A power back door for a vehicle, in which the back door is automatically opened and closed by left and right power back door drive devices, each of the power back door drive devices including an inner tube encompassed by an outer tube; a motor; a spindle screw that is turned by the motor; a spindle nut attached to one end of the inner tube and travels up and down the spindle screw to cause the inner tube to extend and contract relative to the outer tube over a length of travel; and a stopper fixed to the end of the spindle screw to stop the movement of the spindle nut. A length of travel of the left power back door drive device is different than a length of travel of the right power back door drive device so as to attenuate bouncing of the back door when the back door reaches a fully opened position.