Abstract:
The present invention relates to a twin-frame bicycle, particularly to one that can be pedaled by two riders in side-by-side coordination so that the riders and the twin-frame bicycle will incline integrally without tumbling or overturning. At least one articulation mechanism is disposed between a primary bicycle body and an auxiliary bicycle body, wherein the articulation mechanism serves in synergy an articulation parallelogram, and a resilient tension mechanism having an elastic element is disposed between the primary bicycle body and the auxiliary bicycle body and counteracts the deformation of the articulation mechanism to maintain equilibrium.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a bicycle, particularly to a twin-frame bicycle. 
         [0003]    2. Description of the Related Art 
         [0004]    Bicycle is a very popular human-powered transportation means. A bicycle is unstable when standing still and must rely on the rider to maintain equilibrium. Upon driving turns, inertia makes the bicycle body keep forwarding along the original direction. If the center of gravity is too high, bicycle speed is too fast or the bicycle turns urgently, even friction force is enough to offer the necessary centripetal force, the centrifugal moment may cause the bicycle body to rotate and overturn. 
         [0005]    When a bicycle turns, by inclination of the driver&#39;s body and the bicycle body, the reversed moment can be created to reduce the centrifugal moment which may cause the bicycle to overturn outwards. Because of this, bicycle is naturally more stable in motion than tricycle or four-wheeled vehicle. Currently, one kind of dual-riding bicycle of tandem type already exists in the market. The two driver saddles of a tandem bicycle are arranged one after the other instead of side by side. Just like the conventional bicycle, the tandem bicycle must rely on the rider to maintain equilibrium when standing still. Another kind of dual-riding bicycle, of parallel type, also exists. However, the structure of a parallel dual-riding bicycle links two independent bicycles by a fixing rod, which results in awkward action at high speed or when turning. At high speed, sharp turn or bumpy road may cause the parallel dual-riding bicycle to overturn. Thus, it is only good for children learning to walk. 
         [0006]    Consequently, there is a need for a twin-frame bicycle to solve the above-mentioned problems. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a twin-frame bicycle, which includes a primary bicycle body, an auxiliary bicycle body, at least one articulation mechanism and a resilient tension mechanism. The primary bicycle body has a primary bicycle frame, a front wheel, a rear wheel and a pedal mechanism. The front wheel and the rear wheel are connected pivotally to the primary bicycle frame and distanced from each other. The pedal mechanism is used to drive the front wheel or the rear wheel. The auxiliary bicycle body has an auxiliary bicycle frame and a first auxiliary wheel. The auxiliary wheel is connected pivotally to the auxiliary bicycle frame. The articulation mechanism has a first end, a second end and at least two linking portions. The first end and the second end are connected to the primary bicycle frame and the auxiliary bicycle frame, respectively. The linking portions are substantially disposed in parallel between the first end and the second end. The first end, the second end and the linking portions form a movable quadrangle. The resilient tension mechanism has an elastic element disposed between the primary bicycle body and the auxiliary bicycle body. 
         [0008]    The twin-frame bicycle can be pedaled by two riders riding side by side or pedaled individually. Besides providing fun riding, the articulation mechanism linking the primary bicycle body and the auxiliary bicycle body makes the twin-frame bicycle stable and unlikely to tumble or overturn. In addition, since the structure of the auxiliary bicycle body is simple, the cost is saved and the maintenance of the twin-frame bicycle is easy. The structure of the articulation mechanism can be further simplified, thus saving more production cost. Furthermore, by utilizing the articulation mechanism together with the resilient tension mechanism having an elastic element, the twin-frame bicycle can always stand straight up and be controlled much smoothly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is a perspective schematic view of a twin-frame bicycle of the present invention; 
           [0010]      FIG. 1B  is a perspective schematic view of another embodiment of a twin-frame bicycle of the present invention, wherein the auxiliary bicycle frame of the twin-frame bicycle has an additional wheel; 
           [0011]      FIG. 2A  is the right lateral view of  FIG. 1A ; 
           [0012]      FIG. 2B  is the right lateral view of  FIG. 1B ; 
           [0013]      FIG. 3  is the top view of  FIG. 1 ; 
           [0014]      FIG. 4  is an operational view showing the twin-frame bicycle of the present invention during right turn; 
           [0015]      FIG. 5  is an operational view showing the twin-frame bicycle of the present invention passing over a bumpy road; 
           [0016]      FIG. 6  is a schematic view of an articulation mechanism of the present invention; 
           [0017]      FIG. 7  is a front view of the articulation mechanism of the present invention; 
           [0018]      FIG. 8  is an operational view showing the articulation mechanism of  FIG. 7 ; 
           [0019]      FIG. 9  is a schematic view of another embodiment of an articulation mechanism of the present invention; 
           [0020]      FIG. 10  is an operational view showing the articulation mechanism of  FIG. 9 ; 
           [0021]      FIG. 11  is a schematic view of another embodiment of an articulation mechanism of the present invention; 
           [0022]      FIG. 12  is an operational view showing the articulation mechanism of  FIG. 11 ; 
           [0023]      FIG. 13  is a schematic view of another embodiment of an articulation mechanism of the present invention; 
           [0024]      FIG. 14  is the right lateral view of  FIG. 13 ; 
           [0025]      FIG. 15  is a perspective schematic view of another embodiment of a twin-frame bicycle of the present invention; 
           [0026]      FIG. 16  is the right lateral view of  FIG. 15 ; 
           [0027]      FIG. 17  is the top lateral view of  FIG. 15 ; 
           [0028]      FIG. 18  is a cross-sectional view showing a wheel structure of an auxiliary bicycle body according to  FIG. 15  of the present invention; 
           [0029]      FIG. 19  is a schematic view of another embodiment of an auxiliary bicycle body of the present invention; 
           [0030]      FIG. 20  is a schematic view of a further embodiment of a twin-frame bicycle of the present invention; 
           [0031]      FIG. 21  is a schematic view of a gear assembly disposed at a rear folk of the auxiliary bicycle body according to  FIG. 20  of the present invention; 
           [0032]      FIG. 22  is a perspective schematic view of a twin-frame bicycle which has two articulation mechanisms of the present invention; and 
           [0033]      FIGS. 23-26  are schematic views of another embodiment of a twin-frame bicycle of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    As shown in  FIGS. 1A-3  and  7 , a twin-frame bicycle includes a primary bicycle body  1 , an auxiliary bicycle body  3 , an articulation mechanism  5  and a resilient tension mechanism  4 . A saddle  15  of the primary bicycle body  1  and a saddle  35  of the auxiliary bicycle body  3  are arranged in side-by-side juxtaposition instead of one behind the other. 
         [0035]    In this embodiment, the primary bicycle body  1  has a primary bicycle frame  100 , a front wheel  19 , a rear wheel  18  and a pedal mechanism. The front wheel  19  and the rear wheel  18  are connected pivotally to the primary bicycle frame  100  and distanced from each other. The pedal mechanism is used to drive the front wheel  19  or the rear wheel  18 . The auxiliary bicycle body  3  has an auxiliary bicycle frame  310  and a first auxiliary wheel  43 . The first auxiliary wheel  43  is connected pivotally to the auxiliary bicycle frame  310 . The articulation mechanism  5  has a first end  50 , a second end  59  and at least two linking portions  53 ,  54 . The first end  50  and the second end  59  are connected to the primary bicycle frame  100  and the auxiliary bicycle frame  310 , respectively. In this embodiment, the linking portions  53 ,  54  are substantially disposed in parallel between the first end  50  and the second end  59 . The first end  50 , the second end  59  and the linking portions  53 ,  54  form a movable quadrangle. The movable quadrangle can move vertically corresponding to the ground plane. 
         [0036]    The primary bicycle body  1  can be a conventional bicycle having a basic structure. In this embodiment, the primary bicycle frame  100  of the primary bicycle body  1  comprises a cross top bar  11 , a head sleeve tube  12 , a bottom bar  13  and an upright seat bar  14 . The saddle  15  is disposed on the top of the upright seat bar  14 . An axle  23  connected with a front folk  17  and a steering handle  24  is inserted pivotally in the head sleeve tube  12 , and the steering handle  24  and the front folk  17  are disposed pivotally at corresponding ends of the head sleeve tube  12 . The front wheel  19  is connected pivotally to the front folk  17 . The upright seat bar  14  connects a rear folk  16 , and the rear wheel  18  is connected pivotally to the rear folk  16 . Disposed pivotally in a common joint of the bottom bar  13  and the upright seat bar  14  are a front sprocket wheel  20  and a pedal mechanism which is used to drive the front sprocket wheel  20 . In this embodiment, the pedal mechanism comprises a pair of cranks and pedals  25 ,  26 , the cranks are connected to both sides of the center of the front sprocket wheel  20 , and the pedals  25 ,  26  are connected to the cranks. A rear sprocket wheel  21  is disposed pivotally in hub of the rear wheel  18 , and the front sprocket wheel  20  and the rear sprocket wheel  21  are linked by a chain  22  in engagement manner. 
         [0037]    The first auxiliary wheel  43  is connected pivotally to the auxiliary bicycle frame  310 , and the auxiliary bicycle body  3  may not have a front folk, a rear folk, a front wheel and a rear wheel but it still has a cross top bar  31 , a head sleeve tube  32 , a bottom bar  33 , an upright seat bar  34 , a saddle  35 , an active gear  371 , a passive gear  372 , a middle gear  373 , a stem  39 , a steering handle  40  and a pedal mechanism. In this embodiment, the pedal mechanism comprises a pair of cranks  411 ,  421  and a pair of pedals  41 ,  42 , the cranks  411 ,  421  are connected to both sides of the shaft of the first auxiliary wheel  43 , and the pedals  41 ,  42  are connected to the cranks  411 ,  421 , respectively. The steering handle  40  is connected pivotally to the head sleeve tube  32 , the saddle  35  is disposed on the top of the upright seat bar  34 , and the pedal mechanism of the auxiliary bicycle body drives the first auxiliary wheel  43 . 
         [0038]    In other embodiments, the auxiliary bicycle body  3  can further comprise an additional wheel  47  and an adjustable bar  48  (as shown in  FIGS. 1B and 2B ). One end of the adjustable bar  48  is connected to the auxiliary bicycle frame  310 , and the additional wheel  47  is disposed at the other end of the adjustable bar  48 . In this embodiment, the adjustable bar  48  is disposed at the head sleeve tube  32  and the additional wheel  47  is smaller than the first auxiliary wheel  43 . The front wheel  19 , the rear wheel  18  and the first auxiliary wheel  43  contact the ground plane, and the additional wheel  47  may optionally contact the ground plane. Preferably, the additional wheel  47  does not contact the ground plane; however, the additional wheel  47  may contact the ground plane when making a turn or a brake. As a result, the twin-frame bicycle of the invention is highly stable without any possibility of tumbling, even in a high-speed riding and/or a sudden braking. 
         [0039]    The bottom bar  33  and the upright seat bar  34  are not very long and from two ends extend to connect two side bars  331 ,  332  and two upright side bars  341 ,  342 , respectively. The side bars  331 ,  332  and the upright side bars  341 ,  342  are connected two fixing bases  45 ,  46 . The first auxiliary wheel  43  having a rung is disposed pivotally between the fixing bases  45 ,  46 . A shaft  430  is disposed pivotally at the center of the first auxiliary wheel  43 , and the fixing bases  45 ,  46  are disposed at the corresponding ends of the shaft  430 . In this embodiment, two ends of the shaft  430  are connected to the fixing bases  45 ,  46 . The middle gear  373  is disposed between the active gear  371  and the passive gear  372 . The cranks  411 ,  421  connected with the pedals  41 ,  42  are connected to the corresponding cranks  411 ,  421 . The passive gear  372  is disposed pivotally at the shaft  430  and moves along with the rung. Through the cranks  411 ,  421 , the pedals  41 ,  42  drive the active gear  371 , so as to drive the middle gear  373  and the passive gear  372 . It should be noted that in other embodiments, the auxiliary bicycle body  3  can optionally comprise a plurality of middle gear  373  between the active gear  371  and the passive gear  372  according to various designs. 
         [0040]    The resilient tension mechanism  4  has an elastic element  55  disposed between the primary bicycle body  1  and the auxiliary bicycle body  3 . As shown in  FIGS. 1A and 6 , in this embodiment, two ends of the resilient tension mechanism  4  are connected to the first end  50  and the second end  59 , respectively, and the elastic element  55  is substantially parallel to the linking portions  53 ,  54 . It should be noted that in other embodiments, two ends of the resilient tension mechanism  4  can be connected directly to the primary bicycle body  1  and the auxiliary bicycle body  3 , respectively, and the elastic element  55  is substantially orthogonal to the linking portions  53 ,  54 . 
         [0041]    In this embodiment, the first end  50  of the articulation mechanism  5  is the extremity of a left holding base  51 , and the left holding base  51  is screw-fixed to a connecting bulk  10  around the head sleeve tube  12 ; the second end  59  is the extremity of a right holding base  52 , wherein a bolt  301  passes through fixing holes  300  of a connecting bulk  30  around the head sleeve tube  32  and into round holes  520  to fixes the right holding base  52 . The holding bases  51  have a holding cavity  511  and a holding cavity  521 , and the linking portions  53 ,  54  are disposed pivotally in corresponding holding cavities  511 ,  521 . 
         [0042]    As shown in  FIGS. 7 and 8 , in this embodiment, the first end  50  further comprises a first foundation  503 , the second end  59  further comprises a second foundation  593 , and the two ends of the elastic element  55  are connected to the first foundation  503  and the second foundation  593 , respectively. The first foundation  503  and the second foundation  593  are protrusions, and the two ends of the elastic elements  55  are connected to corresponding ends of the protrusions, respectively. When the elastic element  55  deforms, the elastic potential energy caused by the deformed elastic element  55  can provide a require resilient for reinstating the articulation mechanism  5 , so as to balance the primary bicycle body  1  and the auxiliary bicycle body  3 . 
         [0043]    As shown in  FIGS. 9 and 10 , in another embodiment, the resilient tension mechanism  4  comprises a first adjustable unit  551  and a second adjustable unit  552  disposed at two ends of the elastic element  55 . The first adjustable unit  551  comprises a first stepping portion  5511  and a first adjusting portion  5512 , the second adjustable unit  552  comprises a second stepping portion  5521  and a second adjusting portion  5522 . The first foundation  503  comprises a first recess  504 , and the second foundation  593  comprises a second recess  594 . The first stepping portion  5511  and the second stepping portion  5521  penetrate the first foundation  503  and the second foundation  593 , respectively. The first adjusting portion  5512  and the second adjusting portion  5522  are disposed in the first recess  504  and the second recess  594  and connected to the first stepping portion  5511  and the second stepping portion  5521 , respectively. Two ends of the resilient tension mechanism  4  are connected to the primary bicycle body  1  and the auxiliary bicycle body  3 , respectively, and the elastic element  55  is substantially parallel to the linking portions  53 ,  54 . Through adjusting the first stepping portion  5511  and the second stepping portion  5521  by the first adjusting portion  5512  and/or the second adjusting portion  5522 , the length of the elastic element  55  is changed, and the deformed elastic element  55  can provide a require resilient force for reinstating the articulation mechanism  5 , so as to balance the primary bicycle body  1  and the auxiliary bicycle body  3 . 
         [0044]    It should be noted that, as shown in  FIGS. 11 and 12 , in other embodiment, two ends of the resilient tension mechanism  4  can be connected to the at least two linking portions  53 ,  54 , respectively. In this embodiment, the elastic element  55  is substantially orthogonal to the linking portions  53 ,  54 . That is, the first stepping portion  5511  and the second stepping portion  5521  penetrate the linking portions  53 ,  54 , respectively; the first adjusting portion  5512  and the second adjusting portion  5522  are connected to the first stepping portion  5511  and the second stepping portion  5522 , respectively; and the linking portions  53 ,  54  are between the first adjusting portion  5512  and the second adjusting portion  5522 . The first adjusting portion  5512  and/or the second adjusting portion  5522  can adjust the first adjusting portion  5512  and the second adjusting portion  5522  to change the length of the elastic element  55 . Similarly, through adjusting the first stepping portion  5511  and the second stepping portion  5521  by the first adjusting portion  5512  and/or the second adjusting portion  5522 , the length of the elastic element  55  is changed, and the deformed elastic element  55  can provide a require resilient force for reinstating the articulation mechanism  5 , so as to balance the primary bicycle body  1  and the auxiliary bicycle body  3 . 
         [0045]    As shown in  FIGS. 13 and 14 , an articulation mechanism  5  according to another embodiment of the present invention is shown. The articulation mechanism  5  of this embodiment is substantially the same as that in  FIGS. 7 and 8  of the present invention, and the same elements are designated with the same numerals and are therefore not described in detail. The difference between the above two embodiments lies in that, in this embodiment, the articulation mechanism  5  comprises four linking portions  53 ,  54 , wherein two corresponding sides of the first end  50  has two slots  501 ,  502  and two corresponding sides of the second end  59  has two slots  591 ,  592 . The linking portions  53 ,  54  are disposed pivotally in the corresponding slots  501  and  591  and in the corresponding slots  502  and  592  respectively. 
         [0046]    As shown in  FIGS. 5 ,  7  and  8 , when the twin-frame bicycle of the present invention moves over an uneven or bumpy road with some drops in height which make the road surface under the auxiliary bicycle body  3  higher than that under the primary bicycle body  1 , the right end of the linking portions  53 ,  54  will slant downwards at a certain angle to offset the difference in height between the auxiliary bicycle body  3  and the primary bicycle body  1 , and the tolerance between the linking portions  53 ,  54  and the holding bases  51 ,  52  will keep the auxiliary bicycle body  3  and the primary bicycle body  1  moving in parallel even under relative shift disposition or movement. In addition, the relative shift disposition or movement of the articulation mechanism  5  will make the elastic element  55  stretch and the elastic element  55  compress so that the resilient force of the stretched elastic element  55  will help keep the twin-frame bicycle in stable linkage. By the counterforce of the elastic element  55 , both the auxiliary bicycle body  3  and the primary bicycle body  1  are kept stable and linked without inclining or tumbling. When the twin-frame bicycle of the present invention makes a right turn, the movements of the articulation mechanism  5  are as shown in  FIGS. 4 and 8 . When the twin-frame bicycle of the present invention makes a left turn, the movements of the articulation mechanism  5  are the inverse of those shown in  FIGS. 4 and 8  and therefore not described in detail here. When the twin-frame bicycle of the present invention moves on an uneven and bumpy road with some drops in height and makes a left or right turn, the parallelogram synergy formed by the linking portions  53 ,  54  and the holding bases  51 ,  52  is just like an smoothly movable articulated joint, and the confined movement of the linking portions  53 ,  54  limited by the holding bases  51 ,  52  (holding cavities  511 ,  521 ) not only keeps both the auxiliary bicycle body  3  and the primary bicycle body  1  in automatically balanced coordination but also prevents the twin-frame bicycle of the present invention from overturning or tumbling. 
         [0047]    The holding bases  51 ,  52  and the linking portions  53 ,  54  of the present invention can be made of rigid material (such as steel). Furthermore, to avoid strain due to exceeding stress, the contacting areas between both ends of the linking portions  53 ,  54  and the holding bases  51 ,  52  are preferably enlarged. 
         [0048]    The auxiliary bicycle body  3  is arranged in parallel to the primary bicycle body  1  and, the shaft  430  of the first auxiliary wheel  43  is disposed between the shaft  190  of the front wheel  19  and shaft  180  of the rear wheel  18 , i.e., the first auxiliary wheel  43  is disposed between the front wheel  19  and the rear wheel  18 , as shown in  FIGS. 2A and 3 . 
         [0049]    According to the present invention, the front wheel  19 , the rear wheel  18  and the first auxiliary wheel  43  form a triangular base for the twin-frame bicycle and the riders&#39; center of gravity is within the triangular base, which keeps the riders stable without any possibility of tumbling. Furthermore, the auxiliary bicycle body  3  can be chainless, and the twin-frame bicycle of the present invention can comprise two pedal mechanisms. Since the shaft of the active gear  371  may not interfere with the auxiliary wheel  43  (such as by utilizing the arrangement of middle gears  373  disposed between the active gear  371  and the passive gear  372 ), the bottom bracket can be flexibly designed with preferred value (BB value). Therefore, large or small auxiliary wheel can be suitable for the auxiliary bicycle body  3 , and the riders&#39; center of gravity is correspondingly above the fulcrum of the first auxiliary wheel  43 , which keeps the riders comfortable and the twin-frame bicycle stable without any possibility of tumbling. 
         [0050]    The steering handle  24  of the primary bicycle body  1  and the steering handle  40  of the auxiliary bicycle body  3  are adjustable, and this is a known technique and not described in detail here. Since the twin-frame bicycle of the present invention comprises the front wheel  19 , the rear wheel  18  and the first auxiliary wheel  43 , a brake line of the primary bicycle body  1  may be connected to the first auxiliary wheel  43  to brake the first auxiliary wheel  43 , and this is a known technique and not described in detail or shown in the figures here. For safety purpose, a brake handle  101  can control the brake line connected to both the rear wheel  18  of the primary bicycle body  1  and the first auxiliary wheel  43  of the auxiliary bicycle body  3  to brake the twin-frame bicycle. Since the auxiliary bicycle body  3  does not comprise a front wheel and a rear wheel, a brake handle  102  of the primary bicycle body  1  is used only to brake the front wheel  19 . 
         [0051]    Another embodiment of an auxiliary bicycle body  6  is shown in  FIGS. 15 to 18 . Like the above-mentioned auxiliary bicycle body  3 , the auxiliary bicycle body  6  may not comprise a front folk, a front wheel and a rear wheel but comprises a cross top bar  61 , a head sleeve tube  62 , a bottom bar  63 , an upright seat bar  64 , a saddle  65 , a rear folk  66 , a big sprocket wheel  671 , a small sprocket wheel  672 , an axle  69 , a steering handle  70  and a pair of pedals  71 ,  72 . The first auxiliary wheel  73  and an auxiliary sprocket wheel are connected pivotally to the auxiliary bicycle body  6 . In this embodiment, the auxiliary bicycle body  6  further comprises two side bars  631 ,  632  and two upright bars  641 ,  642 . The big sprocket wheel  671  and the first auxiliary wheel  73  are connected pivotally to the auxiliary bicycle body  6 , and the big sprocket wheel  671  can be regarded as the auxiliary sprocket wheel. 
         [0052]    Compared to the above-mentioned auxiliary bicycle body  3 , the auxiliary bicycle body  6  further comprises the rear folk  66 , and two side supporting bar  661 ,  662  are disposed at lower portion of the rear folk  66 . The bottom bar  63  and the upright seat bar  64  are not very long and from two ends extend to connect two side bars  631 ,  632  and two upright side bars  641 ,  642 , respectively. The side bars  631 ,  632 , the upright side bars  641 ,  642  and the side supporting bar  661 ,  662  are connected two fixing base  75 ,  76 , and the first auxiliary wheel  73  having a rung  74  is disposed pivotally between the fixing base  75 ,  76 . In addition, a shaft  730  is disposed pivotally at the center of the first auxiliary wheel  73 . 
         [0053]    Two cranks  711 ,  721  connected with the pedals  71 ,  72  are connected to the corresponding ends of the shaft  730 . Through the cranks  711 ,  721  the pedals  71 ,  72  drive the big sprocket wheel  671 , and the small sprocket wheel  672  is disposed above the rung  74  and moves along with the rung  74 . One end of the rear folk  66  is connected to the upright seat bar  64 , and a speed-changing sprocket wheel assembly  68  is disposed at the other end of the rear folk  66 . The speed-changing sprocket wheel assembly  68  is connected to the side supporting bar  661  which is on the same side as the big sprocket wheel  671 , and the speed-changing sprocket wheel assembly  68  and the big sprocket wheel  671  are disposed at two ends of the side supporting bar  661 . Two sprocket wheels  681 ,  682  of the speed-changing sprocket wheel assembly  68  engage with the big sprocket wheel  671  and the small sprocket wheel  672  through two chains  673 ,  674 . The pedals  71 ,  72  of the auxiliary bicycle body  3  drive the big sprocket wheel  671 , through one chain  673  the big sprocket wheel  671  drives the sprocket wheel  681 , and through another chain  674  of the speed-changing sprocket wheel assembly  68  the sprocket wheel  682  drives the small sprocket wheel  672 ; as the big sprocket wheel  671 , the rung  74  and the first auxiliary wheel  73  are connected pivotally to the auxiliary bicycle body  6  and move accordingly, the riders on the saddles  15 ,  65  only need to pedal the pedals  71 ,  72 , and the twin-frame bicycle will move forward. 
         [0054]    Another embodiment of an auxiliary bicycle body  6  is shown in  FIG. 19 . In this embodiment, the rear folk  66  is no longer needed and the speed-changing sprocket wheel assembly  68  is connected pivotally to the bottom bar  63 . Similarly, the riders only need to pedal the pedals  71 ,  72  to drive the twin-frame bicycle to move forward; meanwhile, production materials and cost are saved. 
         [0055]      FIG. 20  is a schematic view of a further embodiment of a twin-frame bicycle of the present invention.  FIG. 21  is a schematic view of a gear assembly disposed at a rear folk of the auxiliary bicycle body according to  FIG. 20  of the present invention. As shown in  FIGS. 20 and 21 , in this embodiment, an auxiliary bicycle body  81  of the twin-frame bicycle  8  comprises two auxiliary sprocket wheels  82 , a gear assembly  83 , an auxiliary rear folk  84 , a first auxiliary wheel  85  and a pedal mechanism  86 , wherein the diameter of the first auxiliary wheel  85  is smaller than that of a front wheel  88  and a rear wheel  89  of the primary bicycle body  87 , and the gear assembly  83  comprises two active gears  831  and two passive gears  832 . The auxiliary sprocket wheels  82  are connected pivotally to both sides of a shaft of the first auxiliary wheel  85 . The active gears  831  are connected pivotally to the auxiliary rear folk  84  on both sides of the first auxiliary wheel  85 . Each passive gear  832  is disposed between the corresponding active gear  831  and auxiliary sprocket wheel  82 . The pedal mechanism  86  of the auxiliary bicycle body  81  drives the active gears  831  to drive the auxiliary sprocket wheels  82  through the passive gears  832 , and the first auxiliary wheel  85  is driven to rotate. The gear assembly  83  is preferably disposed corresponding above the shaft of the first auxiliary wheel  85 , and the ration of the diameters of the active gear  831  and the auxiliary sprocket wheels  82  is 3:1. In addition, in other applications, the gear assembly  83  can be covered in a housing, and the housing can provide protection for the gear assembly  83 . 
         [0056]    Referring to  FIGS. 6 and 22 , it is to be noted that each of twin-frame bicycles according to the above-mentioned embodiments can comprise two articulation mechanisms  5 , wherein the first end  50  and the second end  59  of one of the articulation mechanisms  5  are connected to the head sleeve tube  12  of the primary bicycle body  1  (the primary bicycle frame) and the head sleeve tube  32  of the auxiliary bicycle body  3  (the auxiliary bicycle frame), and the first end  50  and the second end  59  of the other articulation mechanism  5  are connected respectively to the shaft  180  of the rear wheel  18  and the shaft  430  of the first auxiliary wheel  43 , and thus the shaft  430  of the first auxiliary wheel  43  is disposed substantially along an extension line of the shaft  180  of the rear wheel  18 . 
         [0057]      FIGS. 23-26  are schematic views of another embodiment of a twin-frame bicycle of the present invention. The twin-frame bicycle of the embodiment as shown in  FIG. 23-26  is substantially the same as the twin-frame bicycle of the embodiment as shown in  FIG. 16 , and the same elements are designated with the same numerals. The difference between the above two embodiment lies in that, in the embodiment as shown in  FIG. 23-26 , the auxiliary bicycle body  6  can further comprise a second auxiliary wheel  90 , a front folk  91  and two resilient elements  92 . The first auxiliary wheel  73  and the second auxiliary wheel  90  are connected pivotally to the auxiliary bicycle frame  310  and distanced from each other, and the pedal mechanism  72  of the auxiliary bicycle body  6  is used to drive the first auxiliary wheel  73  (or the pedal mechanism  72  of the auxiliary bicycle body  6  can be used to drive the second auxiliary wheel  90  in other embodiments). The front folk  91  is connected pivotally to the head sleeve tube  62 , the second auxiliary wheel  90  is connected pivotally between two branches of the front folk  91 , and the resilient elements  92  are connected to the respective branches of the front folk  91 . However, it should be noted that in other embodiments, the twin-frame bicycle as shown in  FIG. 23-26  can comprise two articulation mechanisms  5  (referring to  FIG. 22 ). 
         [0058]    Preferably, the steering handle  40  of the auxiliary bicycle body  6  is fixed at the head sleeve tube  62  and non-rotatable; and the front folk  91  is connected pivotally to the head sleeve tube  62  and rotatable. The steering handle  24  of the primary bicycle body  1  is in charge of turning the twin-frame bicycle. In this embodiment, the steering handle  40  of the auxiliary bicycle body  6  is used for handling but not controlling the twin-frame bicycle. When the steering handle  24  of the primary bicycle body  1  is controlled to make the twin-frame bicycle turn, the steering handle  40  of the auxiliary bicycle body  6  does not move or rotate, and according to the theory of physics, the second auxiliary wheel  90  will rotate and turn due to the turn of the front wheel  19 . The resilient elements  92  can provide a resilient force and keeps the second auxiliary wheel  90  turning naturally by the same angle with the front wheel  19 , synchronously. Furthermore, the twin-frame bicycle of the invention is highly stable without any possibility of tumbling, even in a high-speed riding and/or a sudden braking. 
         [0059]    However, it should be noted that the auxiliary bicycle body  6  as shown in  FIG. 24-27  can be separated from the twin-frame bicycle, to be an individual bicycle (the auxiliary bicycle body  6 ) for one rider&#39;s riding. For the separated individual bicycle (the auxiliary bicycle body  6 ), the steering handle  40  and the front folk  91  of the auxiliary bicycle body  6  may be directly or indirectly connected (for example, through an adjustable connection switching device) so as to move or rotate synchronously. 
         [0060]    In sum, a primary bicycle body (such as a conventional bicycle body) can be linked with an auxiliary bicycle body to form a twin-frame bicycle of the present invention, for two riders to ride side by side in parallel and pedal individually. 
         [0061]    Additionally, the articulation mechanism of the present invention is designed with functions in the three-dimensioned structure. The articulation mechanism has rigidity in horizontal and is free in vertical. That is, the articulation mechanism can not only keep the primary bicycle body and the auxiliary bicycle body in parallel but also allow the primary bicycle body and the auxiliary bicycle body moving stably in different levels (such as driving over an uneven or bumpy road with some drops in height). The wheels of the primary bicycle body and the auxiliary bicycle body can move vertically and in parallel to stabilize and balance the twin-frame bicycle, thus solving the problem of tumbling or overturning when driving over an uneven or bumpy road and/or making a turn. 
         [0062]    Besides providing fun riding, the articulation mechanism linking the primary bicycle body and the auxiliary bicycle body makes the twin-frame bicycle stable and unlikely to tumble or overturn. Moreover, by utilizing the articulation mechanism together with a resilient tension mechanism having an elastic element, the twin-frame bicycle can always stand straight up and be controlled much smoothly. Furthermore, since the structure of the auxiliary bicycle body is simple, the cost is saved and the maintenance of the twin-frame bicycle is easy. 
         [0063]    In addition, since the auxiliary wheel can be disposed between the front wheel and the rear wheel of the primary bicycle body, the center of gravity of the twin-frame bicycle will not be located at the rear, so the saddles need not be disposed much toward the back. Moreover, the primary bicycle body and the auxiliary bicycle body need only one articulation mechanism to link them, and the structure of the twin-frame bicycle of the present invention can be further simplified, thus further saving production cost. 
         [0064]    While the embodiments of the present invention have been illustrated and described, various modifications and improvements can be made by those skilled in the art. The embodiments of the present invention are therefore described in an illustrative but not restrictive sense. It is intended that the present invention is not limited to the particular forms as illustrated, and that all modifications that maintain the spirit and scope of the present invention are within the scope defined in the appended claims.