BACKGROUND OF THE INVENTION 
     The present invention relates to a bicycle power generating arrangement, which comprises a rack mounted on the frame of a bicycle to hold wheel means on the ground, and a dynamo coupled to the wheel means and actuated to generate electricity upon running of the bicycle. 
     Various structures of dynamos for bicycle have been disclosed. FIG. 1 shows a dynamo installed in the frame of a bicycle and pressed on the tire of the front wheel according to the prior art. When rotating the front wheel of the bicycle, the dynamo is driven to generate electricity. This structure of dynamo is not satisfactory in function because much resisting force is produced against the pedaling effort of the bicycle rider. The resisting force comes from two sources. One source of the resisting force is the spring means, which forces the rotor of the dynamo against the tire of the front wheel of the bicycle. Because the spring means imparts a pressure to the dynamo against the tire of the front wheel in direction perpendicular to the steering direction of the bicycle, much resisting force is produced against rotation of the front wheel. 
     The other source of the resisting force is the force required to actuate the dynamo. The output capacity of a dynamo for bicycle may be 6V, 3 W. The power of 3 W is not great. However, because the position of the driving power for the dynamo is at the lateral side of the tire of the front wheel of the bicycle, the arm of force is long, and much resistance is produced when rotating the rotor of the dynamo. Another drawback of this structure of dynamo is that the tire of the front wheel of the bicycle wears quickly with use. Because the rotor of the dynamo rubs on the tire of the front wheel of the bicycle during rotation of the front wheel of the bicycle, the tire of the front wheel of the bicycle wears quickly. Furthermore, because the rotor of the dynamo rubs on the tire of the front wheel of the bicycle, the front wheel of the bicycle tends to be forced out of alignment, causing the bicycle to vibrate during running. FIG. 2 illustrates another structure of dynamo according to the prior art. This structure of dynamo is installed in the spokes of the front wheel of the bicycle, and actuated to generate electricity upon running of the bicycle. This structure of dynamo also has drawbacks. Because the dynamo is not detachable, it works upon running of the bicycle. In the daytime, it is not necessary to provide electricity to the lamp of the bicycle, however the dynamo still works. Further, because the revolving speed of the dynamo is equal to the revolving speed of the wheels of the bicycle, the intensity of magnetic field or the length of the enamel wire of the dynamo must be relatively increased so that sufficient output voltage can be obtained (e=Blv, B=intensity of magnetic field, l=length of enamel wire, v=velocity). However, increasing the intensity of magnetic field or the length of the enamel wire of the dynamo relatively increases the manufacturing cost of the dynamo and its dimension. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished to provide a bicycle power generating arrangement, which eliminates the aforesaid drawbacks. According to one aspect of the present invention, the bicycle power generating arrangement comprises a dynamo, and wheel means dragged from the frame of the bicycle and rotated over the ground to actuate the dynamo upon running of the bicycle. According to another aspect of the present invention, lock means is provided and adapted to lock the wheel means in a lifted position, preventing the wheel means from rotation during running of the bicycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a dynamo installed in the frame of a bicycle and pressed on the tire of the front wheel according to the prior art. 
     FIG. 2 illustrates another structure of dynamo installed in the spokes of the front wheel of a bicycle according to the prior art. 
     FIG. 3 is a sectional view showing a dynamo coupled between two wheels according to the present invention. 
     FIG. 4 is a side view of the present invention. 
     FIG. 5 is similar to FIG. 4 but showing the control rod lifted, the wheels suspended in the open air above the ground. 
     FIG. 5A is an enlarged view of a part of FIG.  5 . 
     FIG. 6 is a circuit diagram of a voltage stabilizing circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 3 and 4, a dynamo  10  is mounted on a rack  2  on the frame of a bicycle, and actuated by wheels  1  to convert mechanical energy into electric energy. When riding the bicycle, the wheels  1  move over the ground and rotate, causing the dynamo  10  to convert mechanical energy into electric energy. 
     Referring to FIG. 3 again, the dynamo  10  comprises an axle  11  fixedly connected between the wheels  1 , a magnet  101  mounted on and rotated with the axle  11 , two windings  102  fixedly disposed at two opposite sides of the magnet  101 . When rotating the wheels  1 , the magnet  101  is rotated with the axle  11  relative to the windings  102 , thereby causing electricity to be produced. 
     Referring to FIGS. 4,  5  and  5 A, the rack  2  comprises a front mounting frame  21  fixedly fastened to the down tube of the bicycle, a rear mounting frame  22  connected between a rear part of the front mounting frame  21  and the chain stays of the bicycle, a movable frame  23  having one end pivoted to the rear mounting frame  22  and an opposite end connected to the shell of the dynamo  10 , an upper barrel  25  and a lower barrel  26  fixedly fastened to the seat tube of the bicycle at different elevations and arranged in a line, a tubular control rod  24  inserted through the upper barrel  25  and the lower barrel  26  and pivoted to the movable frame  23 , and a spring hook  27  mounted in the control rod  24  near the top end of the control rod  24 . The spring hook  27  comprises a handle portion  271  and a bevel guide face  272 . When not in use, the control rod  24  is pulled upwards to lift the movable frame  23 . When lifting the control rod  24 , the bevel guide face  272  of the spring hook  27  is moved with the control rod  24  over the inside wall of the upper barrel  25  without hindering upward movement of the control rod  24 . When passed over the upper barrel  25 , the spring hook  27  immediately returns to its former position and stopped above the top side  251  of the upper barrel  25  to prohibit the control rod  24  from downward movement. At this time, the movable frame  23  is turned to the upper position, and the wheels  1  are lifted from the ground and suspended in the open air (see FIG.  5 ). When in use, the handle portion  271  is operated with the hand to force the spring hook  27  back to the inside of the control rod  24  and to disengage the spring hook  27  from the upper barrel  25 , enabling the control rod  24  to be lowered in the barrels  25  and  26 , and at the same time the movable frame  23  is turned downwards to lower the dynamo  10  and to force the wheels  1  into contact with the ground (see FIG.  4 ). When propelling the bicycle, the wheels I are continuously rotated, thereby causing the dynamo  10  to generate electricity. 
     Referring to FIGS. 4 and 5 again, a shock-absorbing spring  231  is connected between the movable frame  23  and the shell of the dynamo  10 . The shock-absorbing spring  231  absorbs shocks when the wheels  1  move on the ground, preventing the wheels  1  from damage. 
     Referring to FIG.  6  and FIGS. 3 and 4 again, a rechargeable battery box  3  is installed in the bicycle frame to receive electricity generated from the dynamo  10 . The rechargeable battery box  3  comprises a voltage stabilizing circuit formed of a bridge rectifier and a zener diode. The voltage stabilizing circuit of the rechargeable battery box  3  stabilizes electricity generated by the dynamo  10 , and then provides stabilized electricity to the lamp or other electric apparatus attached to the bicycle, or directly charges stabilized electricity to the rechargeable battery in the rechargeable battery box  3 . 
     While only one embodiment of the present invention has been shown and described, it will be understood that various modifications and changes could be made thereunto without departing from the spirit and scope of the invention disclosed.
A power generating device that includes a dynamo, a rack mounted on the frame of a bicycle to hold the dynamo, and a wheel attached to the rack. When the device is engaged, the wheel contacts the ground and rotates to drive the dynamo while the bicycle is in motion, causing the dynamo to generate electricity.