Typically the travel cycle of the Super Duty rear axle is an arch.
Fabtechs exclusive traction bar shackle system allows the bars to float with the arched travel of the rear axle. Track Bar Bracket;Rear;For Use w/Fabtech Rear Leaf Springs Or AMP Steps;California Residents: WARNING Fabtech Track Bar Bracket - FTS92025 This provides a non-binding, smooth highway ride, while still providing positive resistance to torque loads, reducing axle wrap and increasing tire traction..