One of the most annoying things that can happen to any car is when it picks up some kind of vibration. While a vibration is not usually a safety issue unless it becomes very bad, a shaking car can be no fun at all to drive, and can sometimes be screamingly frustrating to diagnose which of the many complex components that govern the car's contact with the road is causing the shake.
For a car to run smoothly at speed requires the contact with the road and the transmission of the contact forces to be achieved within very tight tolerances. The majority of vibration problems are caused by the wheels or tires being out of tolerance in some way, usually because of an impact. When I go to diagnose a vibration, I always check the wheels first, then the tires, followed by alignment and suspension. Alignment and suspension issues will require other articles, so we'll address how to diagnose wheels and tires first. I generally start with a couple of questions for the driver:
Do you feel the vibration in the steering wheel or in the seat?
The answer can give us an idea of whether the vibration is coming from the front end, which will generally transmit vibration directly to the steering wheel, or from the back end, which will transmit vibration more through the frame of the car and into the seat. This is not always 100% indicative, as there are a number of variables involved in car vibrations. Certain alignment issues in the back end can cause the steering wheel to vibrate as it shakes the car from side to side, for instance.
Do you feel the vibration at a certain rate of speed?
Many people come to me already saying, “I get this weird shake between X and Y miles per hour.” I am immediately fairly certain that either a wheel is bent or a tire is out of round. A vibration that has a “sweet spot” at a certain speed range is a classic symptom of harmonic modulation caused by a small bend. A wheel and tire assembly that is out of round will have a specific harmonic frequency as it spins, depending on how many bends, the severity of the bends, tire wear and other factors. As the speed changes, the harmonic changes, or modulates, as well. At certain speed ranges this modulation can reach a frequency that will overwhelm the vibration-dampening capacities of the suspension. That's the point at which you begin to feel a vibration in the car that was previously being damped out.
Do you feel a vibration in the brake pedal under hard braking?
If under moderate to stiff braking pressure you can feel the brake pedal shake under your foot, this is a good indication that what you have is a warped brake rotor or other brake-related issues. The brake rotor must be either replaced or re-lathed to make it perfectly flat.