Vibration Symptoms

Most vibrations consist of movements back and forth or up and down that repeat. Every time the vibrating component goes through its complete range of motion and returns to the starting point is called a cycle. The rate at which these cycles occur within a given time is called the frequency. Frequency is measured in cycles per second or Hertz (Hz). One cycle per second equals one Hertz (Hz). Once the frequency of a vibration is known, calculations can be done to determine the system that is the source of the concern.

Order of Vibration 

The order of a vibration refers to how often the vibration is present in one revolution of the component. For example, a vibration that is present once each revolution of a component would be a first order vibration. A vibration present twice each revolution of the component would be a second order vibration. Vibration orders do not have to be whole numbers, they can have decimal values such as 1.5 order vibration or 3.08 order vibration.

The concept of order of vibration is important to remember when the measured frequency of a vibration does not seem to match the frequency calculations of any of the likely systems or components. As the order increases, the frequency of the vibration will also increase by a multiple of that number.

For example, vibration may be present where the frequency is measured at 14 Hertz (Hz). After doing the necessary calculations it is determined the first order tire and wheel frequency is 7 Hertz (Hz) and the first order driveshaft frequency is 22 Hertz (Hz). Based on this information it can be determined the vibration is most likely a second order tire and wheel vibration: 7 Hertz (Hz) (first order tire and wheel frequency) multiplied by 2 (second order) equals 14 Hertz (Hz) (second order tire and wheel frequency).

Relationship of Vibration Frequency to Order of Vibration 

After carrying out the road test as described in this article, the vibration was determined to be either vehicle-speed related or electric motor-speed related. That determination will identify the vibration frequency calculations that should be done.

In calculating and using frequency readings it is important to remember the direct relationship between Hertz (Hz) and RPM. One Hertz (Hz) is equal to 60 RPM. This is easy to remember. Think of Hertz (Hz) as cycles per second. There are 60 seconds in a minute, therefore multiply the Hertz (Hz) reading by 60 to get RPM. Conversely, divide RPM by 60 to get Hertz (Hz).

Use the Frequency and RPM Calculations Worksheet to calculate system/component frequencies. The worksheet provides the necessary steps to determine each system/component group frequency.

Frequency Calculations 

Calculating Tire and Wheel Frequency 

For a vibration concern, use the vehicle speed to determine tire/wheel frequency and RPM. Calculate the tire and wheel RPM and frequency by carrying out and following:

• Measure the diameter of the tire.
• Record the speed at which the vibration occurs.
• Obtain the corresponding tire and wheel RPM and frequency from the Tire Speed and Frequency Chart.
  • If the vehicle speed is not listed, divide the vehicle speed at which the vibration occurs by 16 km/h (10 mph). Multiply that number by the 16 km/h (10 mph) tire RPM listed for that tire diameter in the chart. Then divide that number by 60. For example: a 40 MPH (Miles Per Hour) vibration with 835 mm (33 in) tires. 40 divided by 10 = 4. Multiply 4 by 105 = 420 RPM. Divide 420 RPM by 60 seconds = 7 Hertz (Hz) at 40 MPH (Miles Per Hour).

Tire Speed and Frequency Chart