Folding mechanism and folding semitrailer

Abstract

The invention discloses a folding mechanism and a folding semitrailer, and relates to the technical field of semitrailers, the folding mechanism comprises a fixed frame and a folding frame, the rear end of the fixed frame is rotatably connected with the front end of the folding frame through a hinge assembly, at least one fixed plate is fixed to the rear end of the fixed frame, and at least one protruding part is arranged on the rear end face of the fixed plate; a plug connector is mounted on the fixing plate in a sliding manner, and comprises a limiting head which is positioned above the lug boss and is matched with the groove; movable plates in one-to-one correspondence with the fixed plates are fixed to the front end of the folding frame, and grooves matched with the protruding parts are formed in the front end faces of the movable plates; the folding frame has an unfolding state and an overturning folding state. In the unfolded state, the protruding parts and the grooves are matched to limit axial movement, and surface contact rigid fixing is achieved in combination with the bolt assembly; and in the overturning and folding state, the limiting head is matched with the groove to realize mechanical locking, so that the reliability is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of semi-trailer technology, and in particular to a folding mechanism and a folding semi-trailer, which is suitable for the folding frame structure of cargo transport vehicles and can effectively reduce energy consumption and tire wear during empty return trips. Background Technology

[0002] Semi-trailers, as core equipment in modern logistics and transportation systems, have been widely used in long-distance freight, port transshipment, and engineering logistics due to their significant advantages such as high load-bearing capacity, high transportation efficiency, and good adaptability. In actual operation, semi-trailers often face one-way load transport and empty return trips. In these situations, the large size of the trailer not only creates significant air resistance and increases fuel consumption but also leads to unnecessary tire wear, significantly increasing operating costs.

[0003] To address the aforementioned technical problems, existing technologies have proposed various folding semi-trailer solutions. For example, patent number CN109591891B discloses a folding mechanism and a folding semi-trailer, which designs the rear half of the vehicle body with tires as a foldable structure, achieving folding of the vehicle body through a support frame, hydraulic rods, and a traction unit. However, this solution has serious safety hazards: its locking mechanism relies entirely on magnetic control of the locking pin to achieve the locking function. Once there is a power outage, the electromagnet ages, or the magnet fails, the locking function will be directly lost, leading to the failure of the hinge joint's fixation. This could potentially cause a major safety accident due to the folding frame unexpectedly unfolding during operation.

[0004] Another patent application, number 201621168026.8, discloses a foldable semi-trailer frame, which achieves folding and unfolding of the frame through a first hydraulic cylinder, a second hydraulic cylinder, and an auxiliary wheel system assembly. However, this technical solution has the following drawbacks: First, the structure cannot achieve a complete 180-degree fold, and the vehicle height remains relatively high after folding, limiting the reduction in air resistance; second, maintaining the folded and unfolded state relies entirely on the continuous pressure of the hydraulic cylinder. Under the severe vibration conditions of long-distance transportation, the hydraulic system is subjected to alternating loads, which can easily cause damage to seals, hydraulic oil leakage, or cylinder fatigue damage, leading to system failure; third, the introduction of the hydraulic system increases the overall vehicle weight, maintenance costs, and failure rate.

[0005] Therefore, there is an urgent need in this field for a folding mechanism and a folding semi-trailer that is simple and reliable in structure, has high mechanical locking security, can achieve 180-degree full folding and does not need to rely on a hydraulic system to maintain the locking state. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a folding mechanism and a folding semi-trailer. It adopts a purely mechanical locking structure, which completely solves the problems of easy failure of electromagnetic locking and easy damage of hydraulic system. It achieves 180-degree full folding and significantly improves the safety, reliability and economy of folding semi-trailers.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A folding mechanism includes a fixed frame and a folding frame. The rear end of the fixed frame is rotatably connected to the front end of the folding frame via a hinge assembly. At least one fixed plate is fixed to the rear end of the fixed frame, and at least one protrusion is provided on the rear end face of the fixed plate. A connector is slidably mounted on the fixed plate, and the connector includes a limiting head located above the protrusion and adapted to a groove. The front end of the folding frame is fixed with a movable plate corresponding to each of the fixed plates, and the front end face of the movable plate is provided with a groove adapted to the protrusion. The folding frame has an unfolded state and a flipped-folded state. When in the unfolded state, the bearing surface of the folding frame and the bearing surface of the fixed frame are on the same horizontal plane. The protrusion is inserted into the groove and fits against the inner wall of the groove to restrict the folding frame from moving along the hinge axis. The front end face of the movable plate fits against the rear end face of the fixed plate and is fixedly connected by bolt assembly. When in the flip-folded state, the folding frame rotates 180 degrees around the hinge assembly, the bearing surface of the folding frame is in contact with the bearing surface of the fixed frame and both are in a horizontal position, the limiting head moves into the groove and is in contact with the inner wall of the groove to restrict the movement of the movable plate and maintain a stable flip-folded state.

[0008] Preferably, the fixed plate includes a fixed horizontal plate and a fixed vertical plate fixedly connected to the fixed horizontal plate, the fixed horizontal plate being perpendicular to the fixed vertical plate; the movable plate includes a movable horizontal plate and a movable vertical plate fixedly connected to the movable horizontal plate, the movable horizontal plate being perpendicular to the movable vertical plate; the fixed horizontal plate is rotatably connected to the movable horizontal plate via the hinge assembly. The fixed plate adopts an L-shaped structure with the fixed horizontal plate and the fixed vertical plate perpendicularly arranged, forming a three-dimensional frame support; in the unfolded state, the rear end face of the fixed plate is completely fitted with the front end face of the movable plate, forming a rigid connection through multiple bolt assemblies, resulting in a short load transmission path and high connection rigidity, which can meet the requirements of heavy-duty transportation.

[0009] Preferably, the hinge assembly includes a hinge base, a hinge shaft, and a hinge plate. The hinge base is fixedly mounted on the fixed horizontal plate, and the hinge shaft is rotatably mounted on the hinge base. The hinge plate is fixedly mounted on the hinge shaft, and the hinge plate is fixedly connected to the movable horizontal plate. The hinge plate is rotatably connected to the hinge base through the hinge shaft.

[0010] Preferably, the protrusions are configured as two symmetrically distributed on both sides away from the center of the fixed horizontal plate. A U-shaped component is fixed to the front end of the fixed horizontal plate. The U-shaped component and the protrusions together form a sliding groove, and the connector is slidably installed in the sliding groove. By positioning the connector and the protrusions on both sides away from the center of the fixed horizontal plate, placing them outside the hinge assembly, the movement trajectory of the connector is completely separated from the hinge assembly during the 180-degree rotation of the folding frame and the lifting and lowering of the connector. This effectively avoids movement interference between the mechanisms and ensures the smoothness of the folding and connector operations.

[0011] Preferably, the connector further includes a sliding part fixedly connected to the limiting head, the sliding part being slidably connected to the slide groove, and the lower end of the sliding part extending below the slide groove.

[0012] Preferably, the connector is driven to rise and fall by an adjustment mechanism, which includes a lead screw, a nut assembly, and an operating component. The lead screw is rotatably mounted on the front end face of the fixed horizontal plate, and the nut assembly is threaded onto the lead screw. A connecting plate is fixed to the lower end of the sliding part, and the nut assembly is fixedly connected to the connecting plate. The operating component is mounted on the lower end of the lead screw. At least one bearing seat is mounted on the front end face of the fixed horizontal plate, and the lead screw is rotatably connected to a bearing in the bearing seat. The lead screw can be driven to rotate by rotating a handwheel, enabling rapid rising and falling of the connector and quick fixation in a flipped-folded state, making operation simple and fast.

[0013] Preferably, both the fixed vertical plate and the movable vertical plate have at least one mounting hole. When the folding frame is in the unfolded state, the mounting holes on the fixed vertical plate and the movable vertical plate correspond one-to-one, and the bolt assembly passes through the mounting holes to achieve a fixed connection. The mounting holes facilitate the insertion of the bolt assembly, ensuring reliable locking.

[0014] Preferably, the protrusion has a U-shaped, trapezoidal, or semi-circular structure.

[0015] Preferably, the shape of the rear end face of the limiting head is adapted to the outer contour of the protrusion, so that when the groove fits with the protrusion, it also fits with the limiting head, thereby further enhancing the limiting effect.

[0016] Preferably, both the fixed frame and the folding frame are equipped with at least one pair of tires at their lower ends, so that the vehicle can move in both the unfolded and folded states.

[0017] The present invention also provides a folding semi-trailer, including the folding mechanism described in any of the above claims.

[0018] Compared with the prior art, the beneficial effects of the present invention are: 1. Dual mechanical locking for high safety: In the unfolded state, the invention uses a protrusion and a groove to limit axial movement, combined with a bolt assembly to achieve rigid surface contact fixing; in the flipped-folded state, a limiting head and a groove are used to achieve mechanical locking; the entire locking process does not rely on electromagnetic force or hydraulic pressure, completely avoiding the risk of locking failure due to power failure, magnet failure, or hydraulic leakage, fundamentally eliminating the safety hazard of accidental unfolding of the folding frame, and significantly improving reliability; 2. 180-degree complete folding, significant energy saving effect: Through the layout of the hinge components and the design of the fixed plate and the movable plate, the folding frame can be completely flipped and folded 180 degrees relative to the fixed frame. After folding, the two load-bearing surfaces are tightly attached, the overall height of the vehicle is greatly reduced, the air resistance during the return trip is significantly reduced, the fuel consumption is reduced, and unnecessary wear on the lower tires is avoided, resulting in outstanding economic benefits. 3. Purely mechanical adjustment, low maintenance cost: The screw and nut mechanical adjustment mechanism drives the lifting of the connector. The structure is simple and compact, and there is no need for hydraulic pump station, pipeline and sealing elements. It solves the problem of leakage and damage of hydraulic system under vibration conditions, and greatly reduces maintenance costs and vehicle weight. Attached Figure Description

[0019] Figure 1 Schematic diagram of the folding mechanism provided in the embodiments of this application Figure 1 (Expanded state); Figure 2 Schematic diagram of the folding mechanism provided in the embodiments of this application Figure 2 (In the unfolded state, the limiting head is located within the groove); Figure 3 for Figure 1 Sectional view along line BB; Figure 4 for Figure 3 Another state diagram (the limiting head is located inside the groove); Figure 5 for Figure 3 Another state diagram (flipped / folded state); Figure 6 for Figure 1 Sectional view along line AA; Figure 7 for Figure 1A cross-sectional view along the CC line; Figure 8 For another embodiment in Figure 6 A structural diagram of the viewpoint (the protrusion is a trapezoidal structure); Figure 9 For another embodiment in Figure 7 A structural diagram of the viewpoint (the groove is a trapezoidal structure that fits the protrusion). Figure 10 As another embodiment in Figure 6 A structural diagram of the viewpoint (the protrusion is a semi-circular structure); Figure 11 As another embodiment in Figure 7 A structural diagram of the viewpoint (the groove is a semi-circular structure that fits the protrusion). Figure 12 A schematic diagram of the structure of the folding semi-trailer provided in the embodiment of this application (unfolded state); Figure 13 This is a structural schematic diagram of a folding semi-trailer provided in an embodiment of this application (in a flipped-over folded state). In the diagram: 1. Fixed frame; 2. Folding frame; 3. Hinge assembly; 31. Hinge seat; 32. Hinge shaft; 33. Hinge plate; 4. Fixed plate; 41. Fixed horizontal plate; 42. Fixed vertical plate; 421. Protrusion; 43. U-shaped part; 44. Slide groove; 5. Movable plate; 51. Movable horizontal plate; 52. Movable vertical plate; 521. Groove; 6. Connector; 61. Limiting head; 62. Sliding part; 7. Adjustment mechanism; 71. Lead screw; 72. Nut pair; 73. Operating part; 74. Bearing seat; 75. Connecting plate; 8. Mounting hole. Detailed Implementation

[0020] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0021] like Figures 1 to 7 As shown, this embodiment provides a folding mechanism, including a fixed frame 1 and a folding frame 2. The front end of the fixed frame 1 is used to connect with a tractor, and the rear end of the fixed frame 1 is rotatably connected to the front end of the folding frame 2 via a hinge assembly 3.

[0022] At least one fixing plate 4 is welded to the rear end of the fixed frame 1. In this embodiment, there are two fixing plates 4, located on the left and right sides of the rear end of the fixed frame 1, respectively. At least one protrusion 421 is provided on the rear end surface of the fixing plate 4. In this embodiment, each fixing plate 4 is provided with two protrusions 421, which are symmetrically distributed on both sides away from the center line of the fixed frame 1.

[0023] A connector 6 is slidably mounted on the fixed plate 4. The connector 6 includes a limiting head 61 located above the protrusion 421.

[0024] The front end of the folding frame 2 is welded and fixed with movable plates 5 that correspond one-to-one with the fixed plate 4, such as... Figure 1 and Figure 7 As shown, a groove 521 that matches the protrusion 421 is provided on the front end surface of the movable plate 5.

[0025] like Figure 1 and Figure 3 As shown, the fixed plate 4 includes a fixed horizontal plate 41 and a fixed vertical plate 42 fixedly connected to the fixed horizontal plate 41. A protrusion 421 is located on the fixed vertical plate 42. The fixed horizontal plate 41 and the fixed vertical plate 42 are arranged perpendicularly to each other, forming an L-shaped cross-section. Reinforcing ribs can also be fixed between the fixed horizontal plate 41 and the fixed vertical plate 42 to enhance the strength of the fixed plate 4. The movable plate 5 includes a movable horizontal plate 51 and a movable vertical plate 52 fixedly connected to the movable horizontal plate 51. A groove 521 is located on the movable vertical plate 52. The movable horizontal plate 51 and the movable vertical plate 52 are arranged perpendicularly to each other, also forming an L-shaped structure. Reinforcing ribs can also be fixed between the movable horizontal plate 51 and the movable vertical plate 52 to enhance the fixing strength of the movable plate 5.

[0026] The fixed horizontal plate 41 is rotatably connected to the movable horizontal plate 51 via the hinge assembly 3. When in the unfolded state, the rear end face of the fixed vertical plate 42 is in close contact with the front end face of the movable vertical plate 52.

[0027] like Figures 1 to 4 As shown, when a return trip is required, the bolt assembly is first removed to release the rigid connection between the fixed plate 4 and the movable plate 5. Then, the rear end of the folding frame 2 is lifted using an external hoisting system, and the folding frame 2 is gradually rotated upwards by moving the hoisting system. During the rotation, the protrusion 421 disengages from the groove 521. When the folding frame 2 is rotated to 180 degrees, the bearing surface of the folding frame 2 and the bearing surface of the fixed frame 1 are in contact with each other and are both in a horizontal position. At this time, the folding frame 2 is stacked on top of the fixed frame 1, and the overall height of the vehicle is significantly reduced.

[0028] At this time, the operating adjustment mechanism 7 drives the connector 6 to move downward, causing the limiting head 61 to move into the groove 521. The front end face of the limiting head 61 fits tightly against the inner wall of the groove 521, forming a reliable mechanical limit, preventing the movable plate 5 from rotating relative to the fixed plate 4, thereby firmly locking the folding frame 2 in the flip-folded state. Because a purely mechanical locking mechanism is used, there is no risk of power failure or hydraulic failure, making it extremely safe.

[0029] like Figure 1 and Figure 3 As shown, the hinge assembly 3 includes a hinge base 31, a hinge shaft 32, and a hinge plate 33. The hinge base 31 is fixedly mounted on the fixed horizontal plate 41. The hinge shaft 32 is inserted into the hinge base 31 and has limiting members at both ends to prevent axial movement; or it is rotatably mounted on the hinge base 31 via bearings. Two hinge plates 33 are provided and fixedly mounted on the hinge shaft 32. The hinge plates 33 are fixedly connected to the movable horizontal plate 51 by welding. The hinge plates 33 are rotatably connected to the hinge base 31 via the hinge shaft 32, thereby realizing the rotatable connection between the fixed frame 1 and the folding frame 2.

[0030] like Figure 6 As shown, a U-shaped component 43 is fixedly welded to the front end of the fixed horizontal plate 41. The U-shaped component 43 and the protrusion 421 together form a sliding groove 44. The plug-in component 6 also includes a sliding part 62 fixedly connected to the limiting head 61. The sliding part 62 has a rectangular or circular cross-section and slides in cooperation with the sliding groove 44. The lower end of the sliding part 62 extends below the sliding groove 44. In this embodiment, the sliding part 62 is set to have a rectangular cross-section, which provides good shear resistance.

[0031] The connector 6 is driven to rise and fall via the adjusting mechanism 7. The adjusting mechanism 7 includes a lead screw 71, a nut assembly 72, and an operating component 73. The lead screw 71 is rotatably supported by two bearing seats 74 mounted on the front end face of the fixed horizontal plate 41, and the lead screw 71 is rotatably connected to the bearings inside the bearing seats 74. The nut assembly 72 is threaded onto the lead screw 71, and a connecting plate 75 is fixed to the lower end of the sliding part 62. The nut assembly 72 is welded and fixedly connected to the connecting plate 75. The operating component 73 is installed at the lower end of the lead screw 71, and the operating component 73 can be a handwheel or a handle.

[0032] When the position of the connector 6 needs to be adjusted, the operator rotates the operating component 73, driving the lead screw 71 to rotate. The nut assembly 72 converts the rotational motion into linear motion, which, through the connecting plate 75, drives the sliding part 62 to slide up and down within the slide groove 44, thereby realizing the action of the limit head 61 inserting or withdrawing from the groove 521. Since the two connectors 6 are symmetrically arranged, the two sliding parts 62 can be connected simultaneously through a connecting plate 75, achieving synchronous lifting and lowering on both sides and ensuring balanced force.

[0033] like Figure 6As shown, the protrusion 421 can be designed as a U-shape, trapezoid, or semi-circular structure, and the corresponding groove 521 has a matching shape. In this embodiment, the protrusion 421 is U-shaped. In another embodiment, as shown... Figure 8 and Figure 9 As shown, the protrusion 421 can also be a trapezoidal structure, and the groove 521 has a matching shape. In another embodiment, as... Figure 10 and Figure 11 As shown, the protrusion 421 can also be a semi-circular structure, and the groove 521 has a matching shape.

[0034] In some embodiments, the shape of the rear end face of the limiting head 61 is adapted to the outer contour of the protrusion 421. This design allows the groove 521 to fit against the protrusion 421 and also against the rear end face of the limiting head 61, improving the limiting accuracy and load-bearing capacity.

[0035] Multiple mounting holes 8 are provided on the fixed vertical plate 42 and the movable vertical plate 52. When the folding frame 2 is in the unfolded state, the mounting holes 8 on the fixed horizontal plate 41 and the movable horizontal plate 51 coincide one-to-one, and a fixed connection is achieved by passing high-strength bolts through the mounting holes 8. This connection method is convenient for disassembly and assembly, and allows for easy replacement of vulnerable parts according to wear conditions.

[0036] like Figure 12 and Figure 13 As shown, this embodiment provides a folding semi-trailer, including the folding mechanism as described in Embodiment 1. Tires are mounted on the lower ends of both the fixed frame 1 and the folding frame 2, with two pairs of tires mounted on the lower end of the fixed frame 1 and three pairs of tires mounted on the lower end of the folding frame 2. When in the folded state, the tires under the folding frame 2 flip up and no longer contact the ground, thus avoiding tire wear during the return trip.

[0037] In summary, this invention achieves reliable 180-degree folding of the semi-trailer frame through a mechanical locking structure design, completely solving the safety hazards of electromagnetic and hydraulic locking in existing technologies. It has advantages such as simple structure, convenient operation, safety and reliability, and significant economic benefits, and has broad prospects for promotion and application.

[0038] It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of this invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A folding mechanism, comprising a fixed frame (1) and a folding frame (2), wherein the rear end of the fixed frame (1) is rotatably connected to the front end of the folding frame (2) via a hinge assembly (3), characterized in that, At least one fixed plate (4) is fixed to the rear end of the fixed frame (1). At least one protrusion (421) is provided on the rear end surface of the fixed plate (4). A connector (6) is slidably installed on the fixed plate (4). The connector (6) includes a limiting head (61) located above the protrusion (421) and adapted to the groove (521). The front end of the folding frame (2) is fixed with a movable plate (5) corresponding to the fixed plate (4). The front end surface of the movable plate (5) is provided with a groove (521) adapted to the protrusion (421). The folding frame (2) has an unfolded state and a flipped-folded state: When in the unfolded state, the protrusion (421) is inserted into the groove (521) and fits against the inner wall of the groove (521) to restrict the movement of the folding frame (2). The front end face of the movable plate (5) fits against the rear end face of the fixed plate (4) and is connected by a bolt assembly. When in a flipped-over folded state, the limiting head (61) can move into the groove (521) and fit against the inner wall of the groove (521) to restrict the movement of the movable plate (5).

2. The folding mechanism according to claim 1, characterized in that, The fixed plate (4) includes a fixed horizontal plate (41) and a fixed vertical plate (42) fixedly connected to the fixed horizontal plate (41), the fixed horizontal plate (41) and the fixed vertical plate (42) being arranged perpendicularly; the movable plate (5) includes a movable horizontal plate (51) and a movable vertical plate (52) fixedly connected to the movable horizontal plate (51), the movable horizontal plate (51) and the movable vertical plate (52) being arranged perpendicularly; the fixed horizontal plate (41) is rotatably connected to the movable horizontal plate (51) through the hinge assembly (3).

3. The folding mechanism according to claim 2, characterized in that, The hinge assembly (3) includes a hinge seat (31), a hinge shaft (32), and a hinge plate (33). The hinge seat (31) is fixedly mounted on the fixed horizontal plate (41), and the hinge shaft (32) is rotatably mounted on the hinge seat (31). The hinge plate (33) is fixedly mounted on the hinge shaft (32), and the hinge plate (33) is fixedly connected to the movable horizontal plate (51). The hinge plate (33) is rotatably connected to the hinge seat (31) through the hinge shaft (32).

4. The folding mechanism according to claim 2, characterized in that, The protrusion (421) is provided as two and symmetrically distributed on both sides away from the center of the fixed horizontal plate (41). A U-shaped part (43) is fixed at the front end of the fixed horizontal plate (41). The U-shaped part (43) and the protrusion (421) together form a sliding groove (44). The plug (6) is slidably installed in the sliding groove (44).

5. The folding mechanism according to claim 4, characterized in that, The connector (6) also includes a sliding part (62) fixedly connected to the limiting head (61), the sliding part (62) being slidably connected to the slide groove (44), and the lower end of the sliding part (62) extending below the slide groove (44).

6. The folding mechanism according to claim 5, characterized in that, The connector (6) is driven to rise and fall by the adjustment mechanism (7). The adjustment mechanism (7) includes a lead screw (71), a nut pair (72), an operating component (73), and a bearing seat (74). At least one bearing seat (74) is installed on the front end face of the fixed horizontal plate (41). The lead screw (71) is rotatably connected to the bearing in the bearing seat (74). The nut pair (72) is threaded onto the lead screw (71). A connecting plate (75) is fixed at the lower end of the sliding part (62). The nut pair (72) is fixedly connected to the connecting plate (75). The operating component (73) is installed at the lower end of the lead screw (71).

7. The folding mechanism according to claim 2, characterized in that, Both the fixed vertical plate (42) and the movable vertical plate (52) are provided with at least one mounting hole (8). When the folding frame (2) is in the unfolded state, the mounting holes (8) on the fixed vertical plate (42) and the movable vertical plate (52) correspond one-to-one, and the bolt assembly passes through the mounting holes (8) to achieve a fixed connection.

8. The folding mechanism according to claim 1, characterized in that, The protrusion (421) has a U-shaped, trapezoidal or semi-circular structure.

9. The folding mechanism according to claim 1, characterized in that, When the folding frame (2) is in the unfolded state, it is switched to the flipped folding state by rotating 180 degrees around the hinge assembly (3). At this time, the bearing surface of the folding frame (2) is in contact with the bearing surface of the fixed frame (3) and both are in a horizontal position.

10. A folding semi-trailer, characterized in that, The folding mechanism includes any one of claims 1 to 9, wherein at least one pair of tires are mounted on the lower end of both the fixed frame (1) and the folding frame (2).

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