Automotive vibrations are usually related to the rotating speed of a component. Calculate the speed of these components using either an engine speed method or a vehicle speed method. Use the engine RPM readings taken during the road test in order to diagnose the vibrations that are sensitive to engine speed.

If the vibration is sensitive to vehicle speed, determine the rotational speed of the tires. As long as you operate the vehicle at a constant speed, the tires will operate at a constant speed. This speed is measured in rotations, or cycles per second. The reading is then compared to the frequency of the vibration, which is also measured in cycles per second.

Determine the rotational speed of the tires in revolutions per second, or Hertz (Hz), based on the vehicle speed at which the vibration occurs. In order to determine the rotational speed, use the following procedure:

1. Determine the concern speed -- the vehicle speed at which the vibration occurs.
2. Determine the number of 8 km/h (5 mph) increments:

•	Divide the concern speed by 8 when using km/h.

•	Divide the concern speed by 5 when using mph.

3. Measure the tires rolling circumference.
4. Divide the tires rolling circumference by 88.
5. Multiply the Hertz value by the number of 8 km/h (5 mph) increments.
6. The result is the rotational speed of the tires in Hertz at the concern speed. If this figure matches the vibration frequency, a first-order vibration is present in the tire and wheel assembly.

Sometimes, the tire/wheel vibration may be of a higher-order. In order to compute possible higher-order vibrations, multiply the rotational speed of the tires in Hertz at the concern speed by the order number. If any of these matches the vibration frequency, then a vibration of that particular order is present in one of the tire and wheel assemblies.

Steering and suspension assembly vibrations are the first level of testing for low-frequency vibrations that are sensitive to vehicle speed. The symptoms of a steering/suspension first-order vibration are a shimmy or a shake. This is usually felt in the steering wheel or in the seat. Inspect the following components for wear or damage:

The propeller shaft drives the tires through the rear axle. Knowing the rotational speed of the tires enables you to easily calculate the speed of the propeller shaft. Therefore, you can determine the rotational speed of the propeller shaft by multiplying the rotational speed of the tires by the ratio of the rear axle.

1. Obtain the tire speed by performing the tire speed calculation, described earlier in this section.
2. Determine the axle ratio using one of the following methods:

•	Cross reference the axle information from the service information part label to the parts book.

•	Cross reference the axle identification code stamped on the axle to the parts book.

•	Perform the following calculation:

1.	Make a mark on the tires and the propeller shaft.

2.	Rotate the tires for one complete revolution, counting the number of propeller shaft rotations.

For example, the propeller shaft rotates approximately 3 1/2 times for every tire revolution. The axle ratio is 3.5:1

3. In order to find the propeller shaft rotation speed, multiply the tire speed by the rear axle ratio.

In the earlier example, at a 60 mph concern speed:

12 Hz x 3.5 = 42 Hz

Match the frequency of the vibration from the EVA with the rotational speed of a component (either a tire or a wheel). If none of the frequencies match, do one of the following:

When you have matched the speed of the component with the frequency of the vibration, refer to the service information that outlines the repair procedures for that particular component.