Bad Vibrations

How to find shakes and rattles while the car rolls


No matter what kind of wheel balancer you have, its calibration should be checked at least once a month. The job is easy because only three things are needed: a straight wheel, wheel weights and the owner’s manual.

The procedure varies depending on the manufacturer and the machine, but the basic concept is the same. A wheel of known dimensions is spun and a baseline measurement is taken. Next, a known amount of weight is added to create an imbalance. The wheel is spun again, and if the imbalance matches the added weight, calibration is correct. If it’s not correct, that’s when you need the owner’s manual.

It may be simple, but there are some important details. First of all, the wheel must be straight. Some balancers require a wheel of specific size, and if a cone is used to center the wheel on the balancer shaft, the wheel must be hub centric (many wheels are centered by the studs, not the hub). Some shops keep a new steel wheel as their ‘calibration’ wheel.

Some balancers are shipped with the calibration weights that were used when the machine was built. These must be used to check calibration in the shop too. Whether using those weights or your own, calibration weights are usually fairly hefty, at least 3 ounces (85 grams). This makes the procedure easier and more accurate.

Hub Centered vs Stud Centered

There are two different ways to center the wheel on an axle: hub-centric and stud-centric. Hub-centric wheels fit onto a slight extension of the hub or axle. This keeps the tire mounting bead concentric with the axle. Many aftermarket wheels are hub-centric but have an oversized hub hole, so centering rings are used to make them fit the vehicle. Trucks and SUVs typically use wheels that are centered by the studs. The hole in the center of the wheel may be centered, BUT NOT ALWAYS.

When mounting wheels on a wheel balancer, you need to know which type of wheel you’re working with. For stud-centric wheels, an adapter plate is needed. They are available with every wheel balancer, and there are several aftermarket adapters too. Most are adjustable for different bolt patterns. When mounting a wheel on an adapter, make sure to torque the nuts evenly.

Chassis Vibrations

Wheels and tires are only one possible source of road vibration, especially in our customer's older 3,700-pound full-frame 4WD truck. Vibration can come from other parts of the chassis, including axles or driveshafts, wheel bearings, worn suspension bushings and even faulty motor mounts.

Almost any vibration that can be felt produces noise too. Most techs have used a long screwdriver as a stethoscope to locate noise, but that’s not really possible when the car is being driven. However, there are tools designed for exactly that purpose.

Steelman's Chassis Ears are small radio transmitters that have a clamp with a built-in microphone. With the clamp attached to a suspected problem area, the transmitter is secured to the vehicle and the signal from this electronic stethoscope is sent to a 4-channel receiver. A tech can listen to each transmitter one at a time while someone else drives. It takes practice and experience with known-good vehicles, but this tool can help you quickly find the exact location of any noise or vibration from gears, bearings or suspension pieces while under load. It’s especially useful on trailers (transmitter range is 50 feet).

A Black Art

Like using scanners and scopes to diagnose engine problems, finding a chassis vibration is an art. A bad axle bearing or dragging brake caliper might produce more vibration than noise, and finding it requires imagination and resourcefulness, plus the ability to ‘think outside the box.’ So imagine how you would like to test your theory, then look in your toolbox, look around the shop and look in the pages of this magazine to find the tools that will help you get the job done.


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