Tools Required

J 38792-A Electronic Vibration Analyzer (EVA) 2

A size P235/75R15 tire 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 tire will make TWO complete revolutions in 1 second, 2 Hz, and so on.

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

For example: According to the Tire Rotational Speed table, a P245/45ZR17 tire makes 1.12 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 tire's rotation increases by 1.12 revolutions per second (Hz).

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

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

12 increments X 1.12 Hz = 13.44 Hz, rounded to 13 Hz

4. Compare the rotational speed of the tires at the specific vehicle speed at which the disturbance occurs, to the dominant frequency recorded on the J 38792-A during testing. If the frequencies match, then a first-order disturbance related to the rotation of the tire/wheel assemblies is present.

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

5. To compute higher order tire/wheel assembly rotation related disturbances, multiply the rotational speed of the tires 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 tire/wheel assembly rotation related

13 Hz X 3, for third order = 39 Hz third-order tire/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 tire/wheel assemblies is present.

1. Determine the first order rotational speed of the driveline - transmission output shaft/rear axle pinion gear - in revolutions per second (Hz), based on the first-order rotational speed of the tire/wheel assemblies and the drive axle ratio.

13 Hz X 3.42 drive axle ratio = 44.46 Hz, rounded to 44 Hz first-order driveline rotation related

2. Compare the rotational speed of the driveline - transmission output shaft/rear axle pinion gear - at the specific vehicle speed at which the disturbance occurs, to the dominant frequency recorded on the J 38792-A during testing. If the frequencies match, then a first-order disturbance related to the rotation of the driveline - transmission output shaft/ rear axle pinion gear - is present.

Utilize the following worksheet as an aid in calculating the first, second and third order of tire/wheel assembly rotational speed and the first order of driveline (transmission output shaft/rear axle pinion gear) rotational speed related disturbances that may be present in the vehicle.

If after completing the Tire/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 tire/wheel assembly and/or driveline - transmission output shaft/rear axle pinion gear - 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 Tire/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, tire/wheel assembly or driveline - transmission output shaft/ rear axle pinion gear.