A bent or damaged wheel or rim creates an imbalance in the wheel/tire assembly. If a wheel is bent even slightly it disturbs the perfect weight distribution that was set up when the wheel was balanced. Often the effect is a constant vibration that becomes noticeable at vehicle speeds over 40 mph and changes in magnitude as speed increases.
Vibrations can be caused by tires that are out-of-round or have uneven wear patterns, bulges or flat spots. A heavy or light spot in a tire is a source of imbalance and therefore of vehicle vibration. Such spots prevent a tire from rolling evenly and the tire/wheel assembly undergoes vertical movement.
Vehicle vibration can be related to mechanical conditions in the driveline, such as an imbalanced driveshaft or a worn or incorrectly installed U-joint. Other possible causes include worn shock absorbers, loose or damaged components in the suspension or steering linkage, rotor and axle imbalances and resonant vibrations of the exhaust system.