Diagnosing Wheel Vibration

How to tell if your wheels or tires are bent

Karlis Dambrans/Flickr

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 steering wheel shimmy.

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.

Once we understand the history of the vibration, the next step is to inspect the wheels and tires. The best way to do this is to remove all four wheels and spin the wheel and tire assembly on a balancer. Once the wheel is on the balancer, it should be spun by hand. With the wheel centered and spinning, we look carefully at the outer edges of the wheel on both the inboard and outboard faces. Factory tolerances for a wheel are about .030” (30 thousandths of an inch) both lateral (side to side) and radial (up and down). Most deflections or bends outside that range will be visible to the naked eye while the wheel is spinning centered. If the wheel is straight, the line formed by the outer edges of the rim should be relatively stable, and it should not wobble from side to side.

If the wheel is straight, determine if the tire is out of round. Put your eyes level with the tread surface as it spins and look straight across the surface. Does the tread bounce up and down without an equivalent motion in the wheel? The tire is probably out of round. A steel belt may be bent or delaminated inside the tire, or the tire may be wearing irregularly. Look at the tire straight on; do the tread blocks wiggle from side to side? This indicates that the tires are getting lateral wear, usually as a result of an alignment issue.

Of course, it may be difficult to convince your local tire shop to let you go out back and watch your wheels spin on their balancer. Different shops will have different policies on this, as insurance regulations are often involved. If yours won't, I can only suggest trying smaller shops who can make exceptions if you explain what you're trying to do. Alternately, you can jack up the car or put it on  jack stands, put the transmission in neutral and spin the wheels on the car, or have a friend spin them while you look under the car at the inboard side. It's not as accurate, because the suspension will move a little, but it's a quick and (very) dirty way to get a rough idea.