Pontiac FIREBIRD 1972 Service Manual page 1240

WHEELS AND TIRES 10-3
Fig. 10-2 Characteristic Tread W e a r Pattern of Bias-Belted
Tires
sides of front tire treads. In making a turn to the left,
especially at high speeds, the outside shoulder of the
right tire and the inside shoulder of the left tire take
most of the wear. When making a right-hand turn,
the opposite shoulders of the tires are worn.
3. High crowned roads cause increased wear on the side
of the right front tire. This is particularly true when
there is too much toe-in on front wheels or when
positive camber of right front wheel is greater than
the left.
Cornering wear can usually be differentiated from camber
wear because cornering wear affects both sides of the tire,
giving it a very round apperance. When camber is incor-
rect, it will cause excessive wear only on one side of the
tire tread. Camber wear does not leave the tread rounded
as cornering wear does.
When cornering wear is encountered, the owner should be
shown, by the rough tire surface and rounded shoulders,
that he is severely abrading his tires by fast or sharp turns,
and told that he could greatly prolong the useful life of his
tires by taking the turns a little slower. The tires and
wheels should be rotated and continued in service the
same as with normal camber wear.
TOE-IN OR TOE-OUT MISALIGNMENT WEAR
Front wheels should be straight ahead or toe-in slightly.
When there is excessive toe-in or toe-out, tires will revolve
with a side motion and scrape the tread rubber off. Front
tires will show wear on the outside with a toe-in condition
and on the inside with a toe-out condition. This wear
pattern is reversed when considering toe for rear tires.
UNEVEN TIRE WEAR
Other types of uneven tread wear such as a single spot or
series of cuppings around the tire circumference may also
be noted on some tires. Such uneven wear may be due to
excess toe-in or toe-out with underinflation, uneven cam-
ber, or such irregularities as bent or worn suspension,
wobbly wheels, improper caster, out of round brake
drums, and unequally adjusted brakes.
TREAD WEAR
Tread wear is affected by wheel alignment, cornering, and
inflation pressure as mentioned previously. There are sev-
eral factors which must be considered in analyzing tread
wear.
A careful driver may obtain many times the mileage from
his tires than would be obtained by a severe driver. Also,
tires wear much faster in some localities depending on the
type of road (some are more abrasive than others), condi-
tion of road (rain or snow), the number of sharp turns,
hills or mountains the car must go up or down, and the
prevailing temperature. Fast driving, quick starting, and
hard stopping are generally recognized as a definite cause
of rapid tread wear. By actual test, an increase of 40°F in
temperature reduces tread mileage by 3 3 5%.
TESTING FOR TIRE NOISES
To determine if tires are causing a noise, use the following
procedure:
Snow tires produce a noise which the owner will have to
ignore or overlook. If not equipped with snow tires, drive
the car at various speeds and note the effect of acceleration
and deceleration on noise. Axle and exhaust noise show
definite variations under these conditions while tire noise
will remain constant. Tire noise generally is most pro-
nounced on smooth black top roads at speeds between 15
and 40 miles per hour.
Tire thump is a periodic noise with each revolution of the
wheel. It is prominent only on smooth black top pavement
that is free of surface irregularities. Tire thump may be
checked by driving the car over a smooth black top pave-
ment with tires at recommended inflation pressure, and
again over the same stretch of road with all tires inflated
to 50 lbs. Drop the pressure in one tire at a time to normal
until defective tire is found.
CAUTION: Stn'king any obstructions or rocks
in road with tire at 5Opsi can lead to tire casing
rupture. Operate car with higher than recom-
mended inflation only while testing. Do not
operate car over 50 mph with high tire pressure.
Carefully inspect the tire making the noise for bulges,
irregular wear, low air pressure, toe and heel (saw tooth)
wear, and unusual tread design (ribbed tread gives less
noise than some all-weather treads; mud and snow treads
are very noisy). Checking wheel alignment and rotating
tires will usually cure tire noises unless caused by tire
tread design, heavy irregular tread wear, or tire bulges.