I recently took my car for a major service, followed by wheel alignment. Five hundred kilometres later a vibration appeared on the steering wheel at about 120km/h. What could the problem be? At the workshop and wheel alignment centre they said the car was in perfect condition.
George, assuming the wheels were balanced during the alignment check (the two things are often done together), it would be unusual for them to go out of balance so soon. I suggest you first determine whether it is really a wheel balance problem, by taking the speed up to a point where the vibration is well established, and then quickly depressing the clutch pedal. If the vibration goes away when you do this, you are probably looking at a driveshaft problem rather than wheel imbalance. If the test exonerates the drivetrain, the following come to mind as possible causes:
-A tyre may have lost a balance weight — these things can work loose or be knocked loose by a jolt from a pothole
-Tread separation or some other structural defect could have appeared on a tyre — a bulge in the tread will cause vibration, although not normally speed-related
-One or more shock absorbers may have given up the ghost, allowing excessive wheel bounce — but again, it should be noticeable at lower speeds as well. I recommend that you have the wheels rebalanced, taking the opportunity to give them a close visual and tactile examination while they are on the balancing machine.