A ratchet unit comprising: a freewheel unit being configured to rotate along a first rotational direction or a second rotational direction when receiving a driving power, and having a first circular toothed surface; a ratchet ring having a second circular toothed surface configured to engage with the first circular toothed surface of the freewheel unit, wherein the first circular toothed surface and the second circular toothed surface form a unidirectional ratchet mechanism, such that when the freewheel unit rotate along the first rotational direction, the first circular toothed surface and the second circular toothed surface engage with each other and allow force transmission therebetween, and when the freewheel unit rotates along the second rotational direction, the first circular toothed surface and the second circular toothed surface disengage from each other and disallow force transmission therebetween; a sliding sleeve being telescoped with the ratchet ring in a manner that the sliding sleeve and the ratchet ring are axially slidable yet non-rotatable with respect to each other, such that the ratchet ring is allowed to slide axially against the sliding sleeve between an engaged position and a disengaged position, wherein when the ratchet ring is at the engaged position, the first circular toothed surface and the second circular toothed surface engage with each other, and when the ratchet ring is at the disengaged position, the first circular toothed surface and the second circular toothed surface disengage from each other; a magnetic assembly including a first magnetic part fixed to the ratchet ring, being characterized in: the magnetic assembly further comprising a second magnetic part fixed to a hub body, wherein the first and second magnetic parts face each other with an identical magnetic pole, such that a repulsive force normally exists therebetween and makes the ratchet ring normally stay at the engaged position, wherein when the freewheel unit rotates along the second rotational direction, a force disengaging the first circular toothed surface from the second circular toothed surface surpasses the repulsive force and pushes the ratchet ring toward the disengaged position. a freewheel unit being configured to rotate along a first rotational direction or a second rotational direction when receiving a driving power, and having a first circular toothed surface; a ratchet ring having a second circular toothed surface configured to engage with the first circular toothed surface of the freewheel unit, wherein the first circular toothed surface and the second circular toothed surface form a unidirectional ratchet mechanism, such that when the freewheel unit rotate along the first rotational direction, the first circular toothed surface and the second circular toothed surface engage with each other and allow force transmission therebetween, and when the freewheel unit rotates along the second rotational direction, the first circular toothed surface and the second circular toothed surface disengage from each other and disallow force transmission therebetween; a sliding sleeve being telescoped with the ratchet ring in a manner that the sliding sleeve and the ratchet ring are axially slidable yet non-rotatable with respect to each other, such that the ratchet ring is allowed to slide axially against the sliding sleeve between an engaged position and a disengaged position, wherein when the ratchet ring is at the engaged position, the first circular toothed surface and the second circular toothed surface engage with each other, and when the ratchet ring is at the disengaged position, the first circular toothed surface and the second circular toothed surface disengage from each other; a magnetic assembly including a first magnetic part fixed to the ratchet ring, being characterized in: the magnetic assembly further comprising a second magnetic part fixed to a hub body, wherein the first and second magnetic parts face each other with an identical magnetic pole, such that a repulsive force normally exists therebetween and makes the ratchet ring normally stay at the engaged position, wherein when the freewheel unit rotates along the second rotational direction, a force disengaging the first circular toothed surface from the second circular toothed surface surpasses the repulsive force and pushes the ratchet ring toward the disengaged position. the magnetic assembly further comprising a second magnetic part fixed to a hub body, wherein the first and second magnetic parts face each other with an identical magnetic pole, such that a repulsive force normally exists therebetween and makes the ratchet ring normally stay at the engaged position, wherein when the freewheel unit rotates along the second rotational direction, a force disengaging the first circular toothed surface from the second circular toothed surface surpasses the repulsive force and pushes the ratchet ring toward the disengaged position.