Wheel separations from passenger cars, light trucks and RV’s are reviewed, and the causes are analyzed through component and full vehicle testing. Wheel separations have led to injuries from the vehicle losing control, from the separated wheel colliding with another vehicle or pedestrian, or from another vehicle maneuvering to avoid the projectile. Separations are often soon after a wheel installation. This paper describes the physical evidence often seen after a wheel separation. Interpretation of the evidence through analysis and experiment indicates a low clamping force by the wheel studs and nuts leads to nut detachment or stud fatigue fracture. A low clamping force can result from improperly tightened nuts or from a loss in clamping force due to a very small amount of wear in the mating components clamped by properly tightened studs and nuts. Retightening wheel nuts soon after a wheel change reestablishes any lost clamping force, and so is effective at preventing wheel separations.