In 80 percent to 90 percent of cases, these problems are caused by front wheel imbalances, by bent rims or defective tires, or by out-of-round wheels. The effects usually appear at speeds over 50 mph, and the cause is normally diagnosed during a wheel balance.
When a problem persists, even with balanced wheels and tires, most likely the source is somewhere else in the vehicle's front end. The cause could be imbalanced brake rotors or drums, worn wheel bearings, bad shock absorbers, an out-of-round component, or worn engine or transmission mounts.
Less common causes of these problems are bent a drive shaft, out-of-balance drive shaft on rear-wheel-drive vehicles, excessive drive-shaft angle in 4x4 trucks, defective or misaligned U-joint, defective CV (constant velocity) joint, missing damper weights on the chassis or drivetrain, exhaust vibration, and broken or loose body parts at the front of the car.