Road Test

DESCRIPTION
Vibration, roughness, tramp, shimmy and thump can be caused by excessive tire or wheel runout, worn or cupped tires, or wheel and tire imbalance.

These conditions can also be caused by rough or undulating road surfaces. Driving the vehicle on different types of road surfaces will indicate if the road surfaces are actually causing the condition.

Do not immediately suspect the tires when attempting to diagnose a vibration concern. Other sources of vibration include:
^ loose or worn wheel bearings
^ loose or worn suspension or steering components
^ brake hub and rotor or drum runout
^ loose engine or transmission supports
^ engine-driven accessories

This section covers those vibrations related to the suspension, tires and wheels.

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 (such as shake) or audible (such as tire whine)
^ 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

If the road test indicates the vibration is related to the tires or wheels, refer to Pinpoint Test F. If the road test indicates there is tire whine, but no shake or vibration, the noise originates with 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 axle noise diminishes or changes according to load or speed while 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.

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 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.