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Tire & Wheel Rotational Speed Calculation

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  1. At 5 MPH, a vehicle will travel approximately 88" (7.333 ft) per second. To determine the distance around the tire/wheel combination (circumference) make a vertical mark on the tire with chalk and a corresponding mark on the ground. Move the vehicle forward or backward until mark is pointing towards the ground again (360 degrees) and mark ground again. Measure distance between both marks to determine the circumference around the tire/wheel combination. Divide 88 (inches per second at 5 MPH) by the circumference of each tire to determine the RPS, or Hz at 5 MPH.
  2. Determine the number of increments of 5 MPH that are present, based on the vehicle speed (MPH) at which the disturbance occurs. For example: Assume that a disturbance occurs at a vehicle speed of 60 MPH. A speed of 60 MPH has 12 increments of 5 MPH: 60 MPH divided by 5 MPH = 12 increments.
  3. Determine the rotational speed of the tires in revolutions per second (Hz), at the specific vehicle speed (MPH) at which the disturbance occurs. For example: To determine the tire rotational speed at 60 MPH, multiply the number of increments of 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 EVA 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.
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