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Component Rotational Speed Calculation

Special Tools

GE-38792-A Electronic Vibration Analyser (EVA) 2

For equivalent regional tools, refer to Special Tools and Equipment .

Determining tyre Revolutions Per Second at 8 km/h (5 mph) -- Using EVA

Tyre and wheel assembly rotational speed can be obtained through using the GE-38792-A Electronic Vibration Analyser (EVA) 2. Perform the following steps using the GE-38792-A Electronic Vibration Analyser (EVA) 2 to obtain the rotational speed at 8 km/h (5 mph). Use the Enter key to advance and the Exit key to backup.

1. On the Main Menu screen, select Auto Mode.
2. On the Suspected Source screen, select Vehicle Speed.
3. On the Tyre Info Source screen, select Manual Entry.
4. On the Tyre Width screen, enter the specific width of the tyres.

For example: For a P245/45/R18 tyre, enter 245.

5. On the Aspect Ratio screen, enter the specific aspect ratio of the tyres.

For example: For a P245/45/R18 tyre, enter 0.45.

6. On the Rim Diameter screen, enter the specific rim diameter size.

For example: For a P245/45/R18 tyre, enter 18.0.

7. On the Driveshaft Configuration screen, enter FWD, even if the vehicle is a rear wheel drive.
8. The next screen will display the tyre size just entered for confirmation.

For example: 245 0.45 18.0 -Front Wheel Drive. If the tyre size displayed is correct, press Enter.

9. On the Vehicle Speed Units screen, press Enter, disregard mph or km/h.
10. Press the Exit key several times slowly while watching the backwards progression of the screens. Stop at the Tyre Info Source screen.
11. On the Tyre Info Source screen, select RPS at 5 mph.
12. The next screen will display the revolutions per second (RPS) at 8 km/h (5 mph) for that specific tyre size.

For example: The P245/45/R18 will display 1.08 RPS.

Calculating tyre Revolutions Per Second at 8 km/h (5 mph) -- Without EVA

If the GE-38792-A Electronic Vibration Analyser (EVA) 2 is not available, the tyre and wheel assembly rotational speed can be calculated approximately by performing the following steps.

1. Convert the rim diameter size from inches to centimetres.

For example: For a P245/45/R18 tyre, the rim diameter of 18 in X 2.54 converts to 45.72 cm.

2. Calculate the radius of the rim by dividing the rim diameter by 2.

For example: For a P245/45/R18 tyre, the rim diameter of 18 is converted to 45.72 cm divided by 2 = rim radius 22.86 cm.

3. Calculate the approximate tyre sidewall height by multiplying the specific tyre tread width by the aspect ratio, then reduce 7 percent from the amount by multiplying by 93 percent to approximate load on the tyre reducing the sidewall height.

For example: For a P245/45/R18 tyre, tread width 245 mm X aspect ratio as a decimal 0.45 = 110 mm X 0.93 = approximate sidewall height 102.30 mm.

4. Convert the calculated approximate tyre sidewall height from millimetres to centimetres.

For example: For a P245/45/R18 tyre, approximate sidewall height 102.30 mm converts to 10.23 cm.

5. Calculate the approximate tyre and wheel assembly radius by adding the rim radius and approximate sidewall height, both in cm.

For example: For a P245/45/R18 tyre, rim radius 22.86 cm + 10.23 cm = approximate tyre and wheel assembly radius 33.09 cm.

6. Calculate the approximate circumference of the tyre and wheel assembly by multiplying 2 X pi, or 6.283185 X the approximate tyre and wheel assembly radius.

For example: For a P245/45/R18 tyre, 6.283185 X approximate tyre and wheel assembly radius 33.09 cm = approximate tyre and wheel assembly circumference 207.911 cm.

7. Calculate the approximate revolutions per kilometre by dividing the number of cm in 1 km, 100,000 cm by the approximate tyre and wheel assembly circumference.

For example: For a P245/45/R18 tyre, 100,000 cm divided by approximate tyre and wheel assembly circumference 207.911 cm = approximate revolutions per kilometre 480.975.

8. Calculate the approximate revolutions per second (RPS), or Hz, by dividing the approximate revolutions per kilometre by the number of seconds to travel 1 km at a speed of 8 km per hour, 450 seconds.

For example: For a P245/45/R18 tyre, approximate revolutions per kilometre 480.975 divided by the number of seconds to travel 1 km at a speed of 8 km per hour, 450 seconds = approximate RPS, or Hz 1.069 rounded to 1.07.

A size P235/75R15 tyre rotates ONE complete revolution per second (RPS), or 1 Hz, at a vehicle speed of 8 km/h (5 mph). This means that at 16 km/h (10 mph), the same tyre will make TWO complete revolutions in one second, 2 Hz, and so on.

1. Determine the rotational speed of the tyres in revolutions per second (RPS), or Hertz (Hz), at 8 km/h (5 mph), based on the size of the tyres. Refer to the preceding Tyre Rotational Speed information.

For example: According to the Tyre Rotational Speed information, a P245/45R18 tyre makes 1.08 revolutions per second (Hz) at a vehicle speed of 8 km/h (5 mph). This means that for every increment of 8 km/h (5 mph) in vehicle speed, the tyre's rotation increases by 1.08 revolutions per second, or Hz.

2. Determine the number of increments of 8 km/h (5 mph) that are present, based on the vehicle speed in km/h (mph) at which the disturbance occurs.

For example: Assume that a disturbance occurs at a vehicle speed of 96 km/h (60 mph). A speed of 96 km/h (60 mph) has 12 INCREMENTS of 8 km/h (5 mph):

96 km/h (60 mph) divided by 8 km/h (5 mph) = 12 increments

3. Determine the rotational speed of the tyres in revolutions per second, or Hz, at the specific vehicle speed in km/h (mph) at which the disturbance occurs.

For example: To determine the tyre rotational speed at 96 km/h (60 mph), multiply the number of increments of 8 km/h (5 mph) by the revolutions per second, or Hz, for one increment:

12 increments X 1.08 Hz = 12.96 Hz, rounded to 13 Hz

Note: If the GE-38792-A Electronic Vibration Analyser (EVA) 2 is not available, compare the calculated rotational speed to the frequency range associated with the symptoms of the vibration concern. Refer to Symptoms - Vibration Diagnosis and Correction .

4. Compare the rotational speed of the tyres at the specific vehicle speed at which the disturbance occurs, to the dominant frequency recorded on the GE-38792-A Electronic Vibration Analyser (EVA) 2 during testing. If the frequencies match, then a first-order disturbance related to the rotation of the tyre/wheel assemblies is present.

If the frequencies do not match, then the disturbance may be related to a higher order of tyre/wheel assembly rotation.

5. To compute higher order tyre/wheel assembly rotation related disturbances, multiply the rotational speed of the tyres at the specific vehicle speed at which the disturbance occurs, by the order number:

13 Hz X 2, for second order = 26 Hz second-order tyre/wheel assembly rotation related

13 Hz X 3, for third order = 39 Hz third-order tyre/wheel assembly rotation related

If any of these computations match the frequency of the disturbance, a disturbance of that particular order, relating to the rotation of the tyre/wheel assemblies and/or driveline components, also rotating at the same speed, is present.

Utilise the following worksheet as an aid in calculating the first, second and third order of tyre/wheel assembly rotational speed related disturbances that may be present in the vehicle.

If after completing the Tyre/Wheel Rotation Worksheet, the frequencies calculated do NOT match the dominant frequency of the disturbance recorded during testing, either recheck the data, or attempt to rematch the figures allowing for 1½-8 km/h (1-5 mph) of speedometer error.

If the possible tyre/wheel assembly rotational speed related frequencies still do not match the dominant frequency of the disturbance, the disturbance is most likely torque/load sensitive.

If after completing the Tyre/Wheel Rotation Worksheet, one of the frequencies calculated DOES match the dominant frequency of the disturbance, the disturbance is related to the rotation of that component group - tyre/wheel assembly related.