A flywheel assembly includes a flywheel rotatably mounted on an exercise and a magnetic control device is centrally received in the flywheel for providing damping to the flywheel. The magnetic control device includes two discs abutted each other. Each disc has a wiggle seat rotatable mounted thereon and each wiggle seat is formed with a sector shoe. Each sector has multiple magnets peripherally secured thereon such that the damping acted on the flywheel is adjusted due to the magnets when the two wiggle seats are synchronously wiggled relative to a corresponding one of the two discs.