Vibration Diagnosis

VIBRATION DIAGNOSIS
Wheel imbalance causes most highway speed vibration problems. A vibration can remain after dynamic balancing because:
- A tire is out of round.
- A rim is out of round.
- A tire stiffness variation exists.

Measuring tire and wheel free runout will uncover only part of the problem. All three causes, known as loaded radial runout, must be checked using the method of substituting a known good tire and wheel assembly on the problem vehicle.

Low-speed vibrations, which occur below 64 km/h (40 mph), are usually caused by runout. High-speed vibrations, which occur above 64 km/h (40 mph), can be caused by either imbalance or runout.

Preliminary Checks





Prior to performing any work, always road test the car and perform a careful visual inspection for:
- Obvious tire and wheel runout.
- Obvious drive axle runout.
- Improper tire inflation.
- Incorrect trim height.
- Bent or damaged wheels.
- Debris build-up on the tire or the wheel.
- Irregular or excessive tire wear.
- Improper tire bead seating on the rim.
- Imperfections in the tires, including tread deformations, separations, or bulges from impact damage. Slight sidewall indentations are normal and will not affect ride quality.

Tire Balancing
Balancing is the easiest procedure to perform and should be done first if the vibration occurs at high speeds. Do an off-vehicle, two-plane dynamic balance first to correct any imbalance in the tire and wheel assembly.
An on-vehicle finish balance will correct any brake drum, rotor, or wheel cover imbalance. If balancing does not correct the high-speed vibration, or if the vibration occurs at low speeds, runout is the probable cause.

Runout
Runout can be caused by the tire, the wheel, or the way the wheel is attached to the vehicle. To investigate the possibility of wheel runout, refer to the following procedures as well as the wheel runout diagnosis chart.
1. If runout is suspected, measure the on-vehicle free lateral and free radial runout of the tire and wheel assembly. Both the free lateral and the free radial runout should be less than 1.5 mm (0.06 inch). If either measurement exceeds this number, proceed to step 2.
2. Mount the tire and the wheel on a dynamic balancing machine and remeasure the free lateral and the free radial runout. Record the amount of the free lateral and the free radial runout and the location of the highest measurement. If the free radial or the free lateral runout exceeds 1.3 mm (0.05 inch) at the tire tread, proceed to Step 4.
3. Measure the wheel runout. If the wheel exceeds specifications, replace it.
4. Deflate the tire and match-mount the high radial runout point of the tire to the low radial runout point of the wheel. Reinflate the tire and mount it on the dynamic balancing machine. Measure and record the free radial and the free lateral runout and their locations. In many cases, match mounting the tire on the wheel will bring the tire and wheel assembly's free runout into an acceptable range of 1.3 mm (0.05 inch) or less.
5. If the free runout of the tire and wheel assembly is 1.3 mm (0.05 inch) or less when it was measured off the vehicle, yet exceeds 1.3 mm (0.05 inch) when measured on the vehicle, the attachment of the tire and wheel assembly to the hub is the probable cause of the vibration. Rotate two of the assembly's wheel nuts and recheck the runout. Several positions may have to be tried to find the best location for the nuts.
6. If the tire and wheel assembly free runout cannot be reduced to 11.3 mm (0.05 inch) or less, remove the assembly.
6.1 Measure the hub stud runout using a dial indicator set with a magnetic base.
6.2 Zero the dial indicator set button on one stud.
6.3 Gently lift the set button off the stud. Rotate the flange to position the next stud against the dial indicator set.
6.4 Record the runout of all the studs. The dial indicator should read zero when it is repositioned on the first stud that was checked.
6.5 If the runout exceeds 0.76 mm (0.03 inch), the hub stud or the hub and bearing assembly should be replaced.
Whenever a tire is rotated on the wheel, or whenever a tire or wheel is replaced, rebalance the assembly.





Wheel Runout Diagnosis Chart