a mud flap mounting device for use with a tractor trailer truck is disclosed comprising a frame member adapted for attachment to the tractor truck and having an upper surface with a trough therein , a generally l - shaped support rod having a first leg adapted for attachment to a mud flap and a second leg rotatably mounted to the frame member to rotate between a first position wherein the mud flap is adjacent a wheel to a second position 180 ° therefrom , and biasing means for yieldably maintaining the support rod in the first position and alternatively in the second position .