The pulling arm attaches the trailer to your bicycle rack. The arm is made from 1"x1" steel tubing and 1"x3/16" steel flat bar . The pulling arm attaches to the upright tube on the frame with two pinch bolts. The top pinch bolt goes through the two clamping plates with the 3/8" holes and around the upright tube. This clamps firming on to the upright tube. The bottom pinch bolt tightens against the upright tube and uses the tapped hole. This keeps the arm from twisting on the upright tube. I chose to attach the pulling arm in this way because wanted to be able to adjust the height of the arm in order to keep the trailer platform level. By moving the arm up and down I can adjust for different bike rack heights, so the trailer can always be level on any bike its attached to.