I have never had a car with anything but a stock suspension so I have always went to the suspension/axle. A neighbor goes to the frame. He had a strap loosen slightly which allowed the frame to start moving. A short while later all the straps were loose from the car moving ever so slightly. Of course he was in the left lane going 70 mph when he noticed it. Another buddy tows long travel off road trucks alot. He goes to the frame and the suspension/axle. The suspension/axle is to keep the truck on the trailer and the frame is to keep the truck body from moving......