An adjustable magnetic resistance force of an exercise bike includes two members pivotally connected to a frame of the bike and a casing with magnets received therein is connected to the two members. A plate is connected between a spring and a control cable of the bike. The plate is pivotably connected to one of the two members so that when pulling the control cable, the two members are pivoted and the casing is shifted away from the fly wheel so as to adjust the overlapped portion between the magnets and the fly wheel.
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