For 1990-2009 cars

### Tire and Wheel Rotational Speed Calculation

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 one second, 2 Hz, and so on.

#### Tire Rotational Speed (at 8 km/h [5 mph])

Tire Size | Tread | Revs/Sec (Hertz) at 8 km/h (5 mph)
--- | --- | ---
P175/65R14 | HWY | 1.25
P185/65R14 | HWY | 1.23
Tread Code
HWY | Highway

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 preceding Tire Rotational Speed table.

For example: According to the Tire Rotational Speed table, a P185/65R14 tire makes 1.23 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 1.23 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 1.23 Hz = 14.76 Hz (rounded to 15 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:

15 Hz X 2 (for second order) = 30 Hz second-order tire/wheel assembly rotation related

15 Hz X 3 (for third order) = 45 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 and/or driveline components (also rotating at the same speed) is present.

### Component Rotational Speed Worksheet

Utilize the following worksheet as an aid in calculating the first, second and third order of tire/wheel assembly 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 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 related).