Initial Inspection and Diagnostic Overview

GENERAL INFORMATION
Tire or wheel-induced vibrations can be caused by two factors: imbalance or runout. Low speed vibrations, those at less than 64 km/h (40 mph), are usually runout related. Highway speed vibrations, those above 64 km/h (40 mph), can be caused by either imbalance or runout. Before doing any work, road test the vehicle, and make a careful visual inspection for:
^ Obvious tire and wheel runout.
^ Obvious drive axle runout.
^ Improper inflation pressure.
^ Wrong trim height.
^ Bent wheels.
^ Debris build-up on tire or wheel.
^ Loose or missing wheel weights or wheel nuts.
^ Irregular or excessive tire wear.
^ Improper tire bead seating on rim.
^ Tire damage, such as tread distortions, separations, or bulges from impact damage. Slight sidewall indentations are normal.

After doing the visual inspections, do the procedure in the "Vibration Diagnosis" chart. Before removing a tire and wheel from the vehicle, mark the stud bolt nearest the valve stem position for reference. Also, mark the vehicle position on each tire removed (i.e., right front, left rear, etc.).

BALANCE
Balancing is the easiest procedure to do, and should be done first if the vibration occurs at highway speeds. An off-vehicle, two-plane dynamic balance should be done first to correct an imbalance in the tire and wheel. An on-vehicle finish balance may also be needed. This will correct any brake drum, rotor, or wheel cover imbalance. Follow the correct balancing procedures.

RUNOUT
If balancing does not correct a highway speed vibration, or if the vibration occurs at low speeds, runout is the probable cause. Runout can be caused by the tire, the wheel, or the way the wheel attaches to the vehicle. The following procedure should be used:
1. Measure the free runout of the tire and wheel on the vehicle. (A dial indicator with a roller wheel is preferred, but a dial indicator with a button end may be used).
A. Measure lateral runout (side-to-side) on tire sidewall, as close to tread shoulder as possible.
B. Measure radial runout (up-and-down) on the center tread rib. You may need to wrap tape tightly around the center tread rib for better dial indicator contact.
^ Disregard instantaneous indicator needle jumps due to sidewall depressions, tread blocks, etc.
C. Record the total indicator reading, and locations of the high point of runout.
D. Total tire-wheel on-vehicle runout should be less than 1.5 mm (0.060 inch). If lateral or radial runout exceeds this, go on to Step 2.
2. Mount the tire and wheel on a dynamic balance machine, and measure the amount of runout, as in Step 1.Record amount of tire-wheel runout and its high point locations.
3. Measure wheel runout.
^ If the wheel exceeds specifications, replace it.
^ If the tire and wheel radial or lateral runout exceeds 1.3 mm (0.050 inch) at the tire tread, go on to Step 4.
4. Match mount the high radial runout point of the tire to the low point of the wheel. Reinflate and repeat Steps 2 and 3.
5. If tire and wheel runout is within limits when measured off-vehicle, yet is not when measured on-vehicle, rotate the wheel on the vehicle by two wheel studs and recheck runout. Several positions may have to be tried to find the best mounting position on the hub.
6. If tire and wheel runout cannot be reduced to an acceptable level, do the following:
A. Remove tire and wheel from vehicle.
B. Measure wheel stud runout with a dial indicator. Zero the indicator on one stud, then lift button off stud and rotate flange to position the next stud against the indicator button.
C. Record runout on all studs. Dial indicator should read zero when repositioned on first stud checked. If runout exceeds 0.75 mm (0.030 inch), the hub or axle shaft should be replaced.

When a tire is rotated on the wheel, or a tire or wheel is replaced, the assembly must be rebalanced.

TIRE STIFFNESS VARIATION
In addition to balance and free runout, tire stiffness variation (loaded radial runout) can also cause a vibration. However, this is impossible to measure without a Tire Problem Detector (TPD) or a loaded radial runout buffer. The TPD is a roller drum that slowly rotates the tire while under load and mounted on the vehicle. Tire stiffness variation causes wheel spindle movement which can be measured. The loaded radial runout buffer slowly rotates the tire and wheel off the vehicle under load with a roller drum, and measures tire stiffness variation. It will then "match" the tire to the wheel by buffing off small amounts of rubber from the outer tread rows at the stiff spot. This procedure is usually effective, especially when used as a measuring device and for fine buffing only. Use of the TPD and loaded radial runout buffer provides two methods to measure or correct tire stiffness variation, tire runout and wheel runout at the same time. However, the basic procedure of measuring free runout with a dial indicator, as previously described, is usually more practical, especially when the TPD or runout buffer is unavailable. The free runout of the tire will usually correspond with the tire's stiff spot.

SUBSTITUTION
Substitution of tires and wheels can also be used for vibration diagnosis. Replace each tire and wheel, one at a time, with a known good one, and road test after each change.