Road Test

A tire vibration diagnostic procedure always begins with a road test. The road test and customer interview (if available) will provide much of the information needed to find the source of a vibration.

During the road test, drive the vehicle on a road that is smooth and free of Undulations. If vibration is apparent, note and record the following:
- The speed at which the vibration occurs.
- What type of vibration occurs in each speed range - mechanical or audible.
- How the vibration is affected by changes in vehicle speed, engine speed, and engine torque.
- Type of vibration sensitivity - torque sensitive, vehicle speed sensitive, or engine speed sensitive.

Use the following explanation of terms to help isolate the source of the vibration.

Torque Sensitive
The condition can be improved or worsened by accelerating, decelerating, coasting, maintaining a steady vehicle speed and application of engine torque.

Vehicle Speed Sensitive
The vibration always occurs at the same vehicle speed and is not affected by engine torque, engine speed, or transmission gear selection.

Engine Speed Sensitive
The vibration occurs at varying vehicle speeds when a different transmission gear is selected. It can sometimes be isolated by increasing or decreasing engine speed with the transmission in NEUTRAL, or by stall testing with the transmission in gear. If the condition is engine-speed sensitive, the condition is not related to tires.
If the road test indicates the vibration is related to the tires or wheels, refer to Pinpoint Test G. Refer to Wheels, Tires and Alignment to help pinpoint the cause of the concern. If the road test indicates there is tire whine, but no shake or vibration, the noise originates from the contact between the tire and the road surface.
- A thumping noise usually means the tire has flat or soft spots making a noise as they slap the roadway. Tire whine can be distinguished from axle noise because axle noise diminishes or changes according to load or speed. Tire noise remains the same over a range of speeds.
- To verify that tire noise is not associated with shake or vibration, inflate the tires one at a time and check for a change in the sound. The pitch or whine will change as the increased pressure changes the tire frequency.
- Be sure to deflate tires to their proper pressures after this check is completed.
- A complete road test procedure is provided in Noise, Vibration and Harshness Diagnosis.