Pontiac Fiero 1988 Service Manual page 155

3-14 STEERING, SUSPENSION, TIRES AND WHEELS DIAGNOSIS
VIBRATION COMPLAINT
TIRE-WHEEL-HUB-AXLE
RELATED
Vibrations
that are tire or wheel
induced
can
be caused
by two factors: imbalance or runout.
Low-speed vibrations, those less than 40 mph, are
usually runout related. Highway speed vibrations, those
above 40 mph, can be caused by either imbalance or runout.
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 or propeller shaft runout.
— Proper inflation pressure.
- Wrong trim height.
- Bent wheels.
- Debris build-up on the tire or wheel.
- Loose or missing wheel weights or wheel nuts.
— Irregular or excessive tire wear.
- Proper tire bead seating on rim.
— Damaged tires, such as tread distortions,
separations, or bulges from impact damage. Slight
sidewall indentations are normal and will not affect
ride quality.
Balance is the easiest procedure to perform and should,
therefore, be done first if the vibration occurs at highway
speeds. An off-car two-plane dynamic balance should first
be performed. This will correct any imbalance in the tire and
wheel assembly.
An on-car finish balance may also be required. This
will correct any brake drum, rotor, or wheel cover imbalance.
Follow the balancing procedures outlined in Section 3E.
If balance does not correct the highway speed vibration,
or if the vibration is at low speeds, runout is the probable
cause. Runout can be caused by the tire, wheel, or the way
the wheel attaches to the car. The following procedure
should be used:
A. If runout is suspected, the free runout of the tire
and wheel assembly should first be measured on the car. A
dial indicator with a roller wheel is preferable, but a dial
indicator with button end may be used. Lateral runout (side
to side) should be measured on the tire's sidewall as close to
the tread shoulder as possible. Radial runout (up and down)
should be measured on the center tread rib. Some tread
designs may require tightly wrapping a piece of tape around
the center tread circumference for better dial indicator
contact. For measuring wheel runout follow the "Measuring
Wheel Runout" procedure in Section 3E. Whether measuring
radial or lateral runout, disregard any instantaneous indicator
needle jumps due to sidewall depressions, tread blocks, etc.
Record the total indicator reading, and the location of the
high point of runout. The total tire and wheel on-car runout
should be less than .060", if either measurement exceeds
.060", proceed to Step B.
B. If the on-car radial or lateral runout measured in
Step A exceeds .060", mount the tire and wheel assembly on
a dynamic balance machine and again measure the amount of
runout. Locate on the machine by the wheel's inside center
pilot hole. Using the same procedure as in Step A, record the
amount of tire and wheel runout and its high point location.
Next, measure wheel runout, see Section 3E. If the wheel
exceeds specifications replace the wheel. If the tire and wheel
radial or lateral runout exceeds .050" at the tire tread,
proceed to Step C.
C. If the off-car tire and wheel radial or lateral runout
measured in Step B exceeds .050", match mount the high
radial runout point of tire to low radial runout point of
wheel. Reinflate, mount on the dynamic balance machine,
and again measure and record the radial and lateral runout
and its location, as done in Step B. In many cases, match
mounting the tire on the wheel will bring the assembly's
runout into the acceptable range of less than .050".
D. If the runout of the tire and wheel assembly is
within limits when measured off the car, yet exceeds the
limits when measured on the car, the attachment of the tire
and wheel assembly to the hub is the probable cause. Rotate
the assembly two wheel studs and recheck the runout.
Several positions may have to be tried to find the best
location.
E. If the assembly runout cannot be reduced to an
acceptable level, remove the tire and wheel assembly and
measure wheel stud runout with a dial indicator. Zero
the dial indicator button on one stud. Lift button gently
off stud and rotate flange to position next stud against
dial indicator button. Record the runout on all studs. Dial
indicator should read zero when repositioned on first stud
that was checked. If runout exceeds .030", the hub or axle
shaft should be replaced.
Whenever a tire is rotated on the wheel, or a tire or
wheel is replaced, the assembly must be rebalanced.
In addition to balance and tire and wheel free runout,
tire stiffness variation (loaded radial runout) can also cause
a vibration. However, this is impossible to measure without
a TPD (Tire Problem Detector) or a loaded radial runout
buffer.
The TPD is a roller drum that slowly rotates the tire
while under load and mounted on the car. Tire stiffness
variation causes wheel spindle movement which can be
measured.
The loaded radial runout buffer is a more automated
machine that slowly rotates the tire and wheel off the car
under load with a roller drum and measures the tire's
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.
The TPD and loaded radial runout buffer are two
methods that will measure or correct tire stiffness variation,
tire runout, and wheel runout at the same time. However,
because such equipment is not always available, and both
have their disadvantages, the more basic procedure of
measuring free runout with a dial indicator, as previously
detailed, is usually more practical. The free runout of the
tire will usually correspond with the tire's stiff spot.
The substitution method of vibration diagnosis can also
be used. Install a known good set of tire and wheel
assemblies. If these correct the vibration, the original
assemblies should be reinstalled one at a time until the
vibration returns. This will point out the tire with excess
stiffness variation.
Tire stiffness variation will be higher or lower depending
on the direction of tire rotation.
G35254-3-AN-R2
Fig. 18 Vibration Complaint Chart (2 of 2)