An auto-carriage has a first frame and a second frame pivotally connected to the first frame so that both the front wheels and the rear wheels of the carriage are able to maintain engagement to the ground at all times. Furthermore, a pair of unidirectional bearings are provided to distal ends of an axle, such that the carriage is able to change the moving direction easily. A first linkage connected to a secondary linkage through a universal joint enables the steering force of a user to convert to the driving force to the front wheel bracket so as to change the moving direction of the carriage.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an auto-carriage, and more particularly to
 an auto-carriage having a first frame and a second frame pivotally
 connected with each other so as that the carriage is able to maintain
 engagement with the ground and a differential device enabling the carriage
 to change direction easily.
 2. Description of Related Art
 Pushcart, trolleys, strollers, carriages are all common transportation
 devices used in moving cargo from one place to another. All these
 transportation devices are manually operated and are not easy to be
 manipulated. In order to overcome the labor intensive problem of the
 conventional carriages or trolleys, an automated carrier is introduced to
 the market. The carrier is equipped with an engine so that not much of
 labor work is involved in the operation of the carrier. Furthermore, a
 transmission device is also equipped with the carrier, which enables the
 carrier to change the moving direction while in operation. However, this
 kind of carrier is complicated in structure and costy, which is not
 affordable by small & medium sized enterprises or normal operators.
 It is therefore an objective of the invention to provide an improved
 carriage to mitigate and obviate the aforementioned problems.
 SUMMARY OF THE INVENTION
 The main objective of the invention is to provide an improved auto-carriage
 having sectioned bodies pivotally connected with each other, such that
 different bodies of the carriage are able to maintain engagement with the
 ground no matter what the situation of terrain is.
 Another objective of the invention is that the auto-carriage is equipped
 with a differential device so as to enable the carriage to change
 direction while in movement.
 Other objects, advantages and novel features of the invention will become
 more apparent from the following detailed description when taken in
 conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
 Referring to FIG. 1, FIG. 1A, FIG. 2 and FIG. A, an auto-carriage has a
 first frame (10) and a second frame (40) pivotally connected to the first
 body (10). The first frame (10) centrally defined a space for carrying a
 load so that a user is able to transport the load from one place to
 another. Under the first frame (10), there is firmly provided with a
 cylinder (11) with a shaft (12) rotatably received therein and one end of
 the cylinder (11) is securely connected with a T-shaped steering bar (13).
 The steering bar (13) is provided with a pair of braking handles (14) for
 controlling the braking system (not shown) of the carriage of the
 invention and a control switch (15) for controlling the operation of the
 engine. Due to how the braking handles (14) is connected to the braking
 system and what type of the braking system is being not the focus of the
 invention, detailed description thereof is omitted. The other end of the
 shaft (12) is connected with a linkage (16). The linkage (16) is connected
 with a secondary linkage (161) by means of a universal joint (162) (as
 shown in FIG. 3). The secondary linkage (161) securely engages with a
 U-shaped front wheel bracket (17) that has an inclined support (18)
 integrally formed with the mediate portion of the front wheel bracket (17)
 through a cylindrical housing (19). Near the front end of the first frame
 (10) and corresponding to the cylindrical housing (10), a protrusion (191)
 integrally extends downward from the first frame (10), so that the
 protrusion (191) is able to be rotatably received in the cylindrical
 housing (19).
 Referring to FIGS. 3, 4, 5 and 6, it is noted that when the steering bar
 (13) is steered by a user, the linkage (16) is driven by the pivot of the
 shaft (12) to shift to a direction. By means of the universal joint (162),
 the driving force from the linkage (16) is transmitted to the secondary
 linkage (161) and then the secondary linkage (161) is shifted to an
 opposed direction in relation to the direction of the linkage (16).
 Because the secondary linkage (161) is securely connected with the front
 wheel bracket (17) and the front wheel bracket (17) is then securely
 connected with the inclined support (18), the driving force from the
 linkage (16) will drive the front wheel bracket (17) together with the
 support (18) to pivot in the same direction as that of the linkage (16).
 After a pair of front wheels (24) are rotatably mounted under the front
 wheel bracket (17), the pivotal movement of the front wheel bracket (17)
 together with the inclined support (18) will also drive the front wheels
 (24) to pivot. With reference to FIG. 6A, another preferred embodiment of
 a cross-head universal joint (162') is shown. Incorporating with the
 universal joint (162'), the linkage (16) has a connector (160) securely
 connected to the cross-head universal joint (162') and the secondary
 linkage (161') also has a connector (1610) securely connected to the
 universal joint (162'), such that the pivotal movement of the linkage
 (16') will drive the secondary linkage (161') to pivot and therefore
 drives the front wheel bracket (17) to pivot.
 With reference to FIG. 7, FIG. 7A and FIG. 8, an axle (20) is provided to
 extend through joints between the front wheel bracket (17) and the
 inclined support (18). In the mediate portion of the axle (20) a gear (21)
 is fixedly mounted and having a chain (211) engaged and surrounded the
 gear (21). A sleeve (22) is securely and respectively mounted at two free
 ends of the axle (20) so that an unidirectional bearing (23) is able to be
 received in the sleeve (22) so as to function as a differential device to
 the front wheels (24). It is noted that when the carriage of the invention
 is moving toward a fixed direction, each of the unidirectional bearings
 (23) engages with the respective sleeve (22) so that there is no
 differential effect required. However, when the carriage of the invention
 is turning, one of the front wheels (24) rotates faster than that of the
 other, in this case, the unidirectional bearing (23) provided to the front
 wheel (24) that rotates slower than that of the other fails to engage with
 the sleeve (22). Accordingly, a differential effect is generated to the
 carriage to facilitate the changing of moving direction.
 In order to facilitate the moving of the carriage of the invention, a power
 system is provided to the carriage. The power system includes a battery
 box (31) for receiving therein multiple batteries (not shown), a motor
 regulating device (32) and a motor (33). The motor shaft (not numbered) is
 connected to the gear (21) through the chain (211), such that when the
 motor (33) is in operation, the power of the motor (33) is able to be
 transmitted to the axle (20) to drive the rotation of the front wheels
 (24).
 With reference to FIGS. 9 and 10 and taking reference to FIGS. 1 and 1A,
 each of the braking handles (14) is securely connected to a cord (141)
 that extends into a control box (27) and then engages with a connector
 (28). The connector (28) engages with two secondary cords (141') that
 respectively extend to one of the front wheels (24). To provide braking
 effect to the pair of rear wheels (41), a second control box (27') is
 provided. A connector (28') in the control box (27') has two secondary
 cords (141') that respectively extend to one of the rear wheels (41). With
 such an arrangement, the braking handles (14) are able to control the
 brake to the front wheels and/or the rear wheels (24, 41).
 Beside the first frame (10), the carriage of the invention further has a
 second frame (40) pivotally connected to the first frame (10). The first
 frame (10) has a tube (30) pivotally connected to the second frame (40).
 The second frame (40) has two tubular extensions (300) respectively
 extending from opposed sides of the tube (30) to support one of the
 control boxes (27)
 The second frame (40) further has a pair of rear wheels (41) rotatably
 mounted on opposed sides of the second frame (40) and a platform securely
 mounted between the pair of rear wheels (41) so as to provide a support to
 a user standing on the platform (42).
 It is concluded that the present has the following advantages:
 1 the pivotal connection between the first frame and the second frame
 enables the front and rear wheels of the carriage to engage the ground at
 all times. It is especially useful when the carriage is operated on a
 mountainous area and the road is zigzag.
 2 the steering system enables the user to turn to a desired direction while
 the user is shifting his center of mass. When operated in a mountainous
 area, the user's center of mass often shifts due to the changeable paths.
 Therefore, when the changing of direction is required, the user shifts his
 center of mass will facilitate the changing of the direction.
 3 normally, the differential device in a vehicle is complicated and complex
 and often involves a lot of gears. The carriage of the invention provides
 a simple-structured differential device by using a pair of unidirectional
 bearings to achieve the desired purpose. Therefore, the cost of the
 carriage is greatly reduced.
 It is to be understood, however, that even though numerous characteristics
 and advantages of the present invention have been set forth in the
 foregoing description, together with details of the structure and function
 of the invention, the disclosure is illustrative only, and changes may be
 made in detail, especially in matters of shape, size, and arrangement of
 parts within the principles of the invention to the full extent indicated
 by the broad general meaning of the terms in which the appended claims are
 expressed.