Steering wheel oscillation, or shimmy as it is more commonly called, is defined as the rapid side -to-side movement of the steering wheel while driving. There are several factors that can cause this to happen. The two most common causes are warped brake rotors and separated tires, but there are others as well. Knowing what the cause of the shimmy is allows the problem to be corrected.
Perhaps the most common cause of steering wheel oscillations is a defect in one or more of the tires.
To identify a tire separation, raise the wheels off the ground and spin them one at a time by hand while observing the tread. If the tire tread is separated, a noticeable hop in the tread will be visible. If this is the case, the only repair is a new tire. Moving the tire to the rear of the vehicle will only cause a vibration in the rear that can be felt in the seat while driving.
Troubleshooting balance problems can be a little more difficult, and requires a balance machine to find and correct. One sure sign of a balance problem is a shimmy that starts at a certain vehicle speed, and goes away above or below that speed.
Another common cause of shimmy is an inaccurate wheel alignment. As caster angle is increased positive, there's an increase in directional stability and road feel. Too much of an angle, however, can cause the wheel itself to lose contact with the ground after a bump, and as it contacts the ground again, it will grab and bounce. This is called "washboarding," because it feels like you're driving on a washboard. The only way to pinpoint this as a cause is by checking and correcting any misalignment on an alignment machine, but a common complaint is that the car shimmies after a bump for a few seconds and then the shimmy goes away.
Rarely are steering wheel oscillations caused by worn parts, except that the worn parts can cause excessive tire wear and a separation. Worn parts will however magnify the effects of the oscillation.
Warped brake rotors can cause a shimmy as well. The telltale sign of a warped brake rotor being the cause is that the shimmy will only occur during braking and go away as soon as the brakes are released. If this is the case, the rotors can be turned or replaced as needed.
