Abstract: A transmission assembly has a frame having a through hole, a transmission gear, a driven gear, and a transmission chain surrounding the transmission gear and the driven gear. A torsion spring has a first end and a second end. A swing arm has a round hole, a circular hole, and a through aperture. A shaft rod is inserted in the round hole of the swing arm. The shaft rod passes through the torsion spring and the through hole of the frame. An idle wheel engages with the transmission chain. A bearing is adjacent to the idle wheel. The pivot rod is inserted in the circular hole of the swing arm. The pivot rod passes through the ring and the idle wheel. The first end of the torsion spring is inserted in the through aperture of the swing arm. The second end of the torsion spring is hooked on the frame.