BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a resistance generator adapted to a stationary bicycle exerciser, and more particularly, to one that generates resistance by vortex-induced magnetic loss during the cycling driven by rear wheel of the bicycle. 
     (b) Description of the Prior Art 
     Referring to FIG. 1 of the accompanying drawings, a resistance generator of the prior art is adapted to a bicycle exerciser as disclosed in U.S. Pat. No. 4,826,150 essentially comprised of a main support frame  10 , a girder frame  20  each respectively connected to both ends of the main support frame  10 , a telescopic shaft  30  vertically erected into the main support frame  10 , and a head tube  40  connected at a certain inclination to a front wheel. When the bicycle exerciser is lifted off the ground by the telescopic shaft  30  and the head tube  40 , the resistance generator  50  adapted to the rear end of the main support frame  10  contacts and is driven by a rear wheel of the bicycle exerciser to produce resistance. 
     As illustrated in FIG. 2, said resistance generator  50  by having provided an iron disk  502  and an aluminum disk  503  at an end of a transmission shaft  501 , and multiple magnets  504  fixed to the front of said iron disk  502 . Magnetic north and south poles of those magnets are interlaced with one another. Said aluminum disk  503  is placed in front of those magnets  504 . Another iron disk  506  allowing micro-adjustment is provided inside an outer cover  505  on one side of the resistance generator  50 , and multiple magnets  507  with their magnetic north and south poles are also fixed to the front of the iron disk  506 . Both groups of magnets  504  and  507  are disposed relatively on both sides of the aluminum disk  503 . Once the rear wheel cycles to drive the iron disk  502  and those magnets  504  via the transmission shaft  501 , the resistance is produced due to rejection by opposite magnetism from those magnets  507 . 
     However, the bicycle exerciser generally available in the market today consumes too much space and takes two groups of magnet to produce resistance making it a problem for use and storage and more complicate in the manufacturing process to produce the resistance generator. 
     SUMMARY OF THE INVENTION 
     The primary purpose of the present invention is to provide a resistance generator for a stationary bicycle that produces resistance by vortex magnetic loss. To achieve the purpose, the resistance generator is comprised of a support unit, a transmission unit and a magnetism-controlled unit. In the magnetism-controlled unit, multiple magnets are arranged in a circle with interlaced north and south poles on one side of a fixation disk and linked with an aluminum disk with a transmission shaft from the transmission unit. As is those magnets on the fixation disk are disposed in relation to said aluminum disk, said aluminum disk and those magnets create vortex magnetic loss to produce the resistance when a wheel is cycling to drive said aluminum disk to rotate via the transmission shaft. In turn, the resistance is generated for a rider of the stationary bicycle to achieve fitness training effects. 
     Another purpose of the present invention is to provide a resistance generator of a stationary bicycle with the resistance adjustable. To achieve this purpose, a right support disk is provided to the support unit and one or more than one slope is provided on the flange of the right support disk. An elastic member is provided behind the fixation disk from the magnetism-controlled unit and an ear is provided on the upper edge of said elastic member. Said ear penetrates the slope on the flange of the right support disk to be linked to a control button for adjusting the spacing between the fixation disk and the aluminum disk, thus to vary the amount of the resistance. 
     Another purpose yet of the present invention is to provide a resistance generator that has provided at its bottom a frame to lift the rear wheel of the stationary bicycle off the ground for the rear wheel merely contacts the transmission unit to drive the aluminum disk to rotate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an assembly of a prior art of the present invention; 
     FIG. 2 is a sectional view of a local part of the prior art of the present invention; 
     FIG. 3 is an exploded view of a preferred embodiment of the present invention; 
     FIG. 4 is a perspective view of an assembly of the preferred embodiment of the present invention; 
     FIG. 5 is a schematic view showing that the preferred embodiment of the present invention is applied to a stationary bicycle; 
     FIG. 6 is a sectional view of the assembly of the preferred embodiment of the present invention; and 
     FIG. 7 is a schematic view showing the operation of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 3 through 7, a preferred embodiment of the present invention is essentially comprised of a support unit  1 , a transmission unit  2 , a magnetism-controlled unit  3  and a frame  4 . Within, said support unit  1  including a base plate  11 , a left support disk  12  and a right support disk  13  is provided to support the transmission unit  2 . Said base plate  11  in a shape of a staple (Π) has provided on its front end a T-shape tab  111 . A connection bit  112  is each upward folded respectively from both sides on the upper end of said T-shape tab  111 . An axial hole  121  is provided at the center of, and a positioning block  122  is formed in the lower part behind said left support disk  12 . An axial hole  131  is provided at the center of, and a positioning block  132  is formed in the lower part behind said right support disk  13 . One or more than one slope  133  is provided on a flange of the front of said right support disk  13  and multiple recesses  134  are provided at equal spacing on said slope  133 . Then, both of said positioning blocks  122 ,  132  respectively provided behind the left support disk  12  and the right support disk  13  are abutted on the T-shape bit of the base plate  11 . Finally, both of said left and right support disks  12 ,  13  are respectively connected with screws to both of said positioning blocks  122 ,  132  through said two connection bits  112  provided on both sides of the T-shape tab  111  as illustrated in FIG.  6 . 
     Said transmission unit  2  is essentially comprised of a transmission shaft  21  having provided at its middle section a cylindrical friction roller  22 . Said friction roller  22  is fixed to the transmission shaft by screwing in a countersunk bolt  222  through a screw hole  221  provided sideways on the friction roller  22 . Both ends of the transmission shaft  21  are respectively inserted with a washer  211  and a bearing  212 . A fly wheel  23  to the rear end of the transmission shaft  21  to ensure its firm and consistent turning. 
     Said magnetism-controlled unit  3  for producing the resistance includes an aluminum disk  31 , a fixation ring  32 , a fan  33 , multiple magnets  34 , a fixation disk  35 , an elastic member  36 , a cover  37  and a control button  38 . Within, a through hole  311  is provided at the center of said aluminum disk  31  and multiple holes  312  to permit light are provided in the circumference of said through hole  311 . Said fixation ring  32  in a cylindrical shape has provided at its rear end a flange  321 . Multiple screw holes  322  to match those holes  312  in the aluminum disk  31  are provided on the surface of the flange  321  for the fixation ring  32  to be inserted into the aluminum disk  31  via the through hole  311 . Said fan  33  has multiple blades on its surface and an axial hole  331  at its center to allow insertion by the fixation ring  32 . Multiple through holes  332  to match those holes in the aluminum disk  31  are provided around the axial hole  331  for said fan  33  to be fixed to the front of the aluminum disk  31  by means of multiple bolts  333 . Those magnets  34  each indicating rectangular sheet are arranged in circular with interlaced north and south poles. Said fixation disk  35  related to an iron disk has a sleeve  350  protruding from its rear side, and two ears  351 ,  352  respectively protruding from its upper and lower ends. Said ear  351  is fixed into a control button  38 , which is related to a rectangular block. Multiple positioning tabs  353  are provided on the front surface of the fixation disk  35  to restrict and hold those magnets  34  in position. Said elastic member  36  related to a compression coil is inserted onto the outer circumference of the sleeve  350  protruding from the center of the fixation disk  35  to press against it forward. Said cover  37  in a disk shape has at its inner center protruding a sleeve  371  and one or more than one slope  372  is provided on its flange in relation to the slope  133  provided on the right support disk  13 . Said cover  37  has its sleeve  371  to insert into the elastic member  36  and the sleeve  350  of the fixation disk  35  as illustrated in FIG.  6 . 
     Now referring to FIG. 5, said frame  4  provided to lift the stationary bicycle out of the ground is comprised of two lateral rods  41 ,  41 ′ at the bottom with both ends of each lateral rod respectively protected by a dust cover  411 ,  411 ′. Two struts  42 ,  42 ′ are provided to both ends of both lateral rods  41 ,  41 ′ with said two struts  42 ′ each pivoted to approximately the middle of each strut  42  and linked with a bolt  421 ′ to make the frame  4  foldable. Both struts  42  to the lateral rod  41  are respectively provided with a lateral tube  43  and a threaded rod  44  to be inserted from the outer end into the inner end of the lateral tube  43 . A knob  45  is fixed to the outer end and a retaining sleeve  46  is provided to the inner side of the threaded rod  44 . A fixation knob  47  approximately in triangle is screwed to the threaded rod  44  behind the retaining sleeve  46  for the knob  45  to turn the threaded rod  44 , thus to control the relative tightening by the retaining sleeve  46 . Furthermore, the threaded rod  44  being held in position by the fixation knob  47  is prevented from loosening. 
     As illustrated in FIG. 3 or FIG. 6, the stationary bicycle comprised of the support unit  1 , the transmission unit  2 , the magnetism-controlled unit  3  and the frame  4  has the bearings  212  to the both ends of the transmission shaft  21  of the transmission unit  2  respectively to be provided through the axial holes  121 ,  131  of the left and the right support disks  12 ,  13 . One end of the transmission shaft  21  passes through the left support disk  12  to connect the fly wheel  23  on the outer side of the left support disk  12  while the other end of the transmission shaft  21  passes through the right support disk  13  for the transmission shaft  21  and the friction roller  22  to turn in the middle of the support unit  1 . Said magnetism-controlled unit  3  is fixed to one end of the transmission shaft  21  inside the right support disk  13  with the fixation ring  32  so that both of the aluminum disk  31  and the fan  33  to the fixation ring  32  are driven to turn. Said cover  37  is fixed in relation to the right support disk  13  and one or more than one slot is respectively formed on the slopes  133 ,  372  provided on the flanges of the right support disk  13  and the cover  37  to allow penetration by the upper ear  351  protruding from the fixation disk  35  and the control button  38  to be provided above the magnetism-controlled unit  3 . A spacing is formed to the front magnets  34  in relation to the aluminum disk  31  as the elastic member  36  holds against the fixation disk  35  inside the cover  37 . By means of said spacing, the base plate  11  of the support unit  1  is fixed to one lateral rod  41  of the frame  4  to install the present invention to the stationary bicycle. 
     Both threaded rods  44  on the upper end of the frame  4  are turned for the inner ends of the retaining sleeves  46  to clamp onto both ends of a rear wheel shaft of the stationary bicycle. Then both fixation knobs  47  are tightened up to secure said threaded rods  44 , thus the rear wheel of the stationary bicycle is lifted out of the ground and merely contacts the surface of the friction roller  22  of the transmission unit  2  as illustrated in FIG.  7 . Accordingly, when the rear wheel is cycling, the friction roller  22  and the transmission shaft  21  are synchronously driven to cause the aluminum disk  31  to turn on one side of those magnets  34 . Since said aluminum disk  31  turns in relation to the side of the magnet  34 , a vortex magnetic loss effect is synchronously created to produce resistance to the turning of the transmission unit  2 . The rider of the stationary bicycle has to apply more efforts to keep the rear wheel cycling, thus to achieve the fitness exercise purpose. 
     Now referring to FIG. 3, the adjustment of the amount of the resistance is achieved by changing the axial position of the upper ear  351  of the fixation disk  35  to rest on the slope  133 . Said position of the upper ear  351  resting on the slop  133  is changed by pushing the control button  38  provided above the cover  37 . Upon completing the adjustment, the upper ear  351  is clicked into the proper recess  134  to be held in position. By taking advantage of the upper ear  351  to move against the slope  133  and the plunging by the elastic member  36 , the axial displacement of the fixation disk  35  can be adjusted as desired to control the spacing between the magnet  34  and the aluminum disk  31 . 
     Furthermore, to maintain the strength and service life of each of all the members of the present invention, the fan  33  provided in front of the aluminum disk  31  of the magnetism-controlled unit  3  synchronously turns to directly dispense the heat generated from those magnets  34  and other members to avoid the cover  37  from getting overheated. To ensure firm and consistent operation of the transmission unit  2 , the flywheel  23  is provided at one end of the transmission shaft  21  to avoid vibration and to help increase the resistance for emphasizing the training of the legs of the rider. To save storage space required by the stationary bicycle, the frame  4  can be folded up by means of those lateral rods  41 ′ and struts  42 ′.
A resistance generator adapted to a stationary bicycle exerciser to produce resistance when driven by cycling essentially comprised of a support unit, a transmission unit and a magnetism controlled unit; within, said magnetism controlled unit containing multiples of magnet arranged in circular being provided to a fixation disk and connected with an aluminum disk to a transmission shaft in the transmission unit; and resistance being produced by vortex magnetic loss resulted from those magnets positioned in relation to the rotating aluminum disk as driven by a wheel.