An adjustment device includes a casing to which a movable member and two positioning members are movably connected. Two links are connected between the movable member and the positioning members. Each positioning member has a magnetic member connected thereto and the flywheel has a magnetic operation face. When the movable member moves from a second position to a first position, the links drive the positioning members and the magnetic members from a close-to-center position to an off-center position, and the flywheel cuts the magnetic field of the magnetic members to generate a first resistance to the driving device and the flywheel. When the movable member moves from the first position to the second position, the positioning members and the magnetic members move from the off-center position to the close-to-center position to generate a second resistance that is different from the first resistance.