Component Rotational Speed Calculation
Component Rotational Speed Calculation
Tools Required
J 38792-A Electronic Vibration Analyzer (EVA) 2
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.
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 P235/65R16 tire makes 1.03 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 1.03 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 tires 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 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, or Hz, for one increment:
12 increments X 1.03 Hz = 12.36 Hz, rounded to 12 Hz
Important: If the J 38792-A 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 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:
12 Hz X 2, for second order = 24 Hz second-order tire/wheel assembly rotation related
12 Hz X 3, for third order = 36 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.
Propeller Shaft Rotational Speed
The rear differential assembly is designed to provide on-demand drive torque to the rear tire and wheel assemblies as needed. The differential incorporates clutch packs that are fed differential gear lubricant by gerotor pumps to provide the needed drive torque to the rear wheels. The rear differential assembly is designed to allow varying amounts of slip between the amount of input torque coming from the transfer case through the propeller shaft, and the amount of torque being delivered to the rear tire and wheel assemblies.
The propeller shaft is rotated by the transfer case during any movement of the vehicle. The transfer case is attached directly to the transaxle and incorporates functions of the transaxle extension housing. The transfer case operates using an output ratio that is almost identical to the transaxle final drive ratio. As a result, the propeller shaft rotates at approximately the same speed as the front tire and wheel assemblies.
If the first-order rotational speed of the tires is determined to be 12 Hz, then the first-order rotational speed of the propeller shaft would also be 12 Hz. The slight ratio change that occurs through the transfer case will have little affect on the propeller shaft rotational speed:
The propeller shafts use 2 U-joints. It would be possible to have a second-order propeller shaft rotational disturbance.
To compute a second-order propeller shaft rotation related disturbance, multiply the first order rotational speed of the propeller shaft by the order number of 2:
12 Hz X 2, for second order = 24 Hz second-order propeller shaft rotation related disturbance
The front propeller shaft uses a front constant-velocity (CV) joint. The CV joint uses a 6-ball bearing design. It would be possible to have a sixth-order propeller shaft rotational disturbance.
To compute a sixth-order propeller shaft rotation related disturbance, multiply the first order rotational speed of the propeller shaft by the order number of 6:
12 Hz X 6, for sixth order = 72 Hz sixth-order propeller shaft rotation related disturbance
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 11/2-8 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.
