 D and  14 G, and, incidentally, by slight contour modifications to certain components associated with these rear arms, without, however, these slight modifications inducing significant structural and functional differences. 
     Thus, rather than drive each link block  78 D,  78 G by the pulling and pushing cord  82 D or  82 G as described with regard to  FIGS. 8 and 9 , the variant of  FIGS. 15 and 16  provides for the use of an actuator in the form of a semi-rigid rod  82 D′ or  82 G′, it being noted that only the rod  82 G′ associated with the left rear arm  14 G can be seen in  FIGS. 15 and 16 . Moreover, the corresponding arrangements for the left side of the stroller frame  2  are described in more detail hereinbelow, it being understood that the arrangements of the right side can be deduced by symmetry relative to the plane P. 
     Thus, with regard to  FIGS. 15 and 16 , the semi-rigid rod  82 G′ links in movement the link block  78 G′ and the rear end  34   2  of the shaft  34 . To this end, the rear end  82 G′ 1  of this rod is fixedly joined to a front end head  84 G′ of the link block  78 G′, in this case by having a bulbous shape, received and retained in a complementary recess delimited inside the head  84 G′. Advantageously, the head  84 G′ is fixed relative to the link block  78 G′ by the transfixing locking pin  78 G′ 1 . 
     The running part  82 G′ 2  of the semi-rigid rod  82 G′ is received in a sheath  86 G′ that is functionally similar to the sheath  86 G: this sheath  86 G′ is thus fixedly incorporated inside the arm  14 G with, in particular, its rear end wedged in a fixed front end fitting  80 G′ 5  of the slide  80 G′ whereas this end fitting is passed through from side to side by the rod  82 G′. The flexibility of the running part of the rod  82 G′ 2  is exploited, on the one hand, to adapt the overall longitudinal profile of the rod  82 G′ to the bent part  14 G 2  of the arm  14 G, by bending this profile, and on the other hand, to support, without damage, a slight twist of the rod  82 G′, associated with the helical movement of the link block  78 G′. The result of this last aspect is that the mechanical link between the rod  82 G′ and the link block  78 G′ does not have to be as sophisticated as that between the cord  82 G and the link block  78 G, in particular without the need for a small spherical end piece to be provided at the rear end of this cord. 
     As an example, to obtain sufficient flexibility of the running part of the rod  82 G′ 2 , while guaranteeing sufficient force transmission by this running part of the rod, in a longitudinal direction of the latter, this running part  82 G′ 2  has a cross-shaped transversal section and is made of a synthetic material such as nylon. 
     The front end  82 G′ 3  of the rod  82 G′ is rigidly provided with an insert  82 G′ 4 , for example overmolded with the front end of the rod, as represented in  FIG. 16 . At its end opposite to the rod  82 G′, this insert  82 G′ 4  is fixed to a control wheel  88 ′, arranged at the rear end  34   2  of the shaft  34  and functionally similar to the control wheel  88  shown in  FIGS. 11 and 12 . In the exemplary embodiment considered in  FIG. 16 , the control wheel  88 ′, centered on the axis X 12 -X 12 , is provided, in diametrically opposite areas relative to this axis, with two protruding spheres  88 G′ 1  and  88 D′ 1 , designed to be respectively received, in the manner of a ball joint, in a complementary shell  82 G′ 5  rigidly borne by the insert  82 G′ 4  of the rear end of the corresponding left rod  82 G′ or right rod  82 D′. 
     Thus, as for the control wheel  88 , the rotation of the control wheel  88 ′ by the shaft  34 , over a 90° travel centered on the axis X 12 -X 12 , results, depending on the direction of this rotation, in the semi-rigid rods  82 G′ and  82 D′ being pulled forward or pushed backward in order to actuate the movement of the link blocks  78 G′ and  78 D′ inside their slide  80 G′ and  80 D′. By comparison with the cords  82 G and  82 D, the actuating rods  82 G′ and  82 D′ have the advantage of being standard mechanical components that are widely available on the market, which support, without damage, the transmission of the forces needed to extend and retract the rear wheels  22 G and  22 D. 
       FIGS. 16 to 19  show yet another variant embodiment of the stroller frame  2 , which specifically relates to the mechanical link between the rear end  34   2  of the shaft  34  and the control wheel  88  or  88 ′, this variant being applied here to the control wheel  88 ′ and thus detailed hereinbelow in this context. Rather than rigidly joining this control wheel to the rear end  34   2  of the shaft  34 , as in the embodiment considered in  FIGS. 1 to 12 , a horseshoe-type spring  90  is interposed between them in order to limit the intensity of the torque transmitted between the control wheel  88 ′ and the shaft end  34   2 . 
     More specifically, this spring  90  is conformed to grip, in a manner centered on the axis X 12 -X 12 , a bush  34   3  that is rigidly linked to the shaft end  34   2  and to the central region  88 ′ 1  of the control wheel  88 ′: the stiffness of the spring  90  is engineered to immobilize the relative rotation of the bush  34   3  and the region of the control wheel  88 ′ 1  about the axis X 12 -X 12 , as long as the intensity of the torque transmitted between these two pieces is below a predetermined limit value, as represented in  FIG. 18 . In this way, if a relative torque, about the axis X 12 -X 12 , between the shaft  34  and the control wheel  88  is applied with an intensity above the abovementioned limit value, the spring  90  is deformed elastically to allow a relative free rotation of the shaft  34  on itself and of the control wheel  88 ′ on itself, as represented in  FIG. 19 . 
     In this way, when the user retracts or extends the wheels of the stroller frame  2  by the handle  60 , when this retraction or this extension is inadvertently prevented at one of the rear wheels  22 G and  22 D, for example following the jamming of this wheel against a wall, the kinematic resistance of this wheel induces, on the control wheel  88 ′, a torque differential about the axis X 12 -X 12  relative to the shaft  34 : if the intensity of this torque differential is above the abovementioned limit value, the user continues rotating the shaft  34  without the corresponding force being transmitted to the control wheel  88 ′ and to the rods  82 G′ and  82 D′ by virtue of the spring  90 , thus avoiding damage to this control wheel and these rods. 
     Various arrangements and variants of the stroller frame  2  described hitherto can also be considered. A few examples are given below:
         each wheel  20 ,  22 D and  22 G or at least one of these wheels can be equipped with a rotation immobilizing means in order to immobilize the stroller frame  2  on the ground S for the purpose of parking for a certain time;   the front wheel  20  and/or each of the rear wheels  22 D and  22 G can be replaced by a twinned pair of wheels, or even a twinned wheel train;   moreover, to link the front wheel  20  to the arm  12 , the leg  30  may be replaced by a fork, the two branches of which extend either side of the wheel body  20   2 ;   optionally, the rotation of the shaft  34  by the handle  60  when retracting or extending the wheels can be transmitted, by appropriate arrangements, to the structure  4  of the stroller, in particular in order to respectively assist in the folding and the opening of this structure relative to the frame; and/or   also optionally, the structure of the stroller frame  2  can be reinforced by a cross-member rigidly linking the rear arms  14 D and  14 G, in particular in their rectilinear part  14 D 1  and  14 G 1 ; for example, the opposite ends of such a cross-member are fixedly joined to the respective fixed caps that externally clamp the arm parts  14 D 1  and  14 G 1 , as partially represented in  FIG. 15 , in which the abovementioned cross-member is referenced  92 .