1 GOLF BAG CART FOLDABLE DEVICE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a golf bag cart foldable device, more particularly, to one 5 that features a simultaneous unfolding or folding for its upper cart stand, bottom cart stand and rear wheels, to facilitate the handiness in use. Description of the Prior Art A prior golf bag cart foldable device, for instance, the Utility Patent Application Serial No. 12/248,088, is owned by this applicant, which enables a simultaneous 10 unfolding or folding for its upper cart stand, bottom cart stand and rear wheels, and the cart structure includes a slider set up on the bottom cart stand, where the setup of the slider is limited to the stand with straight tube for its attachment. And that constraint deteriorates considerably in the shaping of the appearance of the cart, which apparently calls for a necessary improvement. 15 The applicant of this invention realized this situation and pushed the inventor ahead with the improvement hard, and eventually the longtime endeavors gave birth to this invention. SUMMARY OF THE INVENTION The objective of this invention is to provide a golf bag cart foldable device, which 20 features a simultaneous unfolding or folding for its upper cart stand, bottom cart stand and rear wheels, to achieve particular handiness in use. To achieve the aforementioned objective, this invention comprises: a bottom cart stand, having a link segment at its top, where both sides of the link segment each 2 is in pinned connection with a rear wheel frame, to link the corresponding rear wheel, and having a bag base at its bottom, where the bag base is connected to a front wheel frame, to link a front wheel; an upper cart stand, in pinned connection with the link segment of the bottom cart stand through a rotator set up at its 5 bottom; a linkage set, comprising a plurality of linkages, which are joined to the upper cart stand, bottom cart stand and rear wheels respectively, to form links between the upper cart stand and bottom cart stand and between the upper cart stand and the rear wheel frame; and a folding joint, set up on the rotator of the upper cart stand, having a buckle that enables a release of the folding joint first, 10 followed by the turning of the upper cart stand, and by means of the link between the linkage set and the rear wheel frame, the unfolding or folding of its upper cart stand, bottom cart stand and rear wheels simultaneously can thus be completed. After that, the folding joint is secured to have the upper cart stand and bottom cart stand locked to each other, for achieving the handy operation. 15 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a three-dimensional diagram of the unfolding according to the present invention; FIG. 2 is a side view of the unfolding according to the present invention; FIG. 3 is a schematic sectional view for the third linkage of the unfolding 20 according to the present invention; FIG. 4 is a three-dimensional exploded view of the folding joint according to the present invention; FIG. 5 is a schematic sectional view at the securing of the folding joint according 3 to the present invention; FIG. 6 is a schematic diagram of the side elevation of the motions of the folding according to the present invention; FIG. 7 is a schematic sectional view at the release of the folding joint according to 5 the present invention; FIG. 8 is a schematic diagram of the motion of the folding for the front wheel according to the present invention; and FIG. 9 is a schematic diagram of the side elevation of the motions of the complete folding according to the present invention. 10 DETAILED DESCRIPTION OF THE INVENTION The structural traits and the functions of this invention are detailed described with reference to the following preferred embodiment and the accompanying drawings, which would help the honorable Examiner to comprehend thoroughly the present invention. 15 Referring to FIGS. 1 - 5, this invention comprises: a bottom cart stand 1, having a link segment 11 at its top, where both sides of the link segment 11 each is in pinned connection with a rear wheel frame 12, which is to link a rear wheel 13; and having a bag base 14 at its bottom, where the base 14 couples a front wheel frame 15, which is to link a front wheel 16. 20 an upper cart stand 2, in pinned connection with the link segment 11 of the bottom cart stand 1 through a rotator 21 set up at its bottom, and pivotally connected to an upper bracket 22 at its top. a linkage set 3, made up of a plurality of linkages, and connected to the upper cart 4 stand 2, the bottom cart stand 1 and the rear wheel frames 12 respectively, to form links between the upper cart stand 2 and bottom cart stand 1 and between the upper cart stand 2 and the rear wheel frames 12; the linkage set 3 comprises: a first linkage 31, a second linkage 32 and a third linkage 33, where the first linkage 5 31 couples to the rotator 21 at its top; the second linkage 32, coupling to the first linkage 31 on its one end, and coupling to the bottom cart stand 1 on its another end, to form a link between the upper cart stand 2 and bottom cart stand 1; the third linkage 33, in spherical connection with the second linkage 32 (shown in FIG. 3) on its one end, and coupling to the rear wheel frames 12 on its another end, to 10 form a link between the upper cart stand 2 and the rear wheel frames 12. a folding joint 4, set up on the rotator 21 of the upper cart stand 2, comprising: a buckle 41, in pinned connection with both sides of the link segment 11 and the rotator 21, having bulges 411 on the inner sides of its both ends; two movable tooth disks 42, coupling to somewhere between the rotator 21 and the buckle 41, 15 where its outer sides (the end sides that match the buckle 41) each is provided with positioning curved trenches 421, which accommodate the corresponding bulges 411 of the buckle 41, to cause a mutual relation of the limit of angle and movement; two rotator tooth disks 211, set up at the outer sides of the bottom of the rotator 21, corresponding to the respective movable tooth disks 42, where the 20 rotator tooth disk 211 and the rotator 21 can be shaped into a single piece; and two springs 43, set up somewhere in between the rotator 21 (also the rotator tooth disk 211) and the movable tooth disk 42, which gives rise to a force to push the movable tooth disks 42 outward, once the buckle 41 is pressed down, and by 5 means of the lodgment of the bulges 411 in the positioning curved trenches 421, the movable tooth disks 42 is pushed inward, which is joined tightly to the rotator tooth disk 211 mutually, the rotator 21 is set to be fixed, and the included angle formed by the upper cart stand 2 and bottom cart stand 1 is then locked; on the 5 contrary, once the buckle 41 is lifted up, the movable tooth disk 42 is pushed outward by means of the resilience of the spring 43, to enable the disk 42 to separate from the rotator tooth disk 211, which makes the rotator 21 free to rotate. According to the aforementioned structural combination, the golf bag cart of this invention exhibits an unfolding state under normal conditions, shown in FIGS. 2 10 & 5, and the buckle 41 presents a pressed-down state at the moment, which helps to lock the relative positions of the upper cart stand 2 and the bottom cart stand 1, and the rear wheel frame 12 is then simultaneously unfolded for a fixing, subject to the interaction among the first linkage 31, the second linkage 32 and the third linkage 33, and there is no need for additional joints to be fixed. 15 Once intending for a folding, the buckle 41 is first pulled up and the upper cart stand 2 is pushed front and turned down, shown in FIGS. 6 & 7, the movable tooth disks 42 is pushed outward by the resilience of the spring 43 at the moment, enabling a separation from the rotator tooth disk 211, which makes the upper cart stand 2 to drive the first linkage 31 through the rotator 21 and to push the second 20 linkage 32, and the second linkage 32 is therefore turning forward; meanwhile, the second linkage 32 pulls the rear wheel frame 12 forward and turns inward through the third linkage 33, which enables the rear wheels 13 to undergo a forward and inward folding, followed by a pressing on the buckle 41 to immobilize the 6 included angle formed by the upper cart stand 2 and the bottom cart stand 1, and lastly the front wheel 16 is turned backward to somewhere below the bottom cart stand 1, to accomplish the folding of the whole bag cart, shown in FIGS. 8 & 9. On the contrary, once the golf bag cart is intended to unfold, the procedure starts 5 by releasing the buckle 41 and turning the upper cart stand 2 up, through the interaction among the first linkage 31, the second linkage 32 and the third linkage 33 with the rear wheel frame 12, the second linkage 32 is turned backward, to unfold the rear wheels 13; at the moment, since the positioning curved trenches 421 introduce constraint on the rotational angle of the upper cart stand 2, the 10 buckle 41 has to be pressed again to lock the upper cart stand 2 and the bottom cart stand 1 to each other if the upper cart stand 2 has been turned to its limit, and finally the front wheel 16 is turned forward to the outer side of the bottom of the bottom cart stand 1, to accomplish the unfolding of the whole bag cart, shown in FIG. 2. 15 In short, the folding joint 4 of this invention is provided with a buckle 41, which enables the folding joint 4 released first before the upper cart stand 2 can be turned (because the rotator tooth disk 211 has been separated from the movable tooth disks 42), by means of the link between the linkage set 3 and the rear wheel frame 12, a simultaneous unfolding or folding for the upper cart stand 2, the 20 bottom cart stand 1 and the rear wheels 12 can be achieved, followed by a locking to the folding joint 4, the upper cart stand 2 and the bottom cart stand 1 are ready to lock each other, to accomplish the purpose of the unfolding or the folding, shown in FIGS. 2 & 9, which substantially facilitates the handiness of the use.
7 From the aforesaid description, this invention features at least the following advantages and functions, which is much creative than the prior art: 1. The upper cart stand 2, the bottom cart stand 1 and the rear wheels 13 can be unfolded or folded simultaneously, which is truly handy in use. 5 2. During the folding of the cart, the upper cart stand 2, the bottom cart stand 1 and the rear wheels 13 are locking to each other, where the cart is not only for contraction in volume but for handy carrying. 3. During the unfolding of the cart, due to the supportive role of the folding joint 4 for having the cart fixed, there is no need for additional joints to be 10 fixed, which is truly handy in use. To sum up, the disclosed concrete structure of the exemplified embodiment of the present invention is not only unknown to the prior art, but surely can accomplish the expected objective and function, which is construed as absolutely novel and creative, and a patent application for the invention is then filed according to the 15 patent law.