A flywheel assembly for exercise devices includes a flywheel. A first disc and a second disc sequentially received in the flywheel and securely attached to each other. A control plate is rotatably sandwiched between the first disc and the second disc. Multiple magnetic devices are respectively and radially connected to the control plate. A shaft centrally extends through the first disc, the second disc and the flywheel, and laterally mounted onto the second disc, wherein the flywheel is rotatable relative to the first disc, the control plate with the magnetic devices and the second disc. A drive device is laterally mounted onto the second disc for driving the control plate and adjusting the damping to the flywheel when the control plate is rotated relative to the first disc and the second disc due to the operation from the drive device.