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A motor drive for a bicycle is provided. The motor drive includes a drive wheel and a motor device driving the drive wheel. The drive wheel contacts the ground and moves the bicycle by rotating friction between the drive wheel and the ground. The motor device includes a fixing portion that attaches

A motor drive for a bicycle is provided. The motor drive includes a drive wheel and a motor device driving the drive wheel. The drive wheel contacts the ground and moves the bicycle by rotating friction between the drive wheel and the ground. The motor device includes a fixing portion that attaches the motor device to the frame of the bicycle, and a driving portion that holds the drive wheel and drives the drive wheel. The driving portion is pivotable with respect to the fixing portion between a parking state and a driving state. The drive wheel is spaced from the ground in the parking state, and contacts the ground in the driving state.

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1. A motor drive for a bicycle, wherein the bicycle comprises a frame, a front wheel and a rear wheel rotatably attached to the frame, a sprocket wheel attached to the frame and two pedals that rotate the sprocket wheel, the motor drive comprising:a) a drive wheel; and b) a motor device driving the

1. A motor drive for a bicycle, wherein the bicycle comprises a frame, a front wheel and a rear wheel rotatably attached to the frame, a sprocket wheel attached to the frame and two pedals that rotate the sprocket wheel, the motor drive comprising:a) a drive wheel; and b) a motor device driving the drive wheel, wherein the drive wheel contacts the ground and moves the bicycle by rotating friction between the drive wheel and the ground, wherein the motor device comprises a fixing portion that attaches the motor device to the frame of the bicycle, and a driving portion that holds the drive wheel and drives the drive wheel, wherein the driving portion is pivotable with respect to the fixing portion between a parking state and a driving state, wherein the drive wheel is spaced from the ground in the parking state, wherein the drive wheel contacts the ground in the driving state. 2. The motor drive of claim 1, further comprising an actuator, wherein the actuator pivots the driving portion between the parking state and the driving state.3. The motor drive of claim 2, wherein the actuator comprises a cable that is connected to the driving portion, and a winch that winds or unwinds the cable.4. The motor drive of claim 3, further comprising a sensor device, wherein the sensor device senses whether the driving portion is in the parking state or in the driving state, wherein the winch of the actuator is stopped when pivoting of the driving portion to the driving state, or to the parking state is completed.5. The motor drive of claim 4, wherein the sensor device comprises a first sensor and a second sensor, wherein the first sensor is positioned so that the driving portion contacts the first sensor when the driving portion is pivoted to the driving state, and wherein the second sensor is positioned so that the driving portion contacts the second sensor when the driving portion is pivoted to the parking state.6. The motor drive of claim 1, further comprising a pressing device that presses the drive wheel to the ground when the driving portion is pivoted to the driving state.7. The motor drive of claim 6, wherein the pressing device is a tension spring, wherein the tension spring comprises a first end and a second end, and wherein the first end is attached to the fixing portion, and the second end is attached to the driving portion.8. The motor drive of claim 1, wherein the driving portion comprises a body rotationally supporting the drive wheel, and wherein the drive wheel comprises a driving motor that is contained inside the drive wheel.9. The motor drive of claim 1, wherein the driving portion comprises a driving motor, a body holding the driving motor, and an extension that extends from the body and rotationally supports the drive wheel.10. The motor drive of claim 9, wherein the extension of the driving portion and the drive wheel are positioned below the sprocket wheel of the bicycle when the motor drive is attached to the bicycle, and wherein the extension and the drive wheel are narrow enough so that the extension and the drive wheel do not interfere with the pedals of the bicycle.11. The motor drive of claim 9, wherein the fixing portion comprises a first half and a second half, wherein the first half comprises a first groove, and the second half comprises a second groove, wherein the first groove and the second groove contact the frame of the bicycle and the first half and the second half are tightened with each other when the fixing portion is fixed to the frame of the bicycle.12. The motor drive of claim 11, wherein the driving motor of the driving portion comprises an electric motor, wherein the body of the driving portion is attached to the first half of the fixing portion, wherein a battery is provided for supplying electric power to the electric motor, and wherein the battery is attached to the second half of the fixing portion.13. The motor drive of claim 12, wherein the body of the driving portion comprises a first projection and the first half of the fixing portion comprises a first slot slidingly receiving the first projection whereby the driving portion is assembled with the fixing portion by sliding the first projection into the first slot, and wherein the battery comprises a second projection and the second half of the fixing portion comprises a second slot slidingly receiving the second projection whereby the battery is assembled with the fixing portion by sliding the second projection into the second slot.14. The motor drive of claim 9, further comprising an operation switch that is installed on a handle of the bicycle, wherein the driving motor of the driving portion comprises an electric motor, wherein operation switch controls start, stop and speed of the electric motor.15. The motor drive of claim 9, further comprising a stop switch that is installed on a handle of the bicycle, wherein the driving motor of the driving portion comprises an electric motor, wherein when a brake of the bicycle is operated, the stop switch disconnects electricity to the electric motor.