NOTICE OF COPYRIGHT 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to any reproduction by anyone of the patent disclosure, as it appears in the United States Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE PRESENT INVENTION 
     1. Field of Invention 
     The present invention relates to vehicle rearview mirror and more particularly, to an auxiliary rearview mirror mounting structure that fits any of a variety of main rearview mirrors, facilitates mounting and dismounting the auxiliary rearview mirror, and enhances the mounting stability. 
     2. Description of Related Arts 
       FIG. 1  illustrates a conventional auxiliary rearview mirror mounting structure mounted at an outer lateral side of a main rearview mirror  30  of a car  20  to provide a wide angle rear view. Adding an auxiliary rearview mirror to each rearview mirror of a trailer would be of a great help. This design of auxiliary rearview mirror mounting structure, as shown in  FIG. 2 , comprises a pivot holder  50  mounted at a rear side of an auxiliary rearview mirror  10 , a pair of support arms  40  pivotally connected to the pivot holder  50 , and two binding straps  60  respectively extended from one support arm  40  and fastenable to the other support arm  40  to secure the two support arms  40  to a main rearview mirror  30 . This design of auxiliary rearview mirror mounting structure can be seen in Taiwan Patent No.I256358. 
       FIG. 3  illustrates another prior art design of auxiliary rearview mirror mounting structure for fastening an auxiliary rearview mirror  10   a  to a main rearview mirror  30 . This design of auxiliary rearview mirror mounting structure comprises a rod member  10   b  having an outer end pivotally connected to the auxiliary rearview mirror  10   a , a plurality of hooking blocks  70  respectively slidably attached to the rod member  10   b  and locked thereto at respective selected locations with a respective lock screw  701 , a strap  60  extended from one side of each hooking block  70  comprises and terminating in a hook  702 , and a tightening-up device  703  mounted at each hooking block  70 . Each tightening-up device  703  comprises a hand screw  7031  threaded through one respective hooking block  70  and terminating in a ball socket  7032 , a rigid pad  7033  pivotally coupled to the ball socket  7032 , and a flexible pad  7034  bonded to the rigid pad  7033 . In installation, hook the hooks  702  on the rim  301  of the main rearview mirror  30 , and then rotate the tightening-up devices  703  to rightly abut the flexible pads  7034  of the tightening-up devices  703  against the main rearview mirror  30 . 
     However, because the straps  60  are made from flexible rubber, elastic fatigue can occur in the straps  60  after a long use. When the straps  60  get loose, they will not be closely attached to the back surface of the main rearview mirror  30 , resulting in mounting instability. Further, if the straps  60  have different tensions, the auxiliary rearview mirror  10   a  and the main rearview mirror  30  will be vibrated during running of the vehicle, leading to image blur and the risk of falling of the auxiliary rearview mirror  10   a  from the main rearview mirror  30 . Further, different main rearview mirrors  30  have a different shape and curvature of rim  301 . The rod member  10   b  is not adjustable to fit the curvatures of the rims  301  of all different main rearview mirrors  30 . Thus, the hooks  702  of the hooking device  70  cannot be positively hooked on the rims  301  of all different main rearview mirrors  30 . Therefore, the conventional auxiliary rearview mirror mounting structure designs still have strong demand for improvement. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention has been accomplished under the circumstances in view. It is therefore the main object of the present invention to provide an auxiliary rearview mirror mounting structure, which is highly adjustable to fit any of a variety of main rearview mirrors and, which facilitates mounting and dismounting the auxiliary rearview mirror, and enhances the mounting stability. 
     To achieve this and other objects of the present invention, an auxiliary rearview mirror mounting structure comprises an auxiliary rearview mirror, two support arms respectively connected to said auxiliary rearview mirror, and a plurality of hooks symmetrically located at the support arms and adapted for hooking on main rearview mirror. Each hook comprises an extension portion extended from the respective support arm, and a hook portion extended from a distal end of the extension portion remote from the respective support arm for hooking on a vehicle rearview mirror. Further, each hook is formed integral with an inner side of one respective support arm. Further, each support arm is equipped with a plurality of tightening-up devices corresponding to respective hooks at the associating support arm. The tightening-up devices are rotatably threaded through the associating support arm from an outer side of the associating support arm toward an inner side thereof. Each tightening-up device comprises a friction pad located at a front end thereof and facing toward the hook portion of one respective hook and adapted for securing the hook portion of one respective hook to a vehicle rearview mirror. Each support arm further comprises a pivot joint pivotally disposed between two hooks. 
     Preferably, each tightening-up device comprises a knob, a screw rod extended from the knob and threaded through the associating support arm and terminating in a ball socket. Further, the friction pad of each tightening-up device is pivotally coupled to the ball socket of the respective tightening-up device. 
     Preferably, the friction pad comprises a rigid pad pivotally coupled to the ball socket of the screw rod of the associating tightening-up device, and a flexible pad bonded to one side of the rigid pad and facing toward the hook portion of one respective hook. 
     Preferably, each tightening-up device further comprises a flexible dust sleeve sleeved onto the associating screw rod and stopped between the associating knob and the respective support arm. 
     Preferably, each support arm comprises a first arm portion pivotally connected with one end thereof to the auxiliary rearview mirror, a second arm portion pivotally connected with one end thereof to an opposite end of the first arm portion remote from the auxiliary rearview mirror, a third arm portion pivotally connected to an opposite end of the second arm portion remote from the first arm portion, a first pivot joint pivotally connected between the second arm portion and the third arm portion, and a second pivot joint pivotally connected between the first arm portion and the second arm portion. Further, the hooks are respectively mounted at the second arm portion and third arm portion of each support arm. 
     Preferably, the auxiliary rearview mirror mounting structure further comprises a flexible cap member capped on the hook portion of each hook. The flexible cap member has a relatively higher front side and a relatively lower rear side. The design of the relatively higher front side and relatively lower rear side of the flexible cap member can match with the gradually outwardly reducing wall thickness design of the peripheral wall of various different commercial main rearview mirrors, enhancing mounting stability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view illustrating an auxiliary rearview mirror mounting structure installed in a rearview mirror of a car according to the prior art. 
         FIG. 2  is an elevational view of the auxiliary rearview mirror mounting structure shown in  FIG. 1 . 
         FIG. 3  is an exploded view of another design of auxiliary rearview mirror mounting structure according to the prior art. 
         FIG. 4  is an exploded view of an auxiliary rearview mirror mounting structure according to the present invention. 
         FIG. 5  is a schematic plain view illustrating the auxiliary rearview mirror mounting structure of the present invention attached to a main rearview mirror before locking. 
         FIG. 6  is a schematic drawing of the present invention, illustrating the tightening-up device rotated inwards/backwards relative to the rim of the main rearview mirror. 
         FIG. 7  is a schematic drawing of the present invention, illustrating the tightening-up device fastened tightly to the rim of the main rearview mirror. 
         FIG. 8  is an elevational view of a flexible cap member for the hook portion of the hook in accordance with the present invention. 
         FIG. 9  corresponds to  FIG. 7 , illustrating the flexible cap member capped on the hook portion of the hook. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 4 , an auxiliary rearview mirror mounting structure A is pivotally connected to a back side of an auxiliary rearview mirror B and fastenable to a rim C 1  of a main rearview mirror C in a motor vehicle to secure the auxiliary rearview mirror mounting structure A to the main rearview mirror C. 
     The auxiliary rearview mirror mounting structure A comprises a pair of support arms  1 . The support arms  1  are made from a hard plastic material, each comprising a first arm portion  11  pivotally connected with its one end to the auxiliary rearview mirror B, a second arm portion  12  pivotally connected with its one end to an opposite end of the first arm portion  11  remote from the auxiliary rearview mirror B, a third arm portion  13  pivotally connected to an opposite end of the second arm portion  12  remote from the first arm portion  11 , a first pivot joint  14  pivotally connected between the second arm portion  12  and the third arm portion  13 , and a second pivot joint  15  pivotally connected between the first arm portion  11  and the second arm portion  12 . Thus, the first arm portion  11 , second arm portion  12  and third arm portion  13  of each support arm  1  can be biased relative to one another to adjust the angular configuration of the respective support arm  1  (see  FIG. 5 ). Each support arm  1  further comprises two hooks  2  respectively integrally extended from the second arm portion  12  and third arm portion  13  thereof at an inner side. Each hook  2  comprises an extension portion  21  extended from the second arm portion  12  and third arm portion  13 , and a hook portion  22  extended from a distal end of the extension portion  21  remote from the second arm portion  12  and third arm portion  13 . 
     The auxiliary rearview mirror mounting structure A further comprises a plurality of tightening-up devices  3  respectively mounted in the support arms  1  corresponding to the respective hooks  2  and rotatable inwards and outwards to lock the support arms  1  to the rim C 1  of the main rearview mirror C or to unlock the support arms  1  from the rim C 1  of the main rearview mirror C. Each tightening-up device  3  comprises a knob  31  disposed outside the respective support arm  1 , a screw rod  32  extended from the knob  31  and threaded through the respective support arm  1  and terminating in a ball socket  321  (see  FIG. 4  and  FIG. 6 ), a friction pad formed of a rigid pad  322  that pivotally coupled to the ball socket  321  of the screw rod  32  and a flexible pad  323  bonded to one side of the rigid pad  322  opposite to the screw rod  32  and facing toward the hook portion  22  of one respective hook  2  at the respective support arm  1 , and a flexible dust sleeve  324  sleeved onto the screw rod  32  and stopped between the knob  31  and the respective support arm  1 . 
     In installation, as shown in  FIG. 5 , bend the support arms  1  to move the hooks  2  subject to the functioning of the respective first pivot joints  14  and second pivot joints  15 , enabling the hook portions  22  of the respective hooks  2  to be hooked on an inner surface of the rim C 1  of the main rearview mirror C. Thereafter, operate the knob  31  of each tightening-up device  3  to rotate the associating screw rod  32  relative to the respective support arm  1  (see  FIG. 6 ), moving the associating flexible pad  323  into abutment against an opposing outer surface of the rim C 1  of the main rearview mirror C (see  FIG. 7 ). Thus, the support arms  1  are locked to the rim C 1  of the main rearview mirror C by the hooks  2  and the tightening-up devices  3 . When wishing to remove the auxiliary rearview mirror mounting structure A from the rim C 1  of the main rearview mirror C, rotate the knob  31  of each tightening-up device  3  in the reversed direction to move the flexible pad  323  of each tightening-up device  3  away from the rim C 1  of the main rearview mirror C. Thus, the mounting and dismounting operations of the auxiliary rearview mirror mounting structure A are quite simple. 
     Further, a flexible cap member  23  (see  FIG. 8 ) made from, for example, rubber, is capped on the hook portion  22  of each hook  2  (see  FIG. 9 ). The flexible cap member  23  is a hollow member defining a relatively higher front side  231  and a relatively lower rear side  232  opposite to the relatively higher front side  231 . In installation, the hook portion  22  of each hook  2  is inserted through the respective flexible cap member  23  in direction from the relatively higher front side  231  toward the relatively lower rear side  232 . In some commercial main rearview mirrors C, the peripheral wall has a thickness gradually reducing in direction from the inner side toward the outer side (see  FIG. 9 ). The design of the relatively higher front side  231  and relatively lower rear side  232  of the flexible cap member  23  can match with the gradually outwardly reducing wall thickness design of the peripheral wall of these commercial main rearview mirrors C. Further, flexible cap member  23  has non-slip ribs  233  located at the inner and outer walls thereof (see  FIG. 8 ) for friction engagement with the rim C 1  positively. 
     In conclusion, the invention provides auxiliary rearview mirror mounting structure A, which comprises two support arms  1  mounted at an auxiliary rearview mirror B, a plurality of hooks  2  located at the support arms  1  for hooking on a rim C 1  of a main rearview mirror C, and a plurality of tightening-up devices  3  for locking the hooks  2  to the rim C 1  of the main rearview mirror C, wherein each support arm  1  comprises first, second and third arm portions  11 ; 12 ; 13  respectively pivotally connected in series with first and second pivot joints  14 ; 15 . Thus, the support arms  1  can be curved, enabling the hooks  2  to be conveniently and positively hooked on the rim C 1  of the main rearview mirror C and then locked thereto by the tightening-up devices  3 . 
     Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.
An auxiliary rearview mirror mounting structure includes two support arms of angularly adjustable multi-segment design pivotally connected to an auxiliary rearview mirror, multiple hooks located at each support arm for hooking on the rim of a main rearview mirror, and tightening-up devices mounted in each support arm corresponding to one respective hook and rotatably movable inwards and backwards for abutting against the rim of the main rearview mirror to lock the hooks to the rim of a main rearview mirror.