The next step after road testing the vehicle is to identify the frequency of the duplicated and abnormal vibration. Use the EVA in order to measure the frequency. If the EVA is not available, the frequency can be categorized into groups according to how the vibration feels or sounds. The majority of vibrations will fit into one of the following categories.

Shake

The shake is a low frequency vibration, typically 5-20 Hz. The shake is sometimes seen in the steering wheel, the seat, or the console. The best description is the feeling from an out-of-round or unbalanced tire. Customers may refer to shake in one of the following terms:

In most cases, damage to or wear of the following components is to blame for the shake:

Roughness

Roughness is a vibration with a slightly higher frequency than the shake, usually 20-50 Hz. Roughness is similar to the feeling you get from holding a jigsaw.

Buzz

Buzz is slightly higher in frequency: 50-100 Hz. A buzz is similar to the feel of an electric razor. You may feel it in your hands through the steering wheel, in the feet through the floor, or in the seat. Inspect the following components for a possible cause:

Tingling

This is the highest frequency that can still be felt. Tingling may sometimes produce a PINS AND NEEDLES sensation. Customers may say the vibration puts their hands or feet to SLEEP.

Boom

Boom is a low frequency interior noise of 20-60 Hz. Sometimes customers complain of a pressure in their ears. Examples of similar noises include a bowling ball rolling down an alley, deep thunder, or a bass drum.

A customer may use the following words to describe boom:

Boom may or may not be accompanied by a perceptible vibration (roughness).

Moan or Drone

Moan or drone is a sustained tone at a low frequency of 60-120 Hz, somewhat higher than boom.

Examples of similar noises include a bumble bee, or blowing air across the top of a soda bottle. Some examples of words to describe moan or drone are humming, buzzing and resonance. Moan or drone may be accompanied by a perceptible buzzing vibration. Inspect the following systems:

Howl

Howl is a noise at mid-range frequency of 120-300 Hz. This sounds like the wind howling.

Whine

Whine is a prolonged, high-pitched sound in the 300-500 Hz range, and is usually related to the meshing gears or gear noise. Similar sounds include mosquitoes, turbine engines, and vacuum cleaners.

At this point in the diagnosis, the vibration has been duplicated, designated as abnormal, identified as being related to engine speed or vehicle speed, and assigned a frequency from the EVA or categorized into a symptom group based on how it feels or sounds.

Automotive vibrations are usually related to the rotating speed of a component. The speed of these components will be calculated using either an engine speed or vehicle speed method. The engine rpm readings taken during the road test will be used in diagnosing vibrations that are engine speed sensitive. If the vibration is vehicle speed sensitive, the rotational speed of the tires needs to be determined. As long as the vehicle is operated at a constant speed, the tires will operate at a constant speed. These 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, follow this procedure:

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

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

•	Divide the complaint speed by 5 when using mph.

3. Determine the vehicle tire size.
4. Locate on the following table, the Hertz value at 8 km/h (5 mph) for that tire size.
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 complaint 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 complaint 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.

We can calculate the speed of the propeller shaft as long as we know the rotational speed of the tires. The propeller shaft drives the tires through the rear axle. Therefore, the rotational speed of the propeller shaft is determined when the rotational speed of the tires is multiplied by the ratio of the rear axle.

1. Insert the correct complaint speed, tire size, and axle ratio on the worksheet.

•	The tire size is always embossed in the sidewall.

•	Axle ratio is located: on the window sticker or factory invoice, option code label, or make a mark on the tires and propeller shaft. Rotate the tires one complete revolution and count the number of propeller shaft rotations.

2. After completing the form, match the frequency of the vibration from the EVA with the rotational speed of a component in the worksheet.
3. If none of the frequencies match, then either recheck the data, or attempt to rematch the figures by allowing for 2.5-8 km/h (1-5 mph) of speedometer error.
4. Refer to the section that oulines the repair procedure for the particular component that matches the frequency of the vibration.

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